| Message ID | 20210602212726.7-1-fuzzybritches0@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v3] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run | expand |
On Wed, Jun 02, 2021 at 09:27:26PM +0000, Kurt Manucredo wrote: > UBSAN: shift-out-of-bounds in kernel/bpf/core.c:1414:2 > shift exponent 248 is too large for 32-bit type 'unsigned int' I'm sorry, but I still do not understand what this changelog text means. Please be very descriptive about what you are doing and why you are doing it. All that is here is a message from a random tool :( thanks, greg k-h
On 6/5/21 8:01 AM, Kurt Manucredo wrote: > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > kernel/bpf/core.c:1414:2. This is not enough. We need more information on why this happens so we can judge whether the patch indeed fixed the issue. > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > missing them and return with error when detected. > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > --- > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > Changelog: > ---------- > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > Fix commit message. > v3 - Make it clearer what the fix is for. > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > check in check_alu_op() in verifier.c. > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > check in ___bpf_prog_run(). > > thanks > > kind regards > > Kurt > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > 1 file changed, 9 insertions(+), 21 deletions(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 94ba5163d4c5..ed0eecf20de5 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > u32_min_val = src_reg.u32_min_value; > u32_max_val = src_reg.u32_max_value; > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > + umax_val >= insn_bitness) { > + /* Shifts greater than 31 or 63 are undefined. > + * This includes shifts by a negative number. > + */ > + verbose(env, "invalid shift %lld\n", umax_val); > + return -EINVAL; > + } I think your fix is good. I would like to move after the following code though: if (!src_known && opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { __mark_reg_unknown(env, dst_reg); return 0; } > + > if (alu32) { > src_known = tnum_subreg_is_const(src_reg.var_off); > if ((src_known && > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > scalar_min_max_xor(dst_reg, &src_reg); > break; > case BPF_LSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } I think this is what happens. For the above case, we simply marks the dst reg as unknown and didn't fail verification. So later on at runtime, the shift optimization will have wrong shift value (> 31/64). Please correct me if this is not right analysis. As I mentioned in the early please write detailed analysis in commit log. Please also add a test at tools/testing/selftests/bpf/verifier/. > if (alu32) > scalar32_min_max_lsh(dst_reg, &src_reg); > else > scalar_min_max_lsh(dst_reg, &src_reg); > break; > case BPF_RSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } > if (alu32) > scalar32_min_max_rsh(dst_reg, &src_reg); > else > scalar_min_max_rsh(dst_reg, &src_reg); > break; > case BPF_ARSH: > - if (umax_val >= insn_bitness) { > - /* Shifts greater than 31 or 63 are undefined. > - * This includes shifts by a negative number. > - */ > - mark_reg_unknown(env, regs, insn->dst_reg); > - break; > - } > if (alu32) > scalar32_min_max_arsh(dst_reg, &src_reg); > else >
On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > kernel/bpf/core.c:1414:2. > > This is not enough. We need more information on why this happens > so we can judge whether the patch indeed fixed the issue. > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > missing them and return with error when detected. > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > --- > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > Changelog: > > ---------- > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > Fix commit message. > > v3 - Make it clearer what the fix is for. > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in check_alu_op() in verifier.c. > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in ___bpf_prog_run(). > > > > thanks > > > > kind regards > > > > Kurt > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 94ba5163d4c5..ed0eecf20de5 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > u32_min_val = src_reg.u32_min_value; > > u32_max_val = src_reg.u32_max_value; > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > + umax_val >= insn_bitness) { > > + /* Shifts greater than 31 or 63 are undefined. > > + * This includes shifts by a negative number. > > + */ > > + verbose(env, "invalid shift %lld\n", umax_val); > > + return -EINVAL; > > + } > > I think your fix is good. I would like to move after I suspect such change will break valid programs that do shift by register. > the following code though: > > if (!src_known && > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > __mark_reg_unknown(env, dst_reg); > return 0; > } > > > + > > if (alu32) { > > src_known = tnum_subreg_is_const(src_reg.var_off); > > if ((src_known && > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > scalar_min_max_xor(dst_reg, &src_reg); > > break; > > case BPF_LSH: > > - if (umax_val >= insn_bitness) { > > - /* Shifts greater than 31 or 63 are undefined. > > - * This includes shifts by a negative number. > > - */ > > - mark_reg_unknown(env, regs, insn->dst_reg); > > - break; > > - } > > I think this is what happens. For the above case, we simply > marks the dst reg as unknown and didn't fail verification. > So later on at runtime, the shift optimization will have wrong > shift value (> 31/64). Please correct me if this is not right > analysis. As I mentioned in the early please write detailed > analysis in commit log. The large shift is not wrong. It's just undefined. syzbot has to ignore such cases.
On 6/5/21 12:10 PM, Alexei Starovoitov wrote: > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >> >> >> >> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>> kernel/bpf/core.c:1414:2. >> >> This is not enough. We need more information on why this happens >> so we can judge whether the patch indeed fixed the issue. >> >>> >>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>> missing them and return with error when detected. >>> >>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>> --- >>> >>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>> >>> Changelog: >>> ---------- >>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>> Fix commit message. >>> v3 - Make it clearer what the fix is for. >>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>> check in check_alu_op() in verifier.c. >>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>> check in ___bpf_prog_run(). >>> >>> thanks >>> >>> kind regards >>> >>> Kurt >>> >>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>> 1 file changed, 9 insertions(+), 21 deletions(-) >>> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>> index 94ba5163d4c5..ed0eecf20de5 100644 >>> --- a/kernel/bpf/verifier.c >>> +++ b/kernel/bpf/verifier.c >>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>> u32_min_val = src_reg.u32_min_value; >>> u32_max_val = src_reg.u32_max_value; >>> >>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>> + umax_val >= insn_bitness) { >>> + /* Shifts greater than 31 or 63 are undefined. >>> + * This includes shifts by a negative number. >>> + */ >>> + verbose(env, "invalid shift %lld\n", umax_val); >>> + return -EINVAL; >>> + } >> >> I think your fix is good. I would like to move after > > I suspect such change will break valid programs that do shift by register. Oh yes, you are correct. We should guard it with src_known. But this should be extremely rare with explicit shifting amount being greater than 31/64 and if it is the case, the compiler will has a warning. > >> the following code though: >> >> if (!src_known && >> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >> __mark_reg_unknown(env, dst_reg); >> return 0; >> } >> >>> + >>> if (alu32) { >>> src_known = tnum_subreg_is_const(src_reg.var_off); >>> if ((src_known && >>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>> scalar_min_max_xor(dst_reg, &src_reg); >>> break; >>> case BPF_LSH: >>> - if (umax_val >= insn_bitness) { >>> - /* Shifts greater than 31 or 63 are undefined. >>> - * This includes shifts by a negative number. >>> - */ >>> - mark_reg_unknown(env, regs, insn->dst_reg); >>> - break; >>> - } >> >> I think this is what happens. For the above case, we simply >> marks the dst reg as unknown and didn't fail verification. >> So later on at runtime, the shift optimization will have wrong >> shift value (> 31/64). Please correct me if this is not right >> analysis. As I mentioned in the early please write detailed >> analysis in commit log. > > The large shift is not wrong. It's just undefined. > syzbot has to ignore such cases. Agree. This makes sense.
On Sat, 5 Jun 2021 10:55:25 -0700, Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > kernel/bpf/core.c:1414:2. > > This is not enough. We need more information on why this happens > so we can judge whether the patch indeed fixed the issue. > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > missing them and return with error when detected. > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > --- > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > Changelog: > > ---------- > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > Fix commit message. > > v3 - Make it clearer what the fix is for. > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in check_alu_op() in verifier.c. > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > check in ___bpf_prog_run(). > > > > thanks > > > > kind regards > > > > Kurt > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 94ba5163d4c5..ed0eecf20de5 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > u32_min_val = src_reg.u32_min_value; > > u32_max_val = src_reg.u32_max_value; > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > + umax_val >= insn_bitness) { > > + /* Shifts greater than 31 or 63 are undefined. > > + * This includes shifts by a negative number. > > + */ > > + verbose(env, "invalid shift %lldn", umax_val); > > + return -EINVAL; > > + } > > I think your fix is good. I would like to move after > the following code though: > > if (!src_known && > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > __mark_reg_unknown(env, dst_reg); > return 0; > } > It can only be right before that code not after. That's the latest. In the case of the syzbot bug, opcode == BPF_LSH and !src_known. Therefore it needs to be before that block of code. > > + > > if (alu32) { > > src_known = tnum_subreg_is_const(src_reg.var_off); > > if ((src_known && > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > scalar_min_max_xor(dst_reg, &src_reg); > > break; > > case BPF_LSH: > > - if (umax_val >= insn_bitness) { > > - /* Shifts greater than 31 or 63 are undefined. > > - * This includes shifts by a negative number. > > - */ > > - mark_reg_unknown(env, regs, insn->dst_reg); > > - break; > > - } > > I think this is what happens. For the above case, we simply > marks the dst reg as unknown and didn't fail verification. > So later on at runtime, the shift optimization will have wrong > shift value (> 31/64). Please correct me if this is not right > analysis. As I mentioned in the early please write detailed > analysis in commit log. > Shouldn't the src reg be changed so that the shift-out-of-bounds can't occur, if return -EINVAL is not what we want here? Changing the dst reg might not help. If I look into kernel/bpf/core.c I can see: DST = DST OP SRC; > Please also add a test at tools/testing/selftests/bpf/verifier/. > I'm going to look into selftests, kind regards thanks, Kurt Manucredo
On Sat, 5 Jun 2021 14:39:57 -0700, Yonghong Song <yhs@fb.com> wrote: > > > > On 6/5/21 12:10 PM, Alexei Starovoitov wrote: > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > >> > >> > >> > >> On 6/5/21 8:01 AM, Kurt Manucredo wrote: > >>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > >>> kernel/bpf/core.c:1414:2. > >> > >> This is not enough. We need more information on why this happens > >> so we can judge whether the patch indeed fixed the issue. > >> > >>> > >>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > >>> missing them and return with error when detected. > >>> > >>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > >>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > >>> --- > >>> > >>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > >>> > >>> Changelog: > >>> ---------- > >>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > >>> Fix commit message. > >>> v3 - Make it clearer what the fix is for. > >>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>> check in check_alu_op() in verifier.c. > >>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>> check in ___bpf_prog_run(). > >>> > >>> thanks > >>> > >>> kind regards > >>> > >>> Kurt > >>> > >>> kernel/bpf/verifier.c | 30 +++++++++--------------------- > >>> 1 file changed, 9 insertions(+), 21 deletions(-) > >>> > >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >>> index 94ba5163d4c5..ed0eecf20de5 100644 > >>> --- a/kernel/bpf/verifier.c > >>> +++ b/kernel/bpf/verifier.c > >>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>> u32_min_val = src_reg.u32_min_value; > >>> u32_max_val = src_reg.u32_max_value; > >>> > >>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > >>> + umax_val >= insn_bitness) { > >>> + /* Shifts greater than 31 or 63 are undefined. > >>> + * This includes shifts by a negative number. > >>> + */ > >>> + verbose(env, "invalid shift %lldn", umax_val); > >>> + return -EINVAL; > >>> + } > >> > >> I think your fix is good. I would like to move after > > > > I suspect such change will break valid programs that do shift by register. > > Oh yes, you are correct. We should guard it with src_known. > But this should be extremely rare with explicit shifting amount being > greater than 31/64 and if it is the case, the compiler will has a > warning. > > > > >> the following code though: > >> > >> if (!src_known && > >> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > >> __mark_reg_unknown(env, dst_reg); > >> return 0; > >> } > >> > >>> + > >>> if (alu32) { > >>> src_known = tnum_subreg_is_const(src_reg.var_off); > >>> if ((src_known && > >>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>> scalar_min_max_xor(dst_reg, &src_reg); > >>> break; > >>> case BPF_LSH: > >>> - if (umax_val >= insn_bitness) { > >>> - /* Shifts greater than 31 or 63 are undefined. > >>> - * This includes shifts by a negative number. > >>> - */ > >>> - mark_reg_unknown(env, regs, insn->dst_reg); > >>> - break; > >>> - } > >> > >> I think this is what happens. For the above case, we simply > >> marks the dst reg as unknown and didn't fail verification. > >> So later on at runtime, the shift optimization will have wrong > >> shift value (> 31/64). Please correct me if this is not right > >> analysis. As I mentioned in the early please write detailed > >> analysis in commit log. > > > > The large shift is not wrong. It's just undefined. > > syzbot has to ignore such cases. > > Agree. This makes sense. Thanks for your input. If you find I should look closer into this bug just let me know. I'd love to help. If not it's fine, too. :-) kind regards, Kurt Manucredo
On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > kernel/bpf/core.c:1414:2. > > > > This is not enough. We need more information on why this happens > > so we can judge whether the patch indeed fixed the issue. > > > > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > > missing them and return with error when detected. > > > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > > --- > > > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > > > Changelog: > > > ---------- > > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > > Fix commit message. > > > v3 - Make it clearer what the fix is for. > > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > check in check_alu_op() in verifier.c. > > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > check in ___bpf_prog_run(). > > > > > > thanks > > > > > > kind regards > > > > > > Kurt > > > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > > index 94ba5163d4c5..ed0eecf20de5 100644 > > > --- a/kernel/bpf/verifier.c > > > +++ b/kernel/bpf/verifier.c > > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > u32_min_val = src_reg.u32_min_value; > > > u32_max_val = src_reg.u32_max_value; > > > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > > + umax_val >= insn_bitness) { > > > + /* Shifts greater than 31 or 63 are undefined. > > > + * This includes shifts by a negative number. > > > + */ > > > + verbose(env, "invalid shift %lld\n", umax_val); > > > + return -EINVAL; > > > + } > > > > I think your fix is good. I would like to move after > > I suspect such change will break valid programs that do shift by register. > > > the following code though: > > > > if (!src_known && > > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > > __mark_reg_unknown(env, dst_reg); > > return 0; > > } > > > > > + > > > if (alu32) { > > > src_known = tnum_subreg_is_const(src_reg.var_off); > > > if ((src_known && > > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > scalar_min_max_xor(dst_reg, &src_reg); > > > break; > > > case BPF_LSH: > > > - if (umax_val >= insn_bitness) { > > > - /* Shifts greater than 31 or 63 are undefined. > > > - * This includes shifts by a negative number. > > > - */ > > > - mark_reg_unknown(env, regs, insn->dst_reg); > > > - break; > > > - } > > > > I think this is what happens. For the above case, we simply > > marks the dst reg as unknown and didn't fail verification. > > So later on at runtime, the shift optimization will have wrong > > shift value (> 31/64). Please correct me if this is not right > > analysis. As I mentioned in the early please write detailed > > analysis in commit log. > > The large shift is not wrong. It's just undefined. > syzbot has to ignore such cases. Hi Alexei, The report is produced by KUBSAN. I thought there was an agreement on cleaning up KUBSAN reports from the kernel (the subset enabled on syzbot at least). What exactly cases should KUBSAN ignore? +linux-hardening/kasan-dev for KUBSAN false positive
On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > <alexei.starovoitov@gmail.com> wrote: > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > > kernel/bpf/core.c:1414:2. > > > > > > This is not enough. We need more information on why this happens > > > so we can judge whether the patch indeed fixed the issue. > > > > > > > > > > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > > > > missing them and return with error when detected. > > > > > > > > Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > > > > Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > > > > --- > > > > > > > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > > > > > > > > Changelog: > > > > ---------- > > > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > > > > Fix commit message. > > > > v3 - Make it clearer what the fix is for. > > > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > > check in check_alu_op() in verifier.c. > > > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > > > > check in ___bpf_prog_run(). > > > > > > > > thanks > > > > > > > > kind regards > > > > > > > > Kurt > > > > > > > > kernel/bpf/verifier.c | 30 +++++++++--------------------- > > > > 1 file changed, 9 insertions(+), 21 deletions(-) > > > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > > > index 94ba5163d4c5..ed0eecf20de5 100644 > > > > --- a/kernel/bpf/verifier.c > > > > +++ b/kernel/bpf/verifier.c > > > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > > u32_min_val = src_reg.u32_min_value; > > > > u32_max_val = src_reg.u32_max_value; > > > > > > > > + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > > > > + umax_val >= insn_bitness) { > > > > + /* Shifts greater than 31 or 63 are undefined. > > > > + * This includes shifts by a negative number. > > > > + */ > > > > + verbose(env, "invalid shift %lld\n", umax_val); > > > > + return -EINVAL; > > > > + } > > > > > > I think your fix is good. I would like to move after > > > > I suspect such change will break valid programs that do shift by register. > > > > > the following code though: > > > > > > if (!src_known && > > > opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > > > __mark_reg_unknown(env, dst_reg); > > > return 0; > > > } > > > > > > > + > > > > if (alu32) { > > > > src_known = tnum_subreg_is_const(src_reg.var_off); > > > > if ((src_known && > > > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > > > > scalar_min_max_xor(dst_reg, &src_reg); > > > > break; > > > > case BPF_LSH: > > > > - if (umax_val >= insn_bitness) { > > > > - /* Shifts greater than 31 or 63 are undefined. > > > > - * This includes shifts by a negative number. > > > > - */ > > > > - mark_reg_unknown(env, regs, insn->dst_reg); > > > > - break; > > > > - } > > > > > > I think this is what happens. For the above case, we simply > > > marks the dst reg as unknown and didn't fail verification. > > > So later on at runtime, the shift optimization will have wrong > > > shift value (> 31/64). Please correct me if this is not right > > > analysis. As I mentioned in the early please write detailed > > > analysis in commit log. > > > > The large shift is not wrong. It's just undefined. > > syzbot has to ignore such cases. > > Hi Alexei, > > The report is produced by KUBSAN. I thought there was an agreement on > cleaning up KUBSAN reports from the kernel (the subset enabled on > syzbot at least). > What exactly cases should KUBSAN ignore? > +linux-hardening/kasan-dev for KUBSAN false positive Can check_shl_overflow() be used at all? Best to just make things readable and compiler-happy, whatever the implementation. :) -- Kees Cook
On 6/9/21 11:20 AM, Kees Cook wrote: > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: >> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov >> <alexei.starovoitov@gmail.com> wrote: >>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>>>> kernel/bpf/core.c:1414:2. >>>> >>>> This is not enough. We need more information on why this happens >>>> so we can judge whether the patch indeed fixed the issue. >>>> >>>>> >>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>>>> missing them and return with error when detected. >>>>> >>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>>>> --- >>>>> >>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>>>> >>>>> Changelog: >>>>> ---------- >>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>>>> Fix commit message. >>>>> v3 - Make it clearer what the fix is for. >>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>> check in check_alu_op() in verifier.c. >>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>> check in ___bpf_prog_run(). >>>>> >>>>> thanks >>>>> >>>>> kind regards >>>>> >>>>> Kurt >>>>> >>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>>>> 1 file changed, 9 insertions(+), 21 deletions(-) >>>>> >>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>>>> index 94ba5163d4c5..ed0eecf20de5 100644 >>>>> --- a/kernel/bpf/verifier.c >>>>> +++ b/kernel/bpf/verifier.c >>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>> u32_min_val = src_reg.u32_min_value; >>>>> u32_max_val = src_reg.u32_max_value; >>>>> >>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>>>> + umax_val >= insn_bitness) { >>>>> + /* Shifts greater than 31 or 63 are undefined. >>>>> + * This includes shifts by a negative number. >>>>> + */ >>>>> + verbose(env, "invalid shift %lld\n", umax_val); >>>>> + return -EINVAL; >>>>> + } >>>> >>>> I think your fix is good. I would like to move after >>> >>> I suspect such change will break valid programs that do shift by register. >>> >>>> the following code though: >>>> >>>> if (!src_known && >>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >>>> __mark_reg_unknown(env, dst_reg); >>>> return 0; >>>> } >>>> >>>>> + >>>>> if (alu32) { >>>>> src_known = tnum_subreg_is_const(src_reg.var_off); >>>>> if ((src_known && >>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>> scalar_min_max_xor(dst_reg, &src_reg); >>>>> break; >>>>> case BPF_LSH: >>>>> - if (umax_val >= insn_bitness) { >>>>> - /* Shifts greater than 31 or 63 are undefined. >>>>> - * This includes shifts by a negative number. >>>>> - */ >>>>> - mark_reg_unknown(env, regs, insn->dst_reg); >>>>> - break; >>>>> - } >>>> >>>> I think this is what happens. For the above case, we simply >>>> marks the dst reg as unknown and didn't fail verification. >>>> So later on at runtime, the shift optimization will have wrong >>>> shift value (> 31/64). Please correct me if this is not right >>>> analysis. As I mentioned in the early please write detailed >>>> analysis in commit log. >>> >>> The large shift is not wrong. It's just undefined. >>> syzbot has to ignore such cases. >> >> Hi Alexei, >> >> The report is produced by KUBSAN. I thought there was an agreement on >> cleaning up KUBSAN reports from the kernel (the subset enabled on >> syzbot at least). >> What exactly cases should KUBSAN ignore? >> +linux-hardening/kasan-dev for KUBSAN false positive > > Can check_shl_overflow() be used at all? Best to just make things > readable and compiler-happy, whatever the implementation. :) This is not a compile issue. If the shift amount is a constant, compiler should have warned and user should fix the warning. This is because user code has something like a << s; where s is a unknown variable and verifier just marked the result of a << s as unknown value. Verifier may not reject the code depending on how a << s result is used. If bpf program writer uses check_shl_overflow() or some kind of checking for shift value and won't do shifting if the shifting may cause an undefined result, there should not be any kubsan warning. >
On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: > On 6/9/21 11:20 AM, Kees Cook wrote: > > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > >> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > >> <alexei.starovoitov@gmail.com> wrote: > >>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > >>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: > >>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > >>>>> kernel/bpf/core.c:1414:2. > >>>> > >>>> This is not enough. We need more information on why this happens > >>>> so we can judge whether the patch indeed fixed the issue. > >>>> > >>>>> > >>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid > >>>>> missing them and return with error when detected. > >>>>> > >>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com > >>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> > >>>>> --- > >>>>> > >>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 > >>>>> > >>>>> Changelog: > >>>>> ---------- > >>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. > >>>>> Fix commit message. > >>>>> v3 - Make it clearer what the fix is for. > >>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>>>> check in check_alu_op() in verifier.c. > >>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary > >>>>> check in ___bpf_prog_run(). > >>>>> > >>>>> thanks > >>>>> > >>>>> kind regards > >>>>> > >>>>> Kurt > >>>>> > >>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- > >>>>> 1 file changed, 9 insertions(+), 21 deletions(-) > >>>>> > >>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > >>>>> index 94ba5163d4c5..ed0eecf20de5 100644 > >>>>> --- a/kernel/bpf/verifier.c > >>>>> +++ b/kernel/bpf/verifier.c > >>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>>>> u32_min_val = src_reg.u32_min_value; > >>>>> u32_max_val = src_reg.u32_max_value; > >>>>> > >>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && > >>>>> + umax_val >= insn_bitness) { > >>>>> + /* Shifts greater than 31 or 63 are undefined. > >>>>> + * This includes shifts by a negative number. > >>>>> + */ > >>>>> + verbose(env, "invalid shift %lld\n", umax_val); > >>>>> + return -EINVAL; > >>>>> + } > >>>> > >>>> I think your fix is good. I would like to move after > >>> > >>> I suspect such change will break valid programs that do shift by register. > >>> > >>>> the following code though: > >>>> > >>>> if (!src_known && > >>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { > >>>> __mark_reg_unknown(env, dst_reg); > >>>> return 0; > >>>> } > >>>> > >>>>> + > >>>>> if (alu32) { > >>>>> src_known = tnum_subreg_is_const(src_reg.var_off); > >>>>> if ((src_known && > >>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, > >>>>> scalar_min_max_xor(dst_reg, &src_reg); > >>>>> break; > >>>>> case BPF_LSH: > >>>>> - if (umax_val >= insn_bitness) { > >>>>> - /* Shifts greater than 31 or 63 are undefined. > >>>>> - * This includes shifts by a negative number. > >>>>> - */ > >>>>> - mark_reg_unknown(env, regs, insn->dst_reg); > >>>>> - break; > >>>>> - } > >>>> > >>>> I think this is what happens. For the above case, we simply > >>>> marks the dst reg as unknown and didn't fail verification. > >>>> So later on at runtime, the shift optimization will have wrong > >>>> shift value (> 31/64). Please correct me if this is not right > >>>> analysis. As I mentioned in the early please write detailed > >>>> analysis in commit log. > >>> > >>> The large shift is not wrong. It's just undefined. > >>> syzbot has to ignore such cases. > >> > >> Hi Alexei, > >> > >> The report is produced by KUBSAN. I thought there was an agreement on > >> cleaning up KUBSAN reports from the kernel (the subset enabled on > >> syzbot at least). > >> What exactly cases should KUBSAN ignore? > >> +linux-hardening/kasan-dev for KUBSAN false positive > > > > Can check_shl_overflow() be used at all? Best to just make things > > readable and compiler-happy, whatever the implementation. :) > > This is not a compile issue. If the shift amount is a constant, > compiler should have warned and user should fix the warning. > > This is because user code has > something like > a << s; > where s is a unknown variable and > verifier just marked the result of a << s as unknown value. > Verifier may not reject the code depending on how a << s result > is used. > > If bpf program writer uses check_shl_overflow() or some kind > of checking for shift value and won't do shifting if the > shifting may cause an undefined result, there should not > be any kubsan warning. I guess the main question: what should happen if a bpf program writer does _not_ use compiler nor check_shl_overflow()?
On 6/9/21 10:32 PM, Dmitry Vyukov wrote: > On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: >> On 6/9/21 11:20 AM, Kees Cook wrote: >>> On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: >>>> On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov >>>> <alexei.starovoitov@gmail.com> wrote: >>>>> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: >>>>>> On 6/5/21 8:01 AM, Kurt Manucredo wrote: >>>>>>> Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() >>>>>>> kernel/bpf/core.c:1414:2. >>>>>> >>>>>> This is not enough. We need more information on why this happens >>>>>> so we can judge whether the patch indeed fixed the issue. >>>>>> >>>>>>> >>>>>>> I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid >>>>>>> missing them and return with error when detected. >>>>>>> >>>>>>> Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com >>>>>>> Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> >>>>>>> --- >>>>>>> >>>>>>> https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 >>>>>>> >>>>>>> Changelog: >>>>>>> ---------- >>>>>>> v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals. >>>>>>> Fix commit message. >>>>>>> v3 - Make it clearer what the fix is for. >>>>>>> v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>>>> check in check_alu_op() in verifier.c. >>>>>>> v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary >>>>>>> check in ___bpf_prog_run(). >>>>>>> >>>>>>> thanks >>>>>>> >>>>>>> kind regards >>>>>>> >>>>>>> Kurt >>>>>>> >>>>>>> kernel/bpf/verifier.c | 30 +++++++++--------------------- >>>>>>> 1 file changed, 9 insertions(+), 21 deletions(-) >>>>>>> >>>>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>>>>>> index 94ba5163d4c5..ed0eecf20de5 100644 >>>>>>> --- a/kernel/bpf/verifier.c >>>>>>> +++ b/kernel/bpf/verifier.c >>>>>>> @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>>>> u32_min_val = src_reg.u32_min_value; >>>>>>> u32_max_val = src_reg.u32_max_value; >>>>>>> >>>>>>> + if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) && >>>>>>> + umax_val >= insn_bitness) { >>>>>>> + /* Shifts greater than 31 or 63 are undefined. >>>>>>> + * This includes shifts by a negative number. >>>>>>> + */ >>>>>>> + verbose(env, "invalid shift %lld\n", umax_val); >>>>>>> + return -EINVAL; >>>>>>> + } >>>>>> >>>>>> I think your fix is good. I would like to move after >>>>> >>>>> I suspect such change will break valid programs that do shift by register. >>>>> >>>>>> the following code though: >>>>>> >>>>>> if (!src_known && >>>>>> opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) { >>>>>> __mark_reg_unknown(env, dst_reg); >>>>>> return 0; >>>>>> } >>>>>> >>>>>>> + >>>>>>> if (alu32) { >>>>>>> src_known = tnum_subreg_is_const(src_reg.var_off); >>>>>>> if ((src_known && >>>>>>> @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, >>>>>>> scalar_min_max_xor(dst_reg, &src_reg); >>>>>>> break; >>>>>>> case BPF_LSH: >>>>>>> - if (umax_val >= insn_bitness) { >>>>>>> - /* Shifts greater than 31 or 63 are undefined. >>>>>>> - * This includes shifts by a negative number. >>>>>>> - */ >>>>>>> - mark_reg_unknown(env, regs, insn->dst_reg); >>>>>>> - break; >>>>>>> - } >>>>>> >>>>>> I think this is what happens. For the above case, we simply >>>>>> marks the dst reg as unknown and didn't fail verification. >>>>>> So later on at runtime, the shift optimization will have wrong >>>>>> shift value (> 31/64). Please correct me if this is not right >>>>>> analysis. As I mentioned in the early please write detailed >>>>>> analysis in commit log. >>>>> >>>>> The large shift is not wrong. It's just undefined. >>>>> syzbot has to ignore such cases. >>>> >>>> Hi Alexei, >>>> >>>> The report is produced by KUBSAN. I thought there was an agreement on >>>> cleaning up KUBSAN reports from the kernel (the subset enabled on >>>> syzbot at least). >>>> What exactly cases should KUBSAN ignore? >>>> +linux-hardening/kasan-dev for KUBSAN false positive >>> >>> Can check_shl_overflow() be used at all? Best to just make things >>> readable and compiler-happy, whatever the implementation. :) >> >> This is not a compile issue. If the shift amount is a constant, >> compiler should have warned and user should fix the warning. >> >> This is because user code has >> something like >> a << s; >> where s is a unknown variable and >> verifier just marked the result of a << s as unknown value. >> Verifier may not reject the code depending on how a << s result >> is used. >> >> If bpf program writer uses check_shl_overflow() or some kind >> of checking for shift value and won't do shifting if the >> shifting may cause an undefined result, there should not >> be any kubsan warning. > > I guess the main question: what should happen if a bpf program writer > does _not_ use compiler nor check_shl_overflow()? If kubsan is not enabled, everything should work as expected even with shl overflow may cause undefined result. if kubsan is enabled, the reported shift-out-of-bounds warning should be ignored. You could disasm the insn to ensure that there indeed exists a potential shl overflow.
On Wed, Jun 09, 2021 at 11:06:31PM -0700, Yonghong Song wrote: > > > On 6/9/21 10:32 PM, Dmitry Vyukov wrote: > > On Thu, Jun 10, 2021 at 1:40 AM Yonghong Song <yhs@fb.com> wrote: > > > On 6/9/21 11:20 AM, Kees Cook wrote: > > > > On Mon, Jun 07, 2021 at 09:38:43AM +0200, 'Dmitry Vyukov' via Clang Built Linux wrote: > > > > > On Sat, Jun 5, 2021 at 9:10 PM Alexei Starovoitov > > > > > <alexei.starovoitov@gmail.com> wrote: > > > > > > On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@fb.com> wrote: > > > > > > > On 6/5/21 8:01 AM, Kurt Manucredo wrote: > > > > > > > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > > > > > > > > kernel/bpf/core.c:1414:2. > > > > > > > [...] > > > > > > > > > > > > > > I think this is what happens. For the above case, we simply > > > > > > > marks the dst reg as unknown and didn't fail verification. > > > > > > > So later on at runtime, the shift optimization will have wrong > > > > > > > shift value (> 31/64). Please correct me if this is not right > > > > > > > analysis. As I mentioned in the early please write detailed > > > > > > > analysis in commit log. > > > > > > > > > > > > The large shift is not wrong. It's just undefined. > > > > > > syzbot has to ignore such cases. > > > > > > > > > > Hi Alexei, > > > > > > > > > > The report is produced by KUBSAN. I thought there was an agreement on > > > > > cleaning up KUBSAN reports from the kernel (the subset enabled on > > > > > syzbot at least). > > > > > What exactly cases should KUBSAN ignore? > > > > > +linux-hardening/kasan-dev for KUBSAN false positive > > > > > > > > Can check_shl_overflow() be used at all? Best to just make things > > > > readable and compiler-happy, whatever the implementation. :) > > > > > > This is not a compile issue. If the shift amount is a constant, > > > compiler should have warned and user should fix the warning. > > > > > > This is because user code has > > > something like > > > a << s; > > > where s is a unknown variable and > > > verifier just marked the result of a << s as unknown value. > > > Verifier may not reject the code depending on how a << s result > > > is used. Ah, gotcha: it's the BPF code itself that needs to catch it. > > > If bpf program writer uses check_shl_overflow() or some kind > > > of checking for shift value and won't do shifting if the > > > shifting may cause an undefined result, there should not > > > be any kubsan warning. Right. > > I guess the main question: what should happen if a bpf program writer > > does _not_ use compiler nor check_shl_overflow()? I think the BPF runtime needs to make such actions defined, instead of doing a blind shift. It needs to check the size of the shift explicitly when handling the shift instruction. > If kubsan is not enabled, everything should work as expected even with > shl overflow may cause undefined result. > > if kubsan is enabled, the reported shift-out-of-bounds warning > should be ignored. You could disasm the insn to ensure that > there indeed exists a potential shl overflow. Sure, but the point of UBSAN is to find and alert about undefined behavior, so we still need to fix this. -- Kees Cook
On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@chromium.org> wrote: > > > > I guess the main question: what should happen if a bpf program writer > > > does _not_ use compiler nor check_shl_overflow()? > > I think the BPF runtime needs to make such actions defined, instead of > doing a blind shift. It needs to check the size of the shift explicitly > when handling the shift instruction. Such ideas were brought up in the past and rejected. We're not going to sacrifice performance to make behavior a bit more 'defined'. CPUs are doing it deterministically. It's the C standard that needs fixing. > Sure, but the point of UBSAN is to find and alert about undefined > behavior, so we still need to fix this. No. The undefined behavior of C standard doesn't need "fixing" most of the time.
On Thu, Jun 10, 2021 at 10:52:37AM -0700, Alexei Starovoitov wrote: > On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <keescook@chromium.org> wrote: > > > > > > I guess the main question: what should happen if a bpf program writer > > > > does _not_ use compiler nor check_shl_overflow()? > > > > I think the BPF runtime needs to make such actions defined, instead of > > doing a blind shift. It needs to check the size of the shift explicitly > > when handling the shift instruction. > > Such ideas were brought up in the past and rejected. > We're not going to sacrifice performance to make behavior a bit more > 'defined'. CPUs are doing it deterministically. What CPUs do is not the whole story. The compiler can assume that the shift amount is less than the width and use that assumption in other places, resulting in other things being miscompiled. Couldn't you just AND the shift amounts with the width minus 1? That would make the shifts defined, and the compiler would optimize out the AND on any CPU that interprets the shift amounts modulo the width anyway (e.g., x86). - Eric
On 15/06/2021 17:42, Kurt Manucredo wrote: > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run() > kernel/bpf/core.c:1414:2. > > The shift-out-of-bounds happens when we have BPF_X. This means we have > to go the same way we go when we want to avoid a divide-by-zero. We do > it in do_misc_fixups(). Shifts by more than insn_bitness are legal in the eBPF ISA; they are implementation-defined behaviour, rather than UB, and have been made legal for performance reasons. Each of the JIT backends compiles the eBPF shift operations to machine instructions which produce implementation-defined results in such a case; the resulting contents of the register may be arbitrary but program behaviour as a whole remains defined. Guard checks in the fast path (i.e. affecting JITted code) will thus not be accepted. The case of division by zero is not truly analogous, as division instructions on many of the JIT-targeted architectures will raise a machine exception / fault on division by zero, whereas (to the best of my knowledge) none will do so on an out-of-bounds shift. (That said, it would be possible to record from the verifier division instructions in the program which are known never to be passed zero as divisor, and eliding the fixup patch in those cases. However, the extra complexity may not be worthwhile.) As I understand it, the UBSAN report is coming from the eBPF interpreter, which is the *slow path* and indeed on many production systems is compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). Perhaps a better approach to the fix would be to change the interpreter to compute "DST = DST << (SRC & 63);" (and similar for other shifts and bitnesses), thus matching the behaviour of most chips' shift opcodes. This would shut up UBSAN, without affecting JIT code generation. -ed
On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > which is the *slow path* and indeed on many production systems is > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > Perhaps a better approach to the fix would be to change the interpreter > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > bitnesses), thus matching the behaviour of most chips' shift opcodes. > This would shut up UBSAN, without affecting JIT code generation. > Yes, I suggested that last week (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even get optimized out when compiling for most CPUs. - Eric
On 6/15/21 9:33 PM, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: >> >> As I understand it, the UBSAN report is coming from the eBPF interpreter, >> which is the *slow path* and indeed on many production systems is >> compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). >> Perhaps a better approach to the fix would be to change the interpreter >> to compute "DST = DST << (SRC & 63);" (and similar for other shifts and >> bitnesses), thus matching the behaviour of most chips' shift opcodes. >> This would shut up UBSAN, without affecting JIT code generation. > > Yes, I suggested that last week > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even > get optimized out when compiling for most CPUs. Did you check if the generated interpreter code for e.g. x86 is the same before/after with that? How does UBSAN detect this in general? I would assume generated code for interpreter wrt DST = DST << SRC would not really change as otherwise all valid cases would be broken as well, given compiler has not really room to optimize or make any assumptions here, in other words, it's only propagating potential quirks under such cases from underlying arch. Thanks, Daniel
On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: > On 6/15/21 9:33 PM, Eric Biggers wrote: > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > > > which is the *slow path* and indeed on many production systems is > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > > > Perhaps a better approach to the fix would be to change the interpreter > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > > > bitnesses), thus matching the behaviour of most chips' shift opcodes. > > > This would shut up UBSAN, without affecting JIT code generation. > > > > Yes, I suggested that last week > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even > > get optimized out when compiling for most CPUs. > > Did you check if the generated interpreter code for e.g. x86 is the same > before/after with that? Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same both before and after (with UBSAN disabled). Here is the patch I used: diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index 5e31ee9f7512..996db8a1bbfb 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) DST = (u32) DST OP (u32) IMM; \ CONT; + /* + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined + * behavior. Normally this won't affect the generated code. + */ +#define ALU_SHIFT(OPCODE, OP) \ + ALU64_##OPCODE##_X: \ + DST = DST OP (SRC & 63);\ + CONT; \ + ALU_##OPCODE##_X: \ + DST = (u32) DST OP ((u32)SRC & 31); \ + CONT; \ + ALU64_##OPCODE##_K: \ + DST = DST OP (IMM & 63); \ + CONT; \ + ALU_##OPCODE##_K: \ + DST = (u32) DST OP ((u32)IMM & 31); \ + CONT; + ALU(ADD, +) ALU(SUB, -) ALU(AND, &) ALU(OR, |) - ALU(LSH, <<) - ALU(RSH, >>) + ALU_SHIFT(LSH, <<) + ALU_SHIFT(RSH, >>) ALU(XOR, ^) ALU(MUL, *) #undef ALU > > How does UBSAN detect this in general? I would assume generated code for > interpreter wrt DST = DST << SRC would not really change as otherwise all > valid cases would be broken as well, given compiler has not really room > to optimize or make any assumptions here, in other words, it's only > propagating potential quirks under such cases from underlying arch. UBSAN inserts code that checks that shift amounts are in range. In theory there are cases where the undefined behavior of out-of-range shift amounts could cause problems. For example, a compiler could make the following function always return true, as it can assume that 'b' is in the range [0, 31]. bool foo(int a, int b, int *c) { *c = a << b; return b < 32; } - Eric
On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: > > On 6/15/21 9:33 PM, Eric Biggers wrote: > > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > > > > which is the *slow path* and indeed on many production systems is > > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > > > > Perhaps a better approach to the fix would be to change the interpreter > > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > > > > bitnesses), thus matching the behaviour of most chips' shift opcodes. > > > > This would shut up UBSAN, without affecting JIT code generation. > > > > > > Yes, I suggested that last week > > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even > > > get optimized out when compiling for most CPUs. > > > > Did you check if the generated interpreter code for e.g. x86 is the same > > before/after with that? > > Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same > both before and after (with UBSAN disabled). Here is the patch I used: > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > index 5e31ee9f7512..996db8a1bbfb 100644 > --- a/kernel/bpf/core.c > +++ b/kernel/bpf/core.c > @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) > DST = (u32) DST OP (u32) IMM; \ > CONT; > > + /* > + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined > + * behavior. Normally this won't affect the generated code. > + */ > +#define ALU_SHIFT(OPCODE, OP) \ > + ALU64_##OPCODE##_X: \ > + DST = DST OP (SRC & 63);\ > + CONT; \ > + ALU_##OPCODE##_X: \ > + DST = (u32) DST OP ((u32)SRC & 31); \ > + CONT; \ > + ALU64_##OPCODE##_K: \ > + DST = DST OP (IMM & 63); \ > + CONT; \ > + ALU_##OPCODE##_K: \ > + DST = (u32) DST OP ((u32)IMM & 31); \ > + CONT; > + > ALU(ADD, +) > ALU(SUB, -) > ALU(AND, &) > ALU(OR, |) > - ALU(LSH, <<) > - ALU(RSH, >>) > + ALU_SHIFT(LSH, <<) > + ALU_SHIFT(RSH, >>) > ALU(XOR, ^) > ALU(MUL, *) > #undef ALU > Note, I missed the arithmetic right shifts later on in the function. Same result there, though. - Eric
On 6/15/21 11:38 PM, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: >> On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: >>> On 6/15/21 9:33 PM, Eric Biggers wrote: >>>> On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: >>>>> >>>>> As I understand it, the UBSAN report is coming from the eBPF interpreter, >>>>> which is the *slow path* and indeed on many production systems is >>>>> compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). >>>>> Perhaps a better approach to the fix would be to change the interpreter >>>>> to compute "DST = DST << (SRC & 63);" (and similar for other shifts and >>>>> bitnesses), thus matching the behaviour of most chips' shift opcodes. >>>>> This would shut up UBSAN, without affecting JIT code generation. >>>> >>>> Yes, I suggested that last week >>>> (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even >>>> get optimized out when compiling for most CPUs. >>> >>> Did you check if the generated interpreter code for e.g. x86 is the same >>> before/after with that? >> >> Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same >> both before and after (with UBSAN disabled). Here is the patch I used: >> >> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c >> index 5e31ee9f7512..996db8a1bbfb 100644 >> --- a/kernel/bpf/core.c >> +++ b/kernel/bpf/core.c >> @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) >> DST = (u32) DST OP (u32) IMM; \ >> CONT; >> >> + /* >> + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined >> + * behavior. Normally this won't affect the generated code. The last one should probably be more specific in terms of 'normally', e.g. that it is expected that the compiler is optimizing this away for archs like x86. Is arm64 also covered by this ... do you happen to know on which archs this won't be the case? Additionally, I think such comment should probably be more clear in that it also needs to give proper guidance to JIT authors that look at the interpreter code to see what they need to implement, in other words, that they don't end up copying an explicit AND instruction emission if not needed there. >> + */ >> +#define ALU_SHIFT(OPCODE, OP) \ >> + ALU64_##OPCODE##_X: \ >> + DST = DST OP (SRC & 63);\ >> + CONT; \ >> + ALU_##OPCODE##_X: \ >> + DST = (u32) DST OP ((u32)SRC & 31); \ >> + CONT; \ >> + ALU64_##OPCODE##_K: \ >> + DST = DST OP (IMM & 63); \ >> + CONT; \ >> + ALU_##OPCODE##_K: \ >> + DST = (u32) DST OP ((u32)IMM & 31); \ >> + CONT; For the *_K cases these are explicitly rejected by the verifier already. Is this required here nevertheless to suppress UBSAN false positive? >> ALU(ADD, +) >> ALU(SUB, -) >> ALU(AND, &) >> ALU(OR, |) >> - ALU(LSH, <<) >> - ALU(RSH, >>) >> + ALU_SHIFT(LSH, <<) >> + ALU_SHIFT(RSH, >>) >> ALU(XOR, ^) >> ALU(MUL, *) >> #undef ALU > > Note, I missed the arithmetic right shifts later on in the function. Same > result there, though. > > - Eric >
On Tue, Jun 15, 2021 at 11:54:41PM +0200, Daniel Borkmann wrote: > On 6/15/21 11:38 PM, Eric Biggers wrote: > > On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: > > > On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: > > > > On 6/15/21 9:33 PM, Eric Biggers wrote: > > > > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > > > > > > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > > > > > > which is the *slow path* and indeed on many production systems is > > > > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > > > > > > Perhaps a better approach to the fix would be to change the interpreter > > > > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > > > > > > bitnesses), thus matching the behaviour of most chips' shift opcodes. > > > > > > This would shut up UBSAN, without affecting JIT code generation. > > > > > > > > > > Yes, I suggested that last week > > > > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even > > > > > get optimized out when compiling for most CPUs. > > > > > > > > Did you check if the generated interpreter code for e.g. x86 is the same > > > > before/after with that? > > > > > > Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same > > > both before and after (with UBSAN disabled). Here is the patch I used: > > > > > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > > > index 5e31ee9f7512..996db8a1bbfb 100644 > > > --- a/kernel/bpf/core.c > > > +++ b/kernel/bpf/core.c > > > @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) > > > DST = (u32) DST OP (u32) IMM; \ > > > CONT; > > > + /* > > > + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined > > > + * behavior. Normally this won't affect the generated code. > > The last one should probably be more specific in terms of 'normally', e.g. that > it is expected that the compiler is optimizing this away for archs like x86. Is > arm64 also covered by this ... do you happen to know on which archs this won't > be the case? > > Additionally, I think such comment should probably be more clear in that it also > needs to give proper guidance to JIT authors that look at the interpreter code to > see what they need to implement, in other words, that they don't end up copying > an explicit AND instruction emission if not needed there. Same result on arm64 with gcc 10.2.0. On arm32 it is different, probably because the 64-bit shifts aren't native in that case. I don't know about other architectures. But there aren't many ways to implement shifts, and using just the low bits of the shift amount is the most logical way. Please feel free to send out a patch with whatever comment you want. The diff I gave was just an example and I am not an expert in BPF. > > > > + */ > > > +#define ALU_SHIFT(OPCODE, OP) \ > > > + ALU64_##OPCODE##_X: \ > > > + DST = DST OP (SRC & 63);\ > > > + CONT; \ > > > + ALU_##OPCODE##_X: \ > > > + DST = (u32) DST OP ((u32)SRC & 31); \ > > > + CONT; \ > > > + ALU64_##OPCODE##_K: \ > > > + DST = DST OP (IMM & 63); \ > > > + CONT; \ > > > + ALU_##OPCODE##_K: \ > > > + DST = (u32) DST OP ((u32)IMM & 31); \ > > > + CONT; > > For the *_K cases these are explicitly rejected by the verifier already. Is this > required here nevertheless to suppress UBSAN false positive? > No, I just didn't know that these constants are never out of range. Please feel free to send out a patch that does this properly. - Eric
On Tue, 15 Jun 2021 15:07:43 -0700, Eric Biggers <ebiggers@kernel.org> wrote: > > On Tue, Jun 15, 2021 at 11:54:41PM +0200, Daniel Borkmann wrote: > > On 6/15/21 11:38 PM, Eric Biggers wrote: > > > On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: > > > > On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: > > > > > On 6/15/21 9:33 PM, Eric Biggers wrote: > > > > > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > > > > > > > > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > > > > > > > which is the *slow path* and indeed on many production systems is > > > > > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > > > > > > > Perhaps a better approach to the fix would be to change the interpreter > > > > > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > > > > > > > bitnesses), thus matching the behaviour of most chips' shift opcodes. > > > > > > > This would shut up UBSAN, without affecting JIT code generation. > > > > > > > > > > > > Yes, I suggested that last week > > > > > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even > > > > > > get optimized out when compiling for most CPUs. > > > > > > > > > > Did you check if the generated interpreter code for e.g. x86 is the same > > > > > before/after with that? > > > > > > > > Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same > > > > both before and after (with UBSAN disabled). Here is the patch I used: > > > > > > > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > > > > index 5e31ee9f7512..996db8a1bbfb 100644 > > > > --- a/kernel/bpf/core.c > > > > +++ b/kernel/bpf/core.c > > > > @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) > > > > DST = (u32) DST OP (u32) IMM; > > > CONT; > > > > + /* > > > > + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined > > > > + * behavior. Normally this won't affect the generated code. > > > > The last one should probably be more specific in terms of 'normally', e.g. that > > it is expected that the compiler is optimizing this away for archs like x86. Is > > arm64 also covered by this ... do you happen to know on which archs this won't > > be the case? > > > > Additionally, I think such comment should probably be more clear in that it also > > needs to give proper guidance to JIT authors that look at the interpreter code to > > see what they need to implement, in other words, that they don't end up copying > > an explicit AND instruction emission if not needed there. > > Same result on arm64 with gcc 10.2.0. > > On arm32 it is different, probably because the 64-bit shifts aren't native in > that case. I don't know about other architectures. But there aren't many ways > to implement shifts, and using just the low bits of the shift amount is the most > logical way. > > Please feel free to send out a patch with whatever comment you want. The diff I > gave was just an example and I am not an expert in BPF. > > > > > > > + */ > > > > +#define ALU_SHIFT(OPCODE, OP) > > > + ALU64_##OPCODE##_X: > > > + DST = DST OP (SRC & 63);> > > + CONT; > > > + ALU_##OPCODE##_X: > > > + DST = (u32) DST OP ((u32)SRC & 31); > > > + CONT; > > > + ALU64_##OPCODE##_K: > > > + DST = DST OP (IMM & 63); > > > + CONT; > > > + ALU_##OPCODE##_K: > > > + DST = (u32) DST OP ((u32)IMM & 31); > > > + CONT; > > > > For the *_K cases these are explicitly rejected by the verifier already. Is this > > required here nevertheless to suppress UBSAN false positive? > > > > No, I just didn't know that these constants are never out of range. Please feel > free to send out a patch that does this properly. > The shift-out-of-bounds on syzbot happens in ALU_##OPCODE##_X only. To pass the syzbot test, only ALU_##OPCODE##_X needs to be guarded. This old patch I tested on syzbot puts a check in all four. https://syzkaller.appspot.com/text?tag=Patch&x=11f8cacbd00000 https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 thanks, kind regards Kurt Manucredo
On 6/16/21 12:07 AM, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 11:54:41PM +0200, Daniel Borkmann wrote: >> On 6/15/21 11:38 PM, Eric Biggers wrote: >>> On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: >>>> On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: >>>>> On 6/15/21 9:33 PM, Eric Biggers wrote: >>>>>> On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: >>>>>>> >>>>>>> As I understand it, the UBSAN report is coming from the eBPF interpreter, >>>>>>> which is the *slow path* and indeed on many production systems is >>>>>>> compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). >>>>>>> Perhaps a better approach to the fix would be to change the interpreter >>>>>>> to compute "DST = DST << (SRC & 63);" (and similar for other shifts and >>>>>>> bitnesses), thus matching the behaviour of most chips' shift opcodes. >>>>>>> This would shut up UBSAN, without affecting JIT code generation. >>>>>> >>>>>> Yes, I suggested that last week >>>>>> (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@gmail.com). The AND will even >>>>>> get optimized out when compiling for most CPUs. >>>>> >>>>> Did you check if the generated interpreter code for e.g. x86 is the same >>>>> before/after with that? >>>> >>>> Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same >>>> both before and after (with UBSAN disabled). Here is the patch I used: >>>> >>>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c >>>> index 5e31ee9f7512..996db8a1bbfb 100644 >>>> --- a/kernel/bpf/core.c >>>> +++ b/kernel/bpf/core.c >>>> @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) >>>> DST = (u32) DST OP (u32) IMM; \ >>>> CONT; >>>> + /* >>>> + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined >>>> + * behavior. Normally this won't affect the generated code. >> >> The last one should probably be more specific in terms of 'normally', e.g. that >> it is expected that the compiler is optimizing this away for archs like x86. Is >> arm64 also covered by this ... do you happen to know on which archs this won't >> be the case? >> >> Additionally, I think such comment should probably be more clear in that it also >> needs to give proper guidance to JIT authors that look at the interpreter code to >> see what they need to implement, in other words, that they don't end up copying >> an explicit AND instruction emission if not needed there. > > Same result on arm64 with gcc 10.2.0. > > On arm32 it is different, probably because the 64-bit shifts aren't native in > that case. I don't know about other architectures. But there aren't many ways > to implement shifts, and using just the low bits of the shift amount is the most > logical way. > > Please feel free to send out a patch with whatever comment you want. The diff I > gave was just an example and I am not an expert in BPF. > >> >>>> + */ >>>> +#define ALU_SHIFT(OPCODE, OP) \ >>>> + ALU64_##OPCODE##_X: \ >>>> + DST = DST OP (SRC & 63);\ >>>> + CONT; \ >>>> + ALU_##OPCODE##_X: \ >>>> + DST = (u32) DST OP ((u32)SRC & 31); \ >>>> + CONT; \ >>>> + ALU64_##OPCODE##_K: \ >>>> + DST = DST OP (IMM & 63); \ >>>> + CONT; \ >>>> + ALU_##OPCODE##_K: \ >>>> + DST = (u32) DST OP ((u32)IMM & 31); \ >>>> + CONT; >> >> For the *_K cases these are explicitly rejected by the verifier already. Is this >> required here nevertheless to suppress UBSAN false positive? > > No, I just didn't know that these constants are never out of range. Please feel > free to send out a patch that does this properly. Summarized and fixed via: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=28131e9d933339a92f78e7ab6429f4aaaa07061c Thanks everyone, Daniel
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 94ba5163d4c5..04e3bf344ecd 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -7880,13 +7880,25 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) return -EINVAL; } - if ((opcode == BPF_LSH || opcode == BPF_RSH || - opcode == BPF_ARSH) && BPF_SRC(insn->code) == BPF_K) { + if (opcode == BPF_LSH || opcode == BPF_RSH || + opcode == BPF_ARSH) { int size = BPF_CLASS(insn->code) == BPF_ALU64 ? 64 : 32; - if (insn->imm < 0 || insn->imm >= size) { - verbose(env, "invalid shift %d\n", insn->imm); - return -EINVAL; + if (BPF_SRC(insn->code) == BPF_K) { + if (insn->imm < 0 || insn->imm >= size) { + verbose(env, "invalid shift %d\n", insn->imm); + return -EINVAL; + } + } + if (BPF_SRC(insn->code) == BPF_X) { + struct bpf_reg_state *src_reg; + + src_reg = ®s[insn->src_reg]; + if (src_reg->umax_value >= size) { + verbose(env, "invalid shift %lld\n", + src_reg->umax_value); + return -EINVAL; + } } }
UBSAN: shift-out-of-bounds in kernel/bpf/core.c:1414:2 shift exponent 248 is too large for 32-bit type 'unsigned int' Reported-and-tested-by: syzbot+bed360704c521841c85d@syzkaller.appspotmail.com Signed-off-by: Kurt Manucredo <fuzzybritches0@gmail.com> --- https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231 Changelog: ---------- v3 - Make it clearer what the fix is for. v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary check in check_alu_op() in verifier.c. v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary check in ___bpf_prog_run(). Hi everyone, I hope this fixes it! kind regards kernel/bpf/verifier.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-)