| Message ID | 1603361762-43062-1-git-send-email-bmeng.cn@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | target/i386: seg_helper: Correct segement selector nullification in the RET/IRET helper | expand |
On Thu, Oct 22, 2020 at 6:16 PM Bin Meng <bmeng.cn@gmail.com> wrote: > > From: Bin Meng <bin.meng@windriver.com> > > Per the SDM, when returning to outer privilege level, for segment > registers (ES, FS, GS, and DS) if the check fails, the segment > selector becomes null, but QEMU clears the base/limit/flags as well > as nullifying the segment selector, which should be a spec violation. > > Real hardware seems to be compliant with the spec, at least on one > Coffee Lake board I tested. > > Signed-off-by: Bin Meng <bin.meng@windriver.com> > --- > > target/i386/seg_helper.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > Ping?
On Mon, Nov 2, 2020 at 4:20 PM Bin Meng <bmeng.cn@gmail.com> wrote: > > On Thu, Oct 22, 2020 at 6:16 PM Bin Meng <bmeng.cn@gmail.com> wrote: > > > > From: Bin Meng <bin.meng@windriver.com> > > > > Per the SDM, when returning to outer privilege level, for segment > > registers (ES, FS, GS, and DS) if the check fails, the segment > > selector becomes null, but QEMU clears the base/limit/flags as well > > as nullifying the segment selector, which should be a spec violation. > > > > Real hardware seems to be compliant with the spec, at least on one > > Coffee Lake board I tested. > > > > Signed-off-by: Bin Meng <bin.meng@windriver.com> > > --- > > > > target/i386/seg_helper.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > Ping? Ping?
On 22/10/20 12:16, Bin Meng wrote: > From: Bin Meng <bin.meng@windriver.com> > > Per the SDM, when returning to outer privilege level, for segment > registers (ES, FS, GS, and DS) if the check fails, the segment > selector becomes null, but QEMU clears the base/limit/flags as well > as nullifying the segment selector, which should be a spec violation. > > Real hardware seems to be compliant with the spec, at least on one > Coffee Lake board I tested. This is all quite messy in QEMU; for performance reasons, it never even checks the flags on memory accesses, only on selector loads. One way to fix it would be to define five extra hflags bits that copy the P bit of DS/ES/SS/FS/GS. gen_lea_v_seg checks if the hflag is clear, and if so it generates a #GP or #SS. Regarding your patch, I think at least the segment should be made "unusable". On Intel processors there is an internal "unusable" flag, on AMD and in QEMU, equivalently, the P bit would be cleared in the flags. As far as I know the difference is only visible with VMX. That is, you'd need something like this: cpu_x86_load_seg_cache(env, seg_reg, 0, env->segs[seg_reg].base, env->segs[seg_reg].limit, env->segs[seg_reg].flags & ~DESC_P_MASK); Thanks, Paolo > > Signed-off-by: Bin Meng <bin.meng@windriver.com> > --- > > target/i386/seg_helper.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c > index be88938..d8766d8 100644 > --- a/target/i386/seg_helper.c > +++ b/target/i386/seg_helper.c > @@ -2108,7 +2108,10 @@ static inline void validate_seg(CPUX86State *env, int seg_reg, int cpl) > if (!(e2 & DESC_CS_MASK) || !(e2 & DESC_C_MASK)) { > /* data or non conforming code segment */ > if (dpl < cpl) { > - cpu_x86_load_seg_cache(env, seg_reg, 0, 0, 0, 0); > + cpu_x86_load_seg_cache(env, seg_reg, 0, > + env->segs[seg_reg].base, > + env->segs[seg_reg].limit, > + env->segs[seg_reg].flags); > } > } > } >
diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c index be88938..d8766d8 100644 --- a/target/i386/seg_helper.c +++ b/target/i386/seg_helper.c @@ -2108,7 +2108,10 @@ static inline void validate_seg(CPUX86State *env, int seg_reg, int cpl) if (!(e2 & DESC_CS_MASK) || !(e2 & DESC_C_MASK)) { /* data or non conforming code segment */ if (dpl < cpl) { - cpu_x86_load_seg_cache(env, seg_reg, 0, 0, 0, 0); + cpu_x86_load_seg_cache(env, seg_reg, 0, + env->segs[seg_reg].base, + env->segs[seg_reg].limit, + env->segs[seg_reg].flags); } } }