| Message ID | 1473449169-16032-1-git-send-email-dave.long@linaro.org |
|---|---|
| State | Superseded |
| Headers | show |
On 09/10/2016 01:48 AM, Masami Hiramatsu wrote: > On Fri, 9 Sep 2016 15:26:09 -0400 > David Long <dave.long@linaro.org> wrote: > >> From: "David A. Long" <dave.long@linaro.org> >> >> Kprobes searches backwards a finite number of instructions to determine if >> there is an attempt to probe a load/store exclusive sequence. It stops when >> it hits the maximum number of instructions or a load or store exclusive. >> However this means it can run up past the beginning of the function and >> start looking at literal constants. This has been shown to cause a false >> positive and blocks insertion of the probe. To fix this, further limit the >> backwards search to stop if it hits a symbol address from kallsyms. The >> presumption is that this is the entry point to this code (particularly for >> the common case of placing probes at the beginning of functions). >> >> This also improves efficiency by not searching code that is not part of the >> function. There may be some possibility that the label might not denote the >> entry path to the probed instruction but the likelihood seems low and this >> is just another example of how the kprobes user really needs to be >> careful about what they are doing. > > Of course user should be careful, but also, in such case, kernel can reject > to probe it. > I'm not exactly sure what you mean. I'm just saying when everything goes right we still cannot promise perfection in detecting a probe within an atomic sequence. This patch will reject a probe that is after a ldx and has no intervening kallsyms label (and assuming it's within the defined maximum count of subsequent instructions). > >> >> Signed-off-by: David A. Long <dave.long@linaro.org> >> --- >> arch/arm64/kernel/probes/decode-insn.c | 48 ++++++++++++++++------------------ >> 1 file changed, 23 insertions(+), 25 deletions(-) >> >> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c >> index 37e47a9..a691112 100644 >> --- a/arch/arm64/kernel/probes/decode-insn.c >> +++ b/arch/arm64/kernel/probes/decode-insn.c >> @@ -16,6 +16,7 @@ >> #include <linux/kernel.h> >> #include <linux/kprobes.h> >> #include <linux/module.h> >> +#include <linux/kallsyms.h> >> #include <asm/kprobes.h> >> #include <asm/insn.h> >> #include <asm/sections.h> >> @@ -122,7 +123,7 @@ arm_probe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi) >> static bool __kprobes >> is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end) >> { >> - while (scan_start > scan_end) { >> + while (scan_start >= scan_end) { >> /* >> * atomic region starts from exclusive load and ends with >> * exclusive store. >> @@ -142,33 +143,30 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi) >> { >> enum kprobe_insn decoded; >> kprobe_opcode_t insn = le32_to_cpu(*addr); >> - kprobe_opcode_t *scan_start = addr - 1; >> - kprobe_opcode_t *scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; >> -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR) >> - struct module *mod; >> -#endif >> - >> - if (addr >= (kprobe_opcode_t *)_text && >> - scan_end < (kprobe_opcode_t *)_text) >> - scan_end = (kprobe_opcode_t *)_text; >> -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR) >> - else { >> - preempt_disable(); >> - mod = __module_address((unsigned long)addr); >> - if (mod && within_module_init((unsigned long)addr, mod) && >> - !within_module_init((unsigned long)scan_end, mod)) >> - scan_end = (kprobe_opcode_t *)mod->init_layout.base; >> - else if (mod && within_module_core((unsigned long)addr, mod) && >> - !within_module_core((unsigned long)scan_end, mod)) >> - scan_end = (kprobe_opcode_t *)mod->core_layout.base; >> - preempt_enable(); >> + kprobe_opcode_t *scan_end = 0; > > Please use NULL for pointer. > A change has been made for v4. >> + unsigned long size = 0, offset = 0; >> + >> + /* >> + * If there's a symbol defined in front of and near enough to >> + * the probe address assume it is the entry point to this >> + * code and use it to further limit how far back we search >> + * when determining if we're in an atomic sequence. If we could >> + * not find any symbol skip the atomic test altogether as we >> + * could otherwise end up searching irrelevant text/literals. >> + * KPROBES depends on KALLSYMS so this last case should never >> + * happen. >> + */ >> + if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) { >> + if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t))) >> + scan_end = addr - (offset / sizeof(kprobe_opcode_t)); >> + else >> + scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; > > } else > return INSN_REJECTED; > > that is what I expected... > > Thank you, > >> } >> -#endif >> decoded = arm_probe_decode_insn(insn, asi); >> >> - if (decoded == INSN_REJECTED || >> - is_probed_address_atomic(scan_start, scan_end)) >> - return INSN_REJECTED; >> + if (decoded != INSN_REJECTED && scan_end) >> + if (is_probed_address_atomic(addr - 1, scan_end)) >> + return INSN_REJECTED; >> >> return decoded; >> } >> -- >> 2.5.0 >> > > Thanks, -dl
On 09/12/2016 12:29 PM, Masami Hiramatsu wrote: > On Sun, 11 Sep 2016 21:53:43 -0400 > David Long <dave.long@linaro.org> wrote: > >> On 09/10/2016 01:48 AM, Masami Hiramatsu wrote: >>> On Fri, 9 Sep 2016 15:26:09 -0400 >>> David Long <dave.long@linaro.org> wrote: >>> >>>> From: "David A. Long" <dave.long@linaro.org> >>>> >>>> Kprobes searches backwards a finite number of instructions to determine if >>>> there is an attempt to probe a load/store exclusive sequence. It stops when >>>> it hits the maximum number of instructions or a load or store exclusive. >>>> However this means it can run up past the beginning of the function and >>>> start looking at literal constants. This has been shown to cause a false >>>> positive and blocks insertion of the probe. To fix this, further limit the >>>> backwards search to stop if it hits a symbol address from kallsyms. The >>>> presumption is that this is the entry point to this code (particularly for >>>> the common case of placing probes at the beginning of functions). >>>> >>>> This also improves efficiency by not searching code that is not part of the >>>> function. There may be some possibility that the label might not denote the >>>> entry path to the probed instruction but the likelihood seems low and this >>>> is just another example of how the kprobes user really needs to be >>>> careful about what they are doing. >>> >>> Of course user should be careful, but also, in such case, kernel can reject >>> to probe it. >>> >> >> I'm not exactly sure what you mean. I'm just saying when everything >> goes right we still cannot promise perfection in detecting a probe >> within an atomic sequence. This patch will reject a probe that is after >> a ldx and has no intervening kallsyms label (and assuming it's within >> the defined maximum count of subsequent instructions). >> > > Hmm, what I meant was the below code. > >>>> + /* >>>> + * If there's a symbol defined in front of and near enough to >>>> + * the probe address assume it is the entry point to this >>>> + * code and use it to further limit how far back we search >>>> + * when determining if we're in an atomic sequence. If we could >>>> + * not find any symbol skip the atomic test altogether as we >>>> + * could otherwise end up searching irrelevant text/literals. >>>> + * KPROBES depends on KALLSYMS so this last case should never >>>> + * happen. >>>> + */ >>>> + if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) { >>>> + if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t))) >>>> + scan_end = addr - (offset / sizeof(kprobe_opcode_t)); >>>> + else >>>> + scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; >>> >>> } else >>> return INSN_REJECTED; >>> >>> that is what I expected... > > As you said above, > >>>> + * KPROBES depends on KALLSYMS so this last case should never >>>> + * happen. > > If it should never happen, it also would be better to reject it because > it is unexpected result. > > Thank you, > OK, cool. Sounds like we're on the same page. -dl
diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c index 37e47a9..a691112 100644 --- a/arch/arm64/kernel/probes/decode-insn.c +++ b/arch/arm64/kernel/probes/decode-insn.c @@ -16,6 +16,7 @@ #include <linux/kernel.h> #include <linux/kprobes.h> #include <linux/module.h> +#include <linux/kallsyms.h> #include <asm/kprobes.h> #include <asm/insn.h> #include <asm/sections.h> @@ -122,7 +123,7 @@ arm_probe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi) static bool __kprobes is_probed_address_atomic(kprobe_opcode_t *scan_start, kprobe_opcode_t *scan_end) { - while (scan_start > scan_end) { + while (scan_start >= scan_end) { /* * atomic region starts from exclusive load and ends with * exclusive store. @@ -142,33 +143,30 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi) { enum kprobe_insn decoded; kprobe_opcode_t insn = le32_to_cpu(*addr); - kprobe_opcode_t *scan_start = addr - 1; - kprobe_opcode_t *scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR) - struct module *mod; -#endif - - if (addr >= (kprobe_opcode_t *)_text && - scan_end < (kprobe_opcode_t *)_text) - scan_end = (kprobe_opcode_t *)_text; -#if defined(CONFIG_MODULES) && defined(MODULES_VADDR) - else { - preempt_disable(); - mod = __module_address((unsigned long)addr); - if (mod && within_module_init((unsigned long)addr, mod) && - !within_module_init((unsigned long)scan_end, mod)) - scan_end = (kprobe_opcode_t *)mod->init_layout.base; - else if (mod && within_module_core((unsigned long)addr, mod) && - !within_module_core((unsigned long)scan_end, mod)) - scan_end = (kprobe_opcode_t *)mod->core_layout.base; - preempt_enable(); + kprobe_opcode_t *scan_end = 0; + unsigned long size = 0, offset = 0; + + /* + * If there's a symbol defined in front of and near enough to + * the probe address assume it is the entry point to this + * code and use it to further limit how far back we search + * when determining if we're in an atomic sequence. If we could + * not find any symbol skip the atomic test altogether as we + * could otherwise end up searching irrelevant text/literals. + * KPROBES depends on KALLSYMS so this last case should never + * happen. + */ + if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) { + if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t))) + scan_end = addr - (offset / sizeof(kprobe_opcode_t)); + else + scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE; } -#endif decoded = arm_probe_decode_insn(insn, asi); - if (decoded == INSN_REJECTED || - is_probed_address_atomic(scan_start, scan_end)) - return INSN_REJECTED; + if (decoded != INSN_REJECTED && scan_end) + if (is_probed_address_atomic(addr - 1, scan_end)) + return INSN_REJECTED; return decoded; }