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SubjectRe: [PATCH v3] arm64: Improve kprobes test for atomic sequence
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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

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