Subject: [PATCH] arm64: Improve kprobes test for atomic sequence

From: [email-masked] (David Long)
Date: Thu, 1 Sep 2016 17:27:59 -0400

On 08/31/2016 10:38 PM, Masami Hiramatsu wrote:
> Hi Dave,
>
> On Wed, 31 Aug 2016 16:52:22 -0400
> David Long <[email-masked]> wrote:
>
>> From: "David A. Long" <[email-masked]>
>>
>> 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.
>
> Hmm, so on aarch64, we can not put a kprobe between load exclusive and
> store exclusive, because kprobe always breaks the atomicity, am I correct?

Yes.

> If so, what happen if any branch in the sequence? e.g.
>
>    load-ex
>    (do something)
> l1:
>    store-ex
> ...
>    load-ex
>    (do something)
>    branch l1;
>

I'm sure atomic code can be constructed in a way which we don't detect, 
and probably can't detect, this is just a "best effort".

>> 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 add a test to see
>> if the typical:
>>
>> 	"stp x29, x30, [sp, #n]!"
>>
>> instruction beginning a function gets hit. This also improves efficiency by
>> not testing code that is not part of the function. There is some
>> possibility that a function will not begin with this instruction, in which
>> case the fixed code will behave no worse than before.
>
> If the function boundary is the problem, why you wouldn't use kallsyms information
> as I did in can_optimize()@arch/x86/kernel/kprobes/opt.c ?
>
>          /* Lookup symbol including addr */
>          if (!kallsyms_lookup_size_offset(paddr, &size, &offset))
>                  return 0;
>
> With this call, symbol start address is (paddr - offset) and end address
> is (paddr - offset + size).
>

Thanks for pointing this out.  I shall work on a V2 patch ASAP.

> Thank you,
>
>>
>> There could also be the case that the stp instruction is found further in
>> the body of the function, which could theoretically allow probing of an
>> atomic squence. The likelihood of this seems low, and this would not be the
>> only aspect of kprobes where the user needs to be careful to avoid
>> problems.
>>
>> Signed-off-by: David A. Long <[email-masked]>
>> ---
>>   arch/arm64/kernel/probes/decode-insn.c | 25 ++++++++++++++++++-------
>>   1 file changed, 18 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
>> index 37e47a9..248e820 100644
>> --- a/arch/arm64/kernel/probes/decode-insn.c
>> +++ b/arch/arm64/kernel/probes/decode-insn.c
>> @@ -122,16 +122,28 @@ 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)
>>   {
>> +	const u32 stp_x29_x30_sp_pre = 0xa9807bfd;
>> +	const u32 stp_ignore_index_mask = 0xffc07fff;
>> +	u32 instruction = le32_to_cpu(*scan_start);
>> +
>>   	while (scan_start > scan_end) {
>>   		/*
>> -		 * atomic region starts from exclusive load and ends with
>> -		 * exclusive store.
>> +		 * Atomic region starts from exclusive load and ends with
>> +		 * exclusive store. If we hit a "stp x29, x30, [sp, #n]!"
>> +		 * assume it is the beginning of the function and end the
>> +		 * search. This helps avoid false positives from literal
>> +		 * constants that look like a load-exclusive, in addition
>> +		 * to being more efficient.
>>   		 */
>> -		if (aarch64_insn_is_store_ex(le32_to_cpu(*scan_start)))
>> +		if ((instruction & stp_ignore_index_mask) == stp_x29_x30_sp_pre)
>>   			return false;
>> -		else if (aarch64_insn_is_load_ex(le32_to_cpu(*scan_start)))
>> -			return true;
>> +
>>   		scan_start--;
>> +		instruction = le32_to_cpu(*scan_start);
>> +		if (aarch64_insn_is_store_ex(instruction))
>> +			return false;
>> +		else if (aarch64_insn_is_load_ex(instruction))
>> +			return true;
>>   	}
>>
>>   	return false;
>> @@ -142,7 +154,6 @@ 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;
>> @@ -167,7 +178,7 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
>>   	decoded = arm_probe_decode_insn(insn, asi);
>>
>>   	if (decoded == INSN_REJECTED ||
>> -			is_probed_address_atomic(scan_start, scan_end))
>> +			is_probed_address_atomic(addr, scan_end))
>>   		return INSN_REJECTED;
>>
>>   	return decoded;
>> --
>> 2.5.0
>>
>
>

Thanks,
-dl