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

From: [email-masked] (David Long)
Date: Sun, 11 Sep 2016 21:53:43 -0400

On 09/10/2016 01:48 AM, Masami Hiramatsu wrote:
> On Fri,  9 Sep 2016 15:26:09 -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.
>> 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 <[email-masked]>
>> ---
>>   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