[PATCH] powerpc: make show_stack's stack walking KASAN-safe

[Daniel Axtens]

"Naveen N. Rao" <naveen.n.rao at linux.ibm.com> writes:

> Daniel Axtens wrote:
>> Make our stack-walking code KASAN-safe by using READ_ONCE_NOCHECK -
>> generic code, arm64, s390 and x86 all do this for similar sorts of
>> reasons: when unwinding a stack, we might touch memory that KASAN has
>> marked as being out-of-bounds. In ppc64 KASAN development, I hit this
>> sometimes when checking for an exception frame - because we're checking
>> an arbitrary offset into the stack frame.
>> 
>> See commit 20955746320e ("s390/kasan: avoid false positives during stack
>> unwind"), commit bcaf669b4bdb ("arm64: disable kasan when accessing
>> frame->fp in unwind_frame"), commit 91e08ab0c851 ("x86/dumpstack:
>> Prevent KASAN false positive warnings") and commit 6e22c8366416
>> ("tracing, kasan: Silence Kasan warning in check_stack of stack_tracer").
>> 
>> Signed-off-by: Daniel Axtens <dja at axtens.net>
>> ---
>>  arch/powerpc/kernel/process.c | 16 +++++++++-------
>>  1 file changed, 9 insertions(+), 7 deletions(-)
>> 
>> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
>> index 89e34aa273e2..430cf06f9406 100644
>> --- a/arch/powerpc/kernel/process.c
>> +++ b/arch/powerpc/kernel/process.c
>> @@ -2151,8 +2151,8 @@ void show_stack(struct task_struct *tsk, unsigned long *stack,
>>  			break;
>>  
>>  		stack = (unsigned long *) sp;
>> -		newsp = stack[0];
>> -		ip = stack[STACK_FRAME_LR_SAVE];
>> +		newsp = READ_ONCE_NOCHECK(stack[0]);
>> +		ip = READ_ONCE_NOCHECK(stack[STACK_FRAME_LR_SAVE]);
>
> Just curious:
> Given that we validate the stack pointer before these accesses, can we 
> annotate show_stack() with __no_sanitize_address instead?
>
> I ask because we have other places where we walk the stack: 
> arch_stack_walk(), as well as in perf callchain. Similar changes will be 
> needed there as well.

Oh good points. Yes, it probably makes most sense to mark all the
functions with __no_sanitize_address, that resolves Christophe's issue
as well. I'll send a v2.

Kind regards,
Daniel

>
>
> - Naveen