Re: [PATCH v10 6/6] powerpc: Book3S 64-bit outline-only KASAN support

[Daniel Axtens]

Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:

> Le 03/02/2021 à 12:59, Daniel Axtens a écrit :
>> Implement a limited form of KASAN for Book3S 64-bit machines running under
>> the Radix MMU, supporting only outline mode.
>> 
>
>> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
>> index a66f435dabbf..9a6fd603f0e7 100644
>> --- a/arch/powerpc/kernel/process.c
>> +++ b/arch/powerpc/kernel/process.c
>> @@ -2157,8 +2157,8 @@ 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]);
>>   		if (!firstframe || ip != lr) {
>>   			printk("%s["REG"] ["REG"] %pS",
>>   				loglvl, sp, ip, (void *)ip);
>> @@ -2176,17 +2176,19 @@ void show_stack(struct task_struct *tsk, unsigned long *stack,
>>   		 * See if this is an exception frame.
>>   		 * We look for the "regshere" marker in the current frame.
>>   		 */
>> -		if (validate_sp(sp, tsk, STACK_INT_FRAME_SIZE)
>> -		    && stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
>> +		if (validate_sp(sp, tsk, STACK_INT_FRAME_SIZE) &&
>> +		    (READ_ONCE_NOCHECK(stack[STACK_FRAME_MARKER]) ==
>> +		     STACK_FRAME_REGS_MARKER)) {
>>   			struct pt_regs *regs = (struct pt_regs *)
>>   				(sp + STACK_FRAME_OVERHEAD);
>>   
>> -			lr = regs->link;
>> +			lr = READ_ONCE_NOCHECK(regs->link);
>>   			printk("%s--- interrupt: %lx at %pS\n",
>> -			       loglvl, regs->trap, (void *)regs->nip);
>> +			       loglvl, READ_ONCE_NOCHECK(regs->trap),
>> +			       (void *)READ_ONCE_NOCHECK(regs->nip));
>>   			__show_regs(regs);
>>   			printk("%s--- interrupt: %lx\n",
>> -			       loglvl, regs->trap);
>> +			       loglvl, READ_ONCE_NOCHECK(regs->trap));
>>   
>>   			firstframe = 1;
>>   		}
>
>
> The above changes look like a bug fix not directly related to KASAN. Should be split out in another 
> patch I think.

That code corresponds to the following part of the patch description:

| - 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 our case we often get this 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")

include/linux/compiler.h describes it as follows:

/*
 * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
 * to hide memory access from KASAN.
 */

So I think it is sufficently connected with KASAN to be in this patch.

Kind regards,
Daniel

>
> Christophe