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
