Re: [PATCH] x86: suppress KMSAN reports in arch_within_stack_frames()

From: Peter Zijlstra
Date: Mon Nov 21 2022 - 05:23:49 EST


On Fri, Nov 18, 2022 at 06:23:05PM +0100, Alexander Potapenko wrote:
> arch_within_stack_frames() performs stack walking and may confuse
> KMSAN by stepping on stale shadow values. To prevent false positive
> reports, disable KMSAN checks in this function.
>
> This fixes KMSAN's interoperability with CONFIG_HARDENED_USERCOPY.
>
> Link: https://github.com/google/kmsan/issues/89
> Link: https://lore.kernel.org/lkml/Y3b9AAEKp2Vr3e6O@sol.localdomain/
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
> arch/x86/include/asm/thread_info.h | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index f0cb881c1d690..f1cccba52eb97 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -163,7 +163,12 @@ struct thread_info {
> * GOOD_FRAME if within a frame
> * BAD_STACK if placed across a frame boundary (or outside stack)
> * NOT_STACK unable to determine (no frame pointers, etc)
> + *
> + * This function reads pointers from the stack and dereferences them. The
> + * pointers may not have their KMSAN shadow set up properly, which may result
> + * in false positive reports. Disable instrumentation to avoid those.
> */
> +__no_kmsan_checks
> static inline int arch_within_stack_frames(const void * const stack,
> const void * const stackend,
> const void *obj, unsigned long len)

Seems OK; but now I'm confused as to the exact distinction between
__no_sanitize_memory and __no_kmsan_checks.

The comments there about seem to suggest __no_sanitize_memory ensures no
instrumentation at all, and __no_kmsan_checks some instrumentation but
doesn't actually check anything -- so what's left then?