linux-kernel - Re: [PATCH] x86: suppress KMSAN reports in arch_within_stack

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Message-ID: <Y3tRgGUKCxUoLeM8@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2022 11:22:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...nel.org>
Subject: Re: [PATCH] x86: suppress KMSAN reports in arch_within_stack_frames()

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@...nel.org>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
>  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?