linux-kernel - Re: [PATCH] x86: kmsan: Fix hook for unaligned accesses

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Message-ID: <CAG_fn=XR6KVQ=DbKZW3kNXsCHgULm2J7i6GCm8CZUjpjuk-d2A@mail.gmail.com>
Date: Fri, 24 May 2024 10:28:05 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Brian Johannesmeyer <bjohannesmeyer@...il.com>
Cc: Marco Elver <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, kasan-dev@...glegroups.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: kmsan: Fix hook for unaligned accesses

On Thu, May 23, 2024 at 11:50 PM Brian Johannesmeyer
<bjohannesmeyer@...il.com> wrote:
>
> When called with a 'from' that is not 4-byte-aligned,
> string_memcpy_fromio() calls the movs() macro to copy the first few bytes,
> so that 'from' becomes 4-byte-aligned before calling rep_movs(). This
> movs() macro modifies 'to', and the subsequent line modifies 'n'.
>
> As a result, on unaligned accesses, kmsan_unpoison_memory() uses the
> updated (aligned) values of 'to' and 'n'. Hence, it does not unpoison the
> entire region.
>
> This patch saves the original values of 'to' and 'n', and passes those to
> kmsan_unpoison_memory(), so that the entire region is unpoisoned.

Nice catch! Does it fix any known bugs?

> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@...il.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>