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

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Message-ID: <ZlEWSIcHXnh/BqbW@rex>
Date: Sat, 25 May 2024 00:35:52 +0200
From: Brian Johannesmeyer <bjohannesmeyer@...il.com>
To: Alexander Potapenko <glider@...gle.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 Fri, May 24, 2024 at 10:28:05AM +0200, Alexander Potapenko wrote:
> Nice catch! Does it fix any known bugs?

Not that I know of. Based on my cursory testing, it seems that
string_memcpy_fromio() is rarely called with an unaligned `from`, so
this is a bit of an edge case.

On that note: I tried creating a unit test for this, to verify that
an unaligned memcpy_fromio() would yield uninitialized data without the
patch, and would yield initialized data with the patch. However, what I
found is that kmsan_unpoison_memory() seems to always unpoison an entire
4-byte word, even if called with a `size` of less than 4. However, this
issue is somewhat unrelated to the patch at hand, so I'll create a
separate patch to demonstrate what I mean.

Thanks,
Brian