linux-kernel - Re: [PATCH v2] x86: disable non-instrumented version of copy

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Date: Wed, 6 Mar 2024 16:09:32 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Alexander Potapenko <glider@...gle.com>, Marco Elver <elver@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev <kasan-dev@...glegroups.com>, 
	LKML <linux-kernel@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86: disable non-instrumented version of copy_mc when
 KMSAN is enabled

On Wed, 6 Mar 2024 at 14:08, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Something like below one?

I'd rather leave the regular fallbacks (to memcpy and copy_to_user())
alone, and I'd just put the

        kmsan_memmove(dst, src, len - ret);

etc in the places that currently just call the MC copy functions.

The copy_mc_to_user() logic is already set up for that, since it has
to do the __uaccess_begin/end().

Changing copy_mc_to_kernel() to look visually the same would only
improve on this horror-show, I feel.

Obviously some kmsan person needs to validate your kmsan_memmove() thing, but

> Can we assume that 0 <= ret <= len is always true?

Yes. It had better be for other reasons.

                  Linus