linux-kernel - Re: [PATCH] kmsan: Fix out-of-bounds access to shadow memory

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Message-ID: <20250911190302.GF1376@sol>
Date: Thu, 11 Sep 2025 12:03:02 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Marco Elver <elver@...gle.com>, kasan-dev@...glegroups.com,
	Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	stable@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] kmsan: Fix out-of-bounds access to shadow memory

On Thu, Sep 11, 2025 at 10:51:45AM -0700, Eric Biggers wrote:
> On Thu, Sep 11, 2025 at 11:09:17AM +0200, Alexander Potapenko wrote:
> > On Wed, Sep 10, 2025 at 9:49 PM Eric Biggers <ebiggers@...nel.org> wrote:
> > >
> > > On Fri, Aug 29, 2025 at 09:45:00AM -0700, Eric Biggers wrote:
> > > > Running sha224_kunit on a KMSAN-enabled kernel results in a crash in
> > > > kmsan_internal_set_shadow_origin():
> > > >
> > > >     BUG: unable to handle page fault for address: ffffbc3840291000
> > > >     #PF: supervisor read access in kernel mode
> > > >     #PF: error_code(0x0000) - not-present page
> > > >     PGD 1810067 P4D 1810067 PUD 192d067 PMD 3c17067 PTE 0
> > > >     Oops: 0000 [#1] SMP NOPTI
> > > >     CPU: 0 UID: 0 PID: 81 Comm: kunit_try_catch Tainted: G                 N  6.17.0-rc3 #10 PREEMPT(voluntary)
> > > >     Tainted: [N]=TEST
> > > >     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
> > > >     RIP: 0010:kmsan_internal_set_shadow_origin+0x91/0x100
> > > >     [...]
> > > >     Call Trace:
> > > >     <TASK>
> > > >     __msan_memset+0xee/0x1a0
> > > >     sha224_final+0x9e/0x350
> > > >     test_hash_buffer_overruns+0x46f/0x5f0
> > > >     ? kmsan_get_shadow_origin_ptr+0x46/0xa0
> > > >     ? __pfx_test_hash_buffer_overruns+0x10/0x10
> > > >     kunit_try_run_case+0x198/0xa00
> > >
> > > Any thoughts on this patch from the KMSAN folks?  I'd love to add
> > > CONFIG_KMSAN=y to my crypto subsystem testing, but unfortunately the
> > > kernel crashes due to this bug :-(
> > >
> > > - Eric
> > 
> > Sorry, I was out in August and missed this email when digging through my inbox.
> > 
> > Curiously, I couldn't find any relevant crashes on the KMSAN syzbot
> > instance, but the issue is legit.
> > Thank you so much for fixing this!
> > 
> > Any chance you can add a test case for it to mm/kmsan/kmsan_test.c?
> 
> Unfortunately most of the KMSAN test cases already fail on upstream,
> which makes it difficult to develop new ones:

The KMSAN test failures bisect to the following commit:

    commit f90b474a35744b5d43009e4fab232e74a3024cae
    Author: Vlastimil Babka <vbabka@...e.cz>
    Date:   Mon Mar 10 13:40:17 2025 +0100

        mm: Fix the flipped condition in gfpflags_allow_spinning()

I'm not sure why.  Apparently something related to lib/stackdepot.c.

Reverting that commit on top of upstream fixes the KMSAN tests.

- Eric