linux-kernel - Re: [PATCH v2] kmsan: Fix out-of-bounds access to shadow memory

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Message-ID: <CAG_fn=UY1HxmxpkM_YFGbr8W272F_bZgZHKiuvbsUjgFCs1RcA@mail.gmail.com>
Date: Wed, 17 Sep 2025 10:47:05 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Eric Biggers <ebiggers@...nel.org>
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
Subject: Re: [PATCH v2] kmsan: Fix out-of-bounds access to shadow memory

On Thu, Sep 11, 2025 at 10:01 PM Eric Biggers <ebiggers@...nel.org> 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
>
> This occurs when memset() is called on a buffer that is not 4-byte
> aligned and extends to the end of a guard page, i.e. the next page is
> unmapped.
>
> The bug is that the loop at the end of
> kmsan_internal_set_shadow_origin() accesses the wrong shadow memory
> bytes when the address is not 4-byte aligned.  Since each 4 bytes are
> associated with an origin, it rounds the address and size so that it can
> access all the origins that contain the buffer.  However, when it checks
> the corresponding shadow bytes for a particular origin, it incorrectly
> uses the original unrounded shadow address.  This results in reads from
> shadow memory beyond the end of the buffer's shadow memory, which
> crashes when that memory is not mapped.
>
> To fix this, correctly align the shadow address before accessing the 4
> shadow bytes corresponding to each origin.
>
> Fixes: 2ef3cec44c60 ("kmsan: do not wipe out origin when doing partial unpoisoning")
> Cc: stable@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...nel.org>
Tested-by: Alexander Potapenko <glider@...gle.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>

Thanks a lot!