linux-cve-announce - CVE-2023-53178: mm: fix zswap writeback race condition

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Message-ID: <2025091555-CVE-2023-53178-9d27@gregkh>
Date: Mon, 15 Sep 2025 16:02:32 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2023-53178: mm: fix zswap writeback race condition

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

mm: fix zswap writeback race condition

The zswap writeback mechanism can cause a race condition resulting in
memory corruption, where a swapped out page gets swapped in with data that
was written to a different page.

The race unfolds like this:
1. a page with data A and swap offset X is stored in zswap
2. page A is removed off the LRU by zpool driver for writeback in
   zswap-shrink work, data for A is mapped by zpool driver
3. user space program faults and invalidates page entry A, offset X is
   considered free
4. kswapd stores page B at offset X in zswap (zswap could also be
   full, if so, page B would then be IOed to X, then skip step 5.)
5. entry A is replaced by B in tree->rbroot, this doesn't affect the
   local reference held by zswap-shrink work
6. zswap-shrink work writes back A at X, and frees zswap entry A
7. swapin of slot X brings A in memory instead of B

The fix:
Once the swap page cache has been allocated (case ZSWAP_SWAPCACHE_NEW),
zswap-shrink work just checks that the local zswap_entry reference is
still the same as the one in the tree.  If it's not the same it means that
it's either been invalidated or replaced, in both cases the writeback is
aborted because the local entry contains stale data.

Reproducer:
I originally found this by running `stress` overnight to validate my work
on the zswap writeback mechanism, it manifested after hours on my test
machine.  The key to make it happen is having zswap writebacks, so
whatever setup pumps /sys/kernel/debug/zswap/written_back_pages should do
the trick.

In order to reproduce this faster on a vm, I setup a system with ~100M of
available memory and a 500M swap file, then running `stress --vm 1
--vm-bytes 300000000 --vm-stride 4000` makes it happen in matter of tens
of minutes.  One can speed things up even more by swinging
/sys/module/zswap/parameters/max_pool_percent up and down between, say, 20
and 1; this makes it reproduce in tens of seconds.  It's crucial to set
`--vm-stride` to something other than 4096 otherwise `stress` won't
realize that memory has been corrupted because all pages would have the
same data.

The Linux kernel CVE team has assigned CVE-2023-53178 to this issue.

Affected and fixed versions
===========================

	Fixed in 6.1.30 with commit 2cab13f500a6333bd2b853783ac76be9e4956f8a
	Fixed in 6.3.4 with commit ba700ea13bf0105a4773c654f7d3bef8adb64ab2
	Fixed in 6.4 with commit 04fc7816089c5a32c29a04ec94b998e219dfb946

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2023-53178
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	mm/zswap.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2cab13f500a6333bd2b853783ac76be9e4956f8a
	https://git.kernel.org/stable/c/ba700ea13bf0105a4773c654f7d3bef8adb64ab2
	https://git.kernel.org/stable/c/04fc7816089c5a32c29a04ec94b998e219dfb946