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

mm: zswap: fix shrinker NULL crash with cgroup_disable=memory

Christian reports a NULL deref in zswap that he bisected down to the zswap
shrinker. The issue also cropped up in the bug trackers of libguestfs [1]
and the Red Hat bugzilla [2].

The problem is that when memcg is disabled with the boot time flag, the
zswap shrinker might get called with sc->memcg == NULL. This is okay in
many places, like the lruvec operations. But it crashes in
memcg_page_state() - which is only used due to the non-node accounting of
cgroup's the zswap memory to begin with.

Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I
was then able to reproduce the crash locally as well.

[1] libguestfs/libguestfs#139
[2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252

References

• https://nvd.nist.gov/vuln/detail/CVE-2024-35846
• https://git.kernel.org/stable/c/682886ec69d22363819a83ddddd5d66cb5c791e1
• https://git.kernel.org/stable/c/b0fdabc908a7f81d12382c87ca9e46a9c2e14042