CVE-2026-64416: mm: swap_cgroup: fix NULL deref in lookup_swap_cgroup_id on swapless host

From: Greg Kroah-Hartman <[email protected]>

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

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

mm: swap_cgroup: fix NULL deref in lookup_swap_cgroup_id on swapless host

lookup_swap_cgroup_id() passes swap_cgroup_ctrl[type].map to
__swap_cgroup_id_lookup() without checking that the type was ever
registered via swap_cgroup_swapon().  On a swapless host every ctrl->map
is NULL, so __swap_cgroup_id_lookup() dereferences NULL + a scaled
swp_offset().

Since commit bea67dcc5eea ("mm: attempt to batch free swap entries for
zap_pte_range()"), zap_pte_range() -> swap_pte_batch() calls
lookup_swap_cgroup_id() on any non-present, non-none PTE that decodes as a
real swap entry, without first validating it against swap_info[].  A
single PTE corrupted into a type-0 swap entry takes the host down at
process exit.

We hit this in production on a swapless 6.12.58 host: ~1s of
"get_swap_device: Bad swap file entry 3f800204222bb" (do_swap_page() being
correctly defensive about the same entry) followed by

  BUG: unable to handle page fault for address: 000003f800204220
  RIP: 0010:lookup_swap_cgroup_id+0x2b/0x60
  Call Trace:
   swap_pte_batch+0xbf/0x230
   zap_pte_range+0x4c8/0x780
   unmap_page_range+0x190/0x3e0
   exit_mmap+0xd9/0x3c0
   do_exit+0x20c/0x4b0

syzbot has reported the identical stack.

The source of the PTE corruption is a separate bug; this change makes the
teardown path as robust as the fault path already is.  Every other caller
of lookup_swap_cgroup_id() is downstream of a get_swap_device() that has
already validated the entry, so the new branch is cold.

The Linux kernel CVE team has assigned CVE-2026-64416 to this issue.


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

	Issue introduced in 6.12 with commit bea67dcc5eea0f6a213897a59b9e644977cd07e6 and fixed in 6.12.97 with commit 818416fef38759f23210de449663cd9d7e293d39
	Issue introduced in 6.12 with commit bea67dcc5eea0f6a213897a59b9e644977cd07e6 and fixed in 6.18.39 with commit b415c00bf23df577a4a95673d00ae76687bcc1d4
	Issue introduced in 6.12 with commit bea67dcc5eea0f6a213897a59b9e644977cd07e6 and fixed in 7.1.4 with commit 6a4196d19f477524d2f92adca90fc1fbe9a0420a
	Issue introduced in 6.12 with commit bea67dcc5eea0f6a213897a59b9e644977cd07e6 and fixed in 7.2-rc1 with commit 63b02a9409cb5180398491b093e48bcb5315f5fb

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-2026-64416
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/swap_cgroup.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/818416fef38759f23210de449663cd9d7e293d39
	https://git.kernel.org/stable/c/b415c00bf23df577a4a95673d00ae76687bcc1d4
	https://git.kernel.org/stable/c/6a4196d19f477524d2f92adca90fc1fbe9a0420a
	https://git.kernel.org/stable/c/63b02a9409cb5180398491b093e48bcb5315f5fb