CVE-2026-68357: watchdog: pretimeout: Fix UAF in watchdog_unregister_governor()

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

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

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

watchdog: pretimeout: Fix UAF in watchdog_unregister_governor()

When a watchdog governor is unregistered, it updates existing watchdog
devices that were using this governor by falling back to `default_gov`.

If the governor being unregistered is currently set as `default_gov`,
the `default_gov` is never cleared.  This leads to 2 use-after-free
issues:
1. New watchdog devices registered after this point will inherit the
   dangling `default_gov`.
2. Existing watchdog devices using the unregistered governor will have
   their `wdd->gov` reassigned to the dangling `default_gov`.

Fix the UAF by clearing `default_gov` if it matches the governor being
unregistered.

The Linux kernel CVE team has assigned CVE-2026-68357 to this issue.


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

	Issue introduced in 4.9 with commit da0d12ff2b829a35e9921918e925d79497b82bef and fixed in 6.6.148 with commit 2e47b91b9b4020fcc01def14d6b6556d66074cf4
	Issue introduced in 4.9 with commit da0d12ff2b829a35e9921918e925d79497b82bef and fixed in 6.12.101 with commit 0ca252720f0e38411cfec3431db9bb1aed0a412c
	Issue introduced in 4.9 with commit da0d12ff2b829a35e9921918e925d79497b82bef and fixed in 6.18.42 with commit 7d1658b066de30f4b23afc14814d22416a971e6e
	Issue introduced in 4.9 with commit da0d12ff2b829a35e9921918e925d79497b82bef and fixed in 7.1.6 with commit 7993d626983cc58fbde9607333cfd2d57725c197
	Issue introduced in 4.9 with commit da0d12ff2b829a35e9921918e925d79497b82bef and fixed in 7.2-rc5 with commit 7362ba0f9c96ac3ad6a2ca3995bd9fc9a28a8661

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-2026-68357
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:
	drivers/watchdog/watchdog_pretimeout.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/2e47b91b9b4020fcc01def14d6b6556d66074cf4
	https://git.kernel.org/stable/c/0ca252720f0e38411cfec3431db9bb1aed0a412c
	https://git.kernel.org/stable/c/7d1658b066de30f4b23afc14814d22416a971e6e
	https://git.kernel.org/stable/c/7993d626983cc58fbde9607333cfd2d57725c197
	https://git.kernel.org/stable/c/7362ba0f9c96ac3ad6a2ca3995bd9fc9a28a8661