linux-cve-announce - CVE-2025-68214: timers: Fix NULL function pointer race in timer_shutdown

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Message-ID: <2025121631-CVE-2025-68214-1871@gregkh>
Date: Tue, 16 Dec 2025 14:57:33 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-68214: timers: Fix NULL function pointer race in timer_shutdown_sync()

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

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

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

timers: Fix NULL function pointer race in timer_shutdown_sync()

There is a race condition between timer_shutdown_sync() and timer
expiration that can lead to hitting a WARN_ON in expire_timers().

The issue occurs when timer_shutdown_sync() clears the timer function
to NULL while the timer is still running on another CPU. The race
scenario looks like this:

CPU0					CPU1
					<SOFTIRQ>
					lock_timer_base()
					expire_timers()
					base->running_timer = timer;
					unlock_timer_base()
					[call_timer_fn enter]
					mod_timer()
					...
timer_shutdown_sync()
lock_timer_base()
// For now, will not detach the timer but only clear its function to NULL
if (base->running_timer != timer)
	ret = detach_if_pending(timer, base, true);
if (shutdown)
	timer->function = NULL;
unlock_timer_base()
					[call_timer_fn exit]
					lock_timer_base()
					base->running_timer = NULL;
					unlock_timer_base()
					...
					// Now timer is pending while its function set to NULL.
					// next timer trigger
					<SOFTIRQ>
					expire_timers()
					WARN_ON_ONCE(!fn) // hit
					...
lock_timer_base()
// Now timer will detach
if (base->running_timer != timer)
	ret = detach_if_pending(timer, base, true);
if (shutdown)
	timer->function = NULL;
unlock_timer_base()

The problem is that timer_shutdown_sync() clears the timer function
regardless of whether the timer is currently running. This can leave a
pending timer with a NULL function pointer, which triggers the
WARN_ON_ONCE(!fn) check in expire_timers().

Fix this by only clearing the timer function when actually detaching the
timer. If the timer is running, leave the function pointer intact, which is
safe because the timer will be properly detached when it finishes running.

The Linux kernel CVE team has assigned CVE-2025-68214 to this issue.


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

	Issue introduced in 6.1.158 with commit 334c33aa487be406a149c8b87c38c8399d2dba8d and fixed in 6.1.159 with commit 1a975716cc8977f461e45e28e3e5977d46ad7a6a
	Issue introduced in 6.2 with commit 0cc04e80458a822300b93f82ed861a513edde194 and fixed in 6.6.118 with commit 6665fbd7730b26d770c232b20d1b907e6a67a914
	Issue introduced in 6.2 with commit 0cc04e80458a822300b93f82ed861a513edde194 and fixed in 6.12.60 with commit 176725f4848376530a0f0da9023f956afcc33585
	Issue introduced in 6.2 with commit 0cc04e80458a822300b93f82ed861a513edde194 and fixed in 6.17.10 with commit a01efa7a780c42ac5170a949bd95c9786ffcc60a
	Issue introduced in 6.2 with commit 0cc04e80458a822300b93f82ed861a513edde194 and fixed in 6.18 with commit 20739af07383e6eb1ec59dcd70b72ebfa9ac362c

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-2025-68214
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:
	kernel/time/timer.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/1a975716cc8977f461e45e28e3e5977d46ad7a6a
	https://git.kernel.org/stable/c/6665fbd7730b26d770c232b20d1b907e6a67a914
	https://git.kernel.org/stable/c/176725f4848376530a0f0da9023f956afcc33585
	https://git.kernel.org/stable/c/a01efa7a780c42ac5170a949bd95c9786ffcc60a
	https://git.kernel.org/stable/c/20739af07383e6eb1ec59dcd70b72ebfa9ac362c