# CVE-2026-89791: perf: Fix use-after-free when perf mmap() revival races with the last munmap()

> A race condition in the Linux kernel perf subsystem allows local attackers to trigger a use-after-free, leading to privilege escalation to root.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89791, DEBIAN-CVE-2026-89791, GHSA-3xg9-4g5f-3mp3, UBUNTU-CVE-2026-89791

## What is CVE-2026-89791?

CVE-2026-89791 is a high-severity use after free vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

perf: Fix use-after-free when perf mmap() revival races with the last munmap()

perf_mmap_close() drops rb->mmap_count *without* holding
event->mmap_mutex (the refcount_dec_and_test() right before the
refcount_dec_and_mutex_lock() of event->mmap_count). A concurrent
perf_mmap_rb() can slot its entire "revival" path into that window
(perf_mmap holds event->mmap_mutex for its whole duration, including
rb_alloc):

  munmap side (perf_mmap_close)          mmap side (perf_mmap_rb)
  -----------------------------------    --------------------------------
  rb->mmap_count 1 -> 0   (no lock)      (holds event->mmap_mutex)
                                         inc_not_zero(rb->mmap_count) fails
                                         ring_buffer_attach(event, NULL)
                                         rb_alloc() + attach new rb
                                         refcount_set(&event->mmap_count, 1)
  lock; event->mmap_count 1 -> 0
  ring_buffer_attach(event, NULL)
  ring_buffer_put() -> frees the *new* rb

The revival's refcount_set(&event->mmap_count, 1) is an invisible
1 -> 1 write: the close frees the just-revived buffer although the
other process still has it mapped -- a page-level use-after-free
allowing local privilege escalation to root by any unprivileged user
(default kernel.perf_event_paranoid=2).

Swap the order of the two counter updates: event->mmap_count is
dropped first via refcount_dec_and_mutex_lock(), so its 1 -> 0
transition and the ring_buffer_attach() stay serialized with
perf_mmap(). rb->mmap_count == 0 then implies every event using the
buffer is detached already, so the result of the rb->mmap_count drop
can gate the remaining teardown directly and detach_rest is no longer
needed.

An earlier fix for this race from Kyle Zeng and David Lee takes
event->mmap_mutex around both counter updates [0]; here the not-last
close stays lockless.

## Which versions are affected by CVE-2026-89791?


## How do I fix CVE-2026-89791?

Apply the vendor's patch.

## Is CVE-2026-89791 being exploited?

There is no known exploitation of CVE-2026-89791 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89791 and fix it if it is affected.

What it is: CVE-2026-89791 is a high-severity use after free vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89791 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89791/ (Markdown: https://insights.sbomify.com/CVE-2026-89791/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89791
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89791
- OSV: https://osv.dev/vulnerability/CVE-2026-89791

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89791/
Summaries are generated and may contain mistakes; the official description above is the record.