# CVE-2026-89654: ceph: fix UAF in check_new_map() on session freed during unlock

> A use-after-free vulnerability in the Linux kernel's Ceph client allows attackers to execute arbitrary code or escalate privileges via a race condition.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-100770, BELL-CVE-2026-89654, DEBIAN-CVE-2026-89654, GHSA-hwg3-wmfr-8q53, UBUNTU-CVE-2026-89654

## What is CVE-2026-89654?

CVE-2026-89654 is a critical use after free vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

ceph: fix UAF in check_new_map() on session freed during unlock

check_new_map() iterates mdsc->sessions[] and for each active session
drops mdsc->mutex to perform per-session operations.  The forced-close
path (rank removed from map) correctly takes a reference on s via
ceph_get_mds_session() before releasing mdsc->mutex, but three other
paths do not:

  Path A (address changed):  mutex_unlock → mutex_lock(&s->s_mutex)
  Path B (reconnect):        mutex_unlock → send_mds_reconnect(mdsc, s)
  Path C (active transition): mutex_unlock → mutex_lock(&s->s_mutex)

Without the extra reference, another thread can acquire mdsc->mutex
during the unlock window, call __unregister_session() which drops the
last reference on s, and free it.  The original thread then accesses
freed memory via s->s_mutex.

Fix by adding ceph_get_mds_session(s) before each mutex_unlock and
ceph_put_mds_session(s) after the corresponding mutex_lock, matching
the pattern already used in the forced-close path.

Race timeline (Path A):

  Thread A (check_new_map)             Thread B (another map update
    holds mdsc->mutex                      or session teardown)
  --------------------------           --------------------------
  s = mdsc->sessions[i]
  (refcount == 1, held only by
   sessions[] array)

  mutex_unlock(&mdsc->mutex)
                               --->    acquires mdsc->mutex
                                       __unregister_session(mdsc, s)
                                         sessions[i] = NULL
                                         ceph_put_mds_session(s)
                                           refcount: 1 -> 0
                                           kfree(s)  <--- freed!

  mutex_lock(&s->s_mutex)
  UAF on freed s->s_mutex

## Which versions are affected by CVE-2026-89654?


## How do I fix CVE-2026-89654?

Apply the vendor's patch.

## Is CVE-2026-89654 being exploited?

There is no known exploitation of CVE-2026-89654 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89654 and fix it if it is affected.

What it is: CVE-2026-89654 is a critical use after free vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89654 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-2026-89654/ (Markdown: https://insights.sbomify.com/CVE-2026-89654/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89654
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89654
- OSV: https://osv.dev/vulnerability/CVE-2026-89654

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89654/
Summaries are generated and may contain mistakes; the official description above is the record.