[CVE-2026-89654][MODERATE 7.0] ceph: fix UAF in check_new_map() on session freed during unlock

[CVE-2026-89654][MODERATE 7.0] ceph: fix UAF in check_new_map() on session freed during unlock

[AL-KERNEL]

CVE: CVE-2026-89654
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ceph: fix UAF in check_new_map() on session freed during unlock
Commit: 21d5be092d947f8d83f76f7ebf3989e7e9230a98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=21d5be092d947f8d83f76f7ebf3989e7e9230a98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89654
Analysis date: Sat, 12 Sep 2026 07:02:12 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89654 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89654

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89654	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89654 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A use after free race exists in the CephFS MDS client check_new_map path because the code can drop mdsc mutex while still using a ceph_mds_session pointer without taking an extra reference. During that unlock window another thread can unregister the same session, drop the last reference, and free the object, after which the original path may access freed memory through s_mutex or later session operations. For the CVSS the PR:N value is used for the paranoid network storage scenario because an attacker does not need local privileges on the victim if they can control or tamper with the relevant Ceph MDS or monitor driven session state. The attack is not expected to be Internet exposed in typical deployments and usually requires access to a Ceph storage cluster network or a compromised cluster endpoint. Impact is at least denial of service via kernel crash. In the worst case this UAF may allow confidentiality or integrity impact due to kernel memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	DANGER DISK RACE UAF KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Applied conservative floor/cap: REAL UAF FLOOR 4 applied, 2 -> 4
* Raw paranoid score: 4
* Paranoid score: 5
* Applied paranoid floor/cap: REAL UAF FLOOR 5 applied, 4 -> 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. The patch directly establishes that `check_new_map()` keeps using `s` after dropping `mdsc->mutex`, while another thread may unregister the same session, drop the last reference, and free it.
* +1 Reliable kernel crash / strong DoS. The stale path can access freed memory through `s->s_mutex`, and other paths pass the stale `ceph_mds_session` into session operations.
* -1 Hard race / timing window. The conservative path requires another thread to unregister and free the session during the specific `mdsc->mutex` unlock window.

Paranoid additional interpretation:

* +2 Network-adjacent storage control path. In a paranoid but still defensible CephFS threat model, a malicious or compromised Ceph cluster endpoint or tampered cluster-control traffic may influence MDS map/session state without local privileges on the victim host. This is not treated as public Internet exposure.
* -1 Race / timing window retained. The bug still depends on winning the unlock window.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Confidentiality and integrity bonuses are not awarded directly, because the established primitive is UAF plus stale structured-object use, not a demonstrated disclosure or write primitive.
* Extra Availability is not awarded, because the crash/DoS is already counted once.
* Concrete repeated release/refcount signal is not awarded. The first release in `__unregister_session()` can free the object, and the exact stale reference survives in `check_new_map()`, but the second stale operation shown is `mutex_lock(&s->s_mutex)` or session processing, not a second concrete `put/free/refcount` mutation on the same dangling reference.
* Structured stale-object leverage is not separately awarded as a point, because attacker-controlled replacement or meaningful post-reuse field control is not established.

3. Reachability analysis

The vulnerable code is in the CephFS client MDS session handling path. It is reached during MDS map/session transitions while the client processes active sessions.

The conservative attacker model is unresolved. The supplied evidence does not prove that a normal local unprivileged user can force the full vulnerable sequence, and it also does not prove that host-admin-equivalent privilege is required. Therefore `HostAdminRequired=UNKNOWN`.

The paranoid attacker model is a network-adjacent Ceph storage scenario. If an attacker controls, compromises, or tampers with relevant Ceph MDS or monitor-driven session state, the victim kernel can be driven into MDS map/session changes over the storage network. This does not imply exposure to arbitrary Internet traffic. CephFS deployments are normally internal storage or data-center networks.

No namespace-scoped privilege chain is established. There is no evidence that this is a user namespace or network namespace reachable operation.

Prior setup is required in the practical sense that the victim must be using CephFS and have active MDS sessions. That is treated as deployment exposure, not as proof of host-admin privilege for the attacker.

Race repeatability is uncertain. Map/session state changes may be repeatable in a controlled cluster scenario, but the exact UAF still depends on an unlock-window race.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free of `struct ceph_mds_session`. The object is an active structured kernel object, not fixed stale storage. The stale use is not limited to a passive diagnostic read. The affected paths may lock `s->s_mutex`, close `s->s_con`, call `send_mds_reconnect(mdsc, s)`, call `ceph_kick_flushing_caps(mdsc, s)`, and update session state after the object may have been freed.

This is `PrimitiveClass=CLASS_B`: a real kernel lifetime/corruption primitive requiring exploitation-leverage analysis. It is not `CLASS_A`, because controlled reclaim, arbitrary write, callback control, refcount takeover, or demonstrated privilege escalation is not established.

This is not merely an ordinary stale cleanup dereference. The stale object participates in active Ceph session handling after a missing reference across an unlock window. However, it is also not a concrete repeated release/refcount case, because the patch does not show a second stale `put/free/refcount` operation performed by the original thread.

Demonstrated consequence: kernel crash or DoS via UAF access.

Speculative consequence: confidentiality or integrity impact may be possible only if allocator reuse or structured replacement can be controlled, but that is not established by the supplied patch.

`ImportantCandidate=YES` because this is a real active-object UAF in a kernel filesystem/network-storage client path, and the post-lifetime exploitation leverage is unresolved enough to require Important-vs-Moderate review.

`AutoDowngradeAllowed=NO` because automatically closing this as ordinary crash-only DoS would discard a concrete active-object UAF with unresolved exploitation surface.

Likely operational severity: Actionable Moderate at minimum, with Strong Important review justified if reachability from malicious Ceph cluster endpoints or attacker-controlled map/session churn is confirmed.

5. One-sentence report phrase

A race in CephFS `check_new_map()` can leave a stale `ceph_mds_session` pointer in use after another thread unregisters and frees the session during an unlock window, causing at least a kernel crash and requiring manual review for possible UAF exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a directly established active-object kernel UAF. Controlled reclaim or privilege escalation is not proven, but the stale session object is used in nontrivial Ceph session processing and should not be automatically downgraded to ordinary DoS.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: ceph: fix UAF in check_new_map() on session freed during unlock
Commit: 21d5be092d947f8d83f76f7ebf3989e7e9230a98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=21d5be092d947f8d83f76f7ebf3989e7e9230a98

Commit description:

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

Cc: stable@vger.kernel.org
Signed-off-by: Xiubo Li <xiubo.li@clyso.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/ceph/mds_client.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=21d5be092d947f8d83f76f7ebf3989e7e9230a98

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89654 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89654

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 4
  Paranoid ActionableScore: 5

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).