[CVE-2026-89655][MODERATE 7.0] ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89655
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock
Commit: 2701431aa3cc8b23efe6890182e7b04f5e76fab5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2701431aa3cc8b23efe6890182e7b04f5e76fab5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89655
Analysis date: Fri, 11 Sep 2026 21:28:34 -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.

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

The original Linux kernel CVE announcement for CVE-2026-89655 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89655

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-2026-89655	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89655 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free can occur in the CephFS client because __kick_flushing_caps iterates ci i_cap_flush_list and drops i_ceph_lock while sending cap messages. During this unlock window, a fast FLUSH_ACK from the MDS can let handle_cap_flush_ack detach and free the same ceph_cap_flush entry, after which the original loop may dereference the freed list node when advancing to the next entry. For the CVSS the PR:N is used in the paranoid network based interpretation because the attacker does not need local privileges on the victim if they control or compromise the Ceph MDS endpoint or can otherwise influence MDS replies. The attack is network reachable only in deployments where the victim CephFS client communicates with an attacker controlled or compromised MDS, and it is not a generic Internet exposed service on most systems. Impact is at least denial of service via kernel crash, and in the paranoid assessment it may allow confidentiality or integrity impact because the bug class is kernel use-after-free rather than a simple NULL dereference.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	DISK INIT RACE UAF KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap changed the score.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap changed the score.
* 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 `cf` can be freed by `handle_cap_flush_ack()` while `__kick_flushing_caps()` still holds the stale iterator object and later dereferences `cf->i_list.next`.
* +1 Reliable kernel crash / strong DoS. A post-free list-node dereference in kernel context can realistically produce an Oops, panic, or memory-corruption crash.
* +2 Network-triggerable in the evaluated CephFS client and MDS trust model. The freeing side is driven by a FLUSH_ACK from the MDS over the Ceph protocol, so a malicious or compromised MDS can influence the vulnerable window without local privileges on the victim.
* -1 Hard timing window. The bug requires the MDS ACK and freeing path to occur while `__kick_flushing_caps()` has dropped `i_ceph_lock` and before it advances the iterator.

Paranoid signals:

* +2 Real UAF / active lifetime violation.
* +1 Reliable kernel crash / strong DoS.
* +2 Network-triggerable through attacker-controlled or compromised MDS replies.
* No timing penalty in the paranoid score because a malicious MDS can deliberately reply quickly and the client may repeatedly generate cap flush traffic.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Concrete repeated release/refcount mutation is not established. The stale operation is a list traversal dereference, not a second `put`, `free`, or refcount decrement on the same object.
* Structured stale-object leverage is not separately awarded because attacker-controlled replacement contents or post-free structured reuse are not established.
* Weak LPE is not awarded solely from the UAF class.
* Extra Availability is not awarded because the same crash is already counted once.
* C/I impact is not awarded in ActionableScore arithmetic merely because a UAF could theoretically corrupt reclaimed memory.

3. Reachability analysis

The most relevant attacker model is a malicious or compromised Ceph MDS endpoint communicating with a victim CephFS client. The MDS can send a fast FLUSH_ACK for a cap flush that the client just emitted, causing `handle_cap_flush_ack()` to detach and free the same `ceph_cap_flush` object while `__kick_flushing_caps()` is between unlock and relock.

This does not require host-admin privileges on the victim once the victim is already a CephFS client of that MDS. Therefore `HostAdminRequired=NO` for the network-based MDS attacker model. The issue is not namespace-scoped, and no namespace capability chain is involved.

The issue is not a generic Internet-exposed service in typical deployments. CephFS clients normally communicate with configured Ceph storage infrastructure, often inside a trusted storage or data-center network. Practical exploitation therefore depends on control of, compromise of, or traffic influence over the MDS path.

Race repeatability is plausible because cap flush traffic can recur, but the exact vulnerable interleaving is timing-dependent. This supports the lower conservative score while still requiring manual review.

4. Primitive and severity interpretation

The exact primitive is a real kernel use-after-free on `struct ceph_cap_flush`. The stale object is an active structured list element in `ci->i_cap_flush_list`. The vulnerable operation is not merely passive metadata access: the iterator advances through `cf->i_list.next` after `cf` may already have been detached and freed.

This is not a concrete repeated release/refcount case. There is no shown second lifetime mutation through the same stale reference. It is instead an active structured list traversal UAF.

Primitive class is `CLASS_B`: a real kernel lifetime primitive exists, but controlled reclaim, arbitrary read/write, callback control, or demonstrated privilege escalation are not established. Demonstrated consequence is at least kernel crash or DoS. Speculative consequence is broader memory corruption if the freed list node is reused in an exploitable way, but that is not proven by the patch.

`ImportantCandidate=YES` because this is a real active-object UAF in a networked filesystem client path where the freeing event can be influenced by an external MDS peer. The exploitability beyond crash is unresolved and should not be auto-closed as ordinary Moderate.

`AutoDowngradeAllowed=NO` because automatic downgrade would discard a concrete kernel UAF with meaningful unresolved lifetime-corruption leverage. This does not prove Important severity by itself, but it requires manual review.

Likely operational severity is Actionable Moderate at minimum, with Strong Important candidacy depending on whether further analysis shows controlled reclaim, repeatable post-free reuse, or another useful corruption primitive.

5. One-sentence report phrase

A CephFS client use-after-free can occur when __kick_flushing_caps drops i_ceph_lock while iterating i_cap_flush_list and a fast MDS FLUSH_ACK frees the current ceph_cap_flush entry, causing a stale list-node dereference and possible kernel crash.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed kernel UAF in a networked filesystem client path with externally influenced lifetime timing, and post-free exploitation leverage is not fully resolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock
Commit: 2701431aa3cc8b23efe6890182e7b04f5e76fab5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2701431aa3cc8b23efe6890182e7b04f5e76fab5

Commit description:

list_for_each_entry() iterates ci->i_cap_flush_list but drops
i_ceph_lock to send cap messages.  During the unlock window,
handle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries
with tid <= flush_tid from the list, release i_ceph_lock, and free
them via ceph_free_cap_flush() outside any lock.  When the original
thread reacquires i_ceph_lock and the for-loop macro advances via
cf = list_next_entry(cf, i_list), it dereferences cf->i_list.next
on freed memory.

The race timeline:

  __kick_flushing_caps()              handle_cap_flush_ack()
  -----------------------             -----------------------
  holds i_ceph_lock        <---
  iterates to cf (tid=10)
  prepares FLUSH message
  drops i_ceph_lock        <---
  __send_cap() ── FLUSH(tid=10)
	                              MDS sends FLUSH_ACK(tid=10)
                           --->       acquires i_ceph_lock
                                      cf->tid(10) <= flush_tid(10),
                                      detaches cf from i_cap_flush_list
                                      drops i_ceph_lock
                                      ceph_free_cap_flush(cf) <- frees it!
  acquires i_ceph_lock     <---
  for-loop advances:
    cf = list_next_entry(cf, i_list)
      -- UAF on freed cf->i_list.next

The cf was just sent by __kick_flushing_caps itself via __send_cap().
The MDS may respond with FLUSH_ACK quickly enough that
handle_cap_flush_ack() frees cf before __kick_flushing_caps can
finish the iteration.

Fix by converting to a manual while loop: save the next pointer
under i_ceph_lock before dropping it, then use the saved pointer
after reacquiring, so the potentially-freed cf is never accessed again.

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/caps.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=2701431aa3cc8b23efe6890182e7b04f5e76fab5

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

The original Linux kernel CVE announcement for CVE-2026-89655 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89655

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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
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please use reply email interface to add additional manual analyses
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