[CVE-2026-89651][MODERATE 7.0] ceph: bound MDSCapAuth path and fs_name decode in handle_session()

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

CVE: CVE-2026-89651
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ceph: bound MDSCapAuth path and fs_name decode in handle_session()
Commit: dc5c7dffda92ef372fa587c444a1c537373df824
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc5c7dffda92ef372fa587c444a1c537373df824
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89651
Analysis date: Sat, 12 Sep 2026 01:03:06 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
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-2026-89651 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89651

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89651	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89651 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-126;CWE-125;*CWE-20;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.6';DESCR 'A remote out-of-bounds read exists in the Ceph kernel client MDS session handling path because handle_session decodes MDSCapAuth match path and fs_name strings with ceph_decode_copy without first checking that the requested length is still inside the message front buffer. A malicious or compromised MDS can trigger this with a crafted CEPH_SESSION_OPEN message during session setup, including the first post-connect message on mount. For the CVSS the PR:N is used because the attacker does not need privileges on the client system and only needs to be reachable as the MDS endpoint or to tamper with that MDS traffic. The issue is network reachable in Ceph deployments where the client connects to an attacker controlled or compromised MDS, but Ceph clusters are usually located on internal storage or data center networks rather than the public Internet. Impact is at least remote denial of service via kernel crash. In the paranoid score, limited confidentiality or integrity impact is kept because kernel memory can be read past the message allocation and copied into cap authorization state, but there is no clear arbitrary write primitive in the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)  SKIP CVE-2026-89651 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ KASAN OOB DISK ERRORPATH IMPROVEONLY KERNEL_PANIC_PLUS_UAF LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::

No floor or cap changed the conservative score. No floor or cap changed the paranoid score.

2. Signal breakdown

Conservative positive signals:

* +2 Remote reachable / directly network-triggerable: a malicious or compromised Ceph MDS can send a crafted CEPH_SESSION_OPEN message to the kernel client during session setup.
* +1 Reliable kernel crash / strong DoS: supplied evidence states KASAN reports slab-out-of-bounds read and that the client can be crashed.
* +1 Weak / indirect corruption candidate: the primitive is an out-of-bounds kernel read from the message front allocation through unchecked ceph_decode_copy length handling.

Conservative negative signals:

* 0 No host-admin penalty: host-admin-equivalent privilege on the victim is not required by the attacker once the client connects to the malicious or compromised MDS.
* 0 No race penalty: the trigger is malformed protocol data during decode, not a timing race.
* 0 No rare-hardware penalty: no special hardware is required.

Paranoid additional signal:

* +1 Limited integrity or confidentiality review concern: the out-of-bounds bytes are copied into kernel-allocated MDSCapAuth path or fs_name state and may be consumed as structured authorization matching data. This does not prove arbitrary write or disclosure, but it is enough to avoid automatic downgrade without manual review.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Real UAF not demonstrated.
* Confidentiality disclosure to attacker not demonstrated.
* Strong integrity impact not demonstrated.
* LPE not awarded.
* Extra Availability not awarded because the kernel crash is already counted once.
* ImportantCandidate is not set because the established primitive is an OOB read and crash path, not a strong corruption, controlled replacement, authorization bypass, or demonstrated privilege escalation primitive.

3. Reachability analysis

The attacker model is a malicious or compromised Ceph MDS, or an attacker able to tamper with MDS traffic. The vulnerable operation is reached by the Ceph kernel client when handling CEPH_SESSION_OPEN with msg_version >= 6. The supplied context says this can occur as the first post-connect message on mount and does not require client-side user interaction after connection.

Host administrator privilege on the client is not required for the malicious MDS to send the triggering message. Prior client configuration or mounting of the Ceph filesystem is a deployment prerequisite, but the attacker action itself is network-side. Namespace-scoped privilege is not relevant to the complete trigger. No special hardware, sysctl, boot mode, or race is required.

The issue is network reachable in practical Ceph deployments where the client connects to an attacker-controlled or compromised MDS. Ceph clusters are usually internal storage or data-center infrastructure rather than public Internet services, but that limits exposure rather than changing the primitive.

4. Primitive and severity interpretation

The exact primitive is an unchecked length-controlled copy from the Ceph message front buffer into cap_auths[i].match.path or cap_auths[i].match.fs_name. If the decoded length exceeds the remaining bytes in the message front, ceph_decode_copy reads past the end of the kvmalloc-backed message allocation.

This is not a UAF. It is not a repeated release or refcount mutation. It is not a controlled write. The demonstrated consequence is remote kernel crash or KASAN slab-out-of-bounds read on the client. The speculative consequence is limited C/I influence because unintended kernel memory can be copied into active cap authorization matching fields, but no disclosure path or attacker-controlled overwrite is established.

PrimitiveClass is CLASS_C because the patch establishes primarily OOB read and crash behavior without an independently useful corruption primitive. ImportantCandidate is NO because no controlled memory corruption, LPE, authorization bypass, or protected-resource theft is established. AutoDowngradeAllowed is NO because this is network-triggerable kernel OOB read in filesystem client protocol handling and the copied data enters authorization-related state, so manual review should not be skipped.

Likely operational severity is actionable Moderate: high CVSS-style availability impact when an exposed or compromised MDS is in the trust path, but not enough evidence for Important-class exploitation beyond DoS.

5. One-sentence report phrase

A malicious or compromised Ceph MDS can send a crafted CEPH_SESSION_OPEN message that causes unchecked MDSCapAuth path or fs_name decoding to read past the message front allocation, leading to a remote kernel client crash and requiring manual review for limited authorization-state side effects.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-triggerable from a malicious or compromised MDS and involves an out-of-bounds kernel read whose result is copied into MDSCapAuth matching state, even though the demonstrated impact is primarily client kernel DoS.

7. Call-site confidence

Call-site confidence: high

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

Patch: ceph: bound MDSCapAuth path and fs_name decode in handle_session()
Commit: dc5c7dffda92ef372fa587c444a1c537373df824
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dc5c7dffda92ef372fa587c444a1c537373df824

Commit description:

handle_session() decodes the MDSCapAuth records carried by a
CEPH_SESSION_OPEN message (msg_version >= 6). For each record the
match.path and match.fs_name byte strings are read by first decoding a
32-bit length and then copying that many bytes with the bare
ceph_decode_copy(). Unlike the surrounding fields, which all use the
_safe decode variants, these two copies are not preceded by a
ceph_decode_need() bounds check, and the enclosing MDSCapAuth and
MDSCapMatch struct_len fields are skipped rather than enforced as an
upper bound. A length larger than the bytes remaining in the message
front makes ceph_decode_copy() read past the end of the front buffer.

The message front is a dedicated allocation (ceph_msg_new2() ->
kvmalloc), so the over-read runs off that object. A malicious or
compromised MDS can trigger this with the first post-connect message on
mount, with no client-side user interaction; under KASAN it is reported
as a slab-out-of-bounds read in handle_session().

Impact: a malicious MDS can force the kernel client to read up to 4 GiB
past the message front allocation during session setup, crashing the
client (out-of-bounds read).

Switch both copies to ceph_decode_copy_safe(), which performs the
ceph_decode_need() bounds check before the copy and branches to the
existing bad label, matching the rest of the decoder and the error path
that frees the partially decoded cap_auths array.

Cc: stable@vger.kernel.org
Fixes: 1d17de9 ("ceph: save cap_auths in MDS client when session is opened")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.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=dc5c7dffda92ef372fa587c444a1c537373df824

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

The original Linux kernel CVE announcement for CVE-2026-89651 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89651

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 8.6

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).