[CVE-2026-52957][MODERATE 7.0] libceph: Fix potential null-ptr-deref in decode_choose_args()

[CVE-2026-52957][MODERATE 7.0] libceph: Fix potential null-ptr-deref in decode_choose_args()

[AL-KERNEL]

CVE: CVE-2026-52957
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: libceph: Fix potential null-ptr-deref in decode_choose_args()
Commit: d55ffad8d422b5d1cc44dad32bd3d25f4471cd9f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d55ffad8d422b5d1cc44dad32bd3d25f4471cd9f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52957
Analysis date: Fri, 26 Jun 2026 14:37:01 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malformed Ceph OSD map containing CRUSH choose_args for a sparse bucket index can trigger a NULL pointer dereference in libceph decode_choose_args and crash a kernel Ceph client when the attacker can control or tamper with monitor-delivered map data.

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

The original Linux kernel CVE announcement for CVE-2026-52957 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52957

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-2026-52957	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A NULL pointer dereference in libceph OSD map decoding can occur because decode_choose_args checks that bucket_index is within max_buckets but did not verify that c buckets bucket_index is non NULL before reading the bucket size. A malformed or corrupted CEPH_MSG_OSD_MAP containing CRUSH choose_args for a sparse bucket index can therefore crash a kernel Ceph client during map decode. For the CVSS the PR:N is used because no local privileges on the victim are needed if the attacker can control or tamper with a Ceph monitor message. The issue is network reachable in Ceph deployments, but practical exploitation usually requires a malicious or compromised monitor, a trusted storage network position, or successful message tampering. Impact is denial of service via kernel crash, with no supported evidence of information disclosure or privilege escalation from this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	REMOTE READ NULLPTR SIMPLEFIX NETWORK LINUS KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2. The affected parser handles CEPH_MSG_OSD_MAP data received by a kernel Ceph client.
* Reliable kernel crash / strong DoS: +1. The patch prevents a NULL pointer dereference when c->buckets[bucket_index] is NULL.
* Availability impact realistic: +1 in the paranoid score. A malicious or corrupted OSD map can plausibly crash the client kernel during decode.
* Important filesystem/storage path: +1 in the paranoid score. This is in libceph OSD map handling, a storage control-plane path used by kernel Ceph clients.
* Generic memory corruption: +0. The demonstrated primitive is NULL pointer dereference, not UAF, OOB write, double-free, or controlled overwrite.
* Privilege escalation plausible: +0. No supported LPE primitive is visible from the patch.
* Confidentiality impact plausible: +0. No read-back or information disclosure path is shown.
* Integrity impact plausible: +0. The patch context supports crash prevention, not data modification or policy bypass.

## 3. Reachability analysis

A remote peer can influence this path only if it can provide or tamper with Ceph OSD map messages, typically by acting as a malicious or compromised monitor, by compromising the storage control plane, or by successful MITM in a trusted Ceph network. No local privileges on the victim host are needed in that attacker model. The path is not Internet-default exposure, but it is realistic in Ceph deployments where kernel clients trust network-delivered cluster maps.

## 4. Severity interpretation

This behaves as an actionable Moderate rather than a strong Important issue. The practical impact is kernel DoS through a network-delivered storage control message. The patch does not support a memory corruption primitive beyond NULL pointer dereference, and there is no evidence of privilege escalation, arbitrary write, UAF reclaim, or information disclosure. The paranoid score reaches 5 because the bug is network reachable in a real storage subsystem and can crash the kernel client.

## 5. One-sentence report phrase

A malformed Ceph OSD map containing CRUSH choose_args for a sparse bucket index can trigger a NULL pointer dereference in libceph decode_choose_args and crash a kernel Ceph client when the attacker can control or tamper with monitor-delivered map data.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. This is network reachable in Ceph deployments and can cause a kernel crash, even though the supported primitive is DoS only and not privilege escalation.

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

Patch: libceph: Fix potential null-ptr-deref in decode_choose_args()
Commit: d55ffad8d422b5d1cc44dad32bd3d25f4471cd9f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d55ffad8d422b5d1cc44dad32bd3d25f4471cd9f

Commit description:

A message of type CEPH_MSG_OSD_MAP contains an OSD map that itself
contains a CRUSH map. When decoding this CRUSH map in crush_decode(), an
array of max_buckets CRUSH buckets is decoded, where some indices may
not refer to actual buckets and are therefore set to NULL. The received
CRUSH map may optionally contain choose_args that get decoded in
decode_choose_args(). When decoding a crush_choose_arg_map, a series of
choose_args for different buckets is decoded, with the bucket_index
being read from the incoming message. It is only checked that the bucket
index does not exceed max_buckets, but not that it doesn't point to an
index with a NULL bucket. If a (potentially corrupted) message contains
a crush_choose_arg_map including such a bucket_index, a null pointer
dereference may occur in the subsequent processing when attempting to
access the bucket with the given index.

This patch fixes the issue by extending the affected check. Now, it is
only attempted to access the bucket if it is not NULL.

Cc: stable@vger.kernel.org
Signed-off-by: Raphael Zimmer <raphael.zimmer@tu-ilmenau.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ceph/osdmap.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=d55ffad8d422b5d1cc44dad32bd3d25f4471cd9f

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

The original Linux kernel CVE announcement for CVE-2026-52957 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52957

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:L/PR:N/UI:N/S:U/C:N/I:N/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).