[CVE-2026-52955][IMPORTANT] libceph: Fix potential out-of-bounds access in crush_decode()

[CVE-2026-52955][IMPORTANT] libceph: Fix potential out-of-bounds access in crush_decode()

[AL-KERNEL]

CVE: CVE-2026-52955
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: libceph: Fix potential out-of-bounds access in crush_decode()
Commit: 6e70ef53e818c53eab28d7b0026b7fd03dddaba5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e70ef53e818c53eab28d7b0026b7fd03dddaba5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52955
Analysis date: Wed, 24 Jun 2026 15:39:45 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised Ceph monitor can send a malformed OSD map with inconsistent CRUSH bucket algorithm fields, causing libceph to process or destroy a bucket using the wrong layout and potentially triggering out-of-bounds kernel memory access and DoS with broader memory-corruption concerns.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-52955 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52955

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: IMPORTANT
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-2026-52955	IMPORTANT	CHECK	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-843;CWE-125;*CWE-787;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A potential out-of-bounds access in libceph crush_decode() occurs because a CEPH_MSG_OSD_MAP CRUSH bucket contains two algorithm fields and the first value is used for allocation while the later b alg value is used for processing and destruction. If a malicious or compromised Ceph monitor provides a map where these values differ, the client may process a bucket as the wrong type and access fields outside the allocated object. For the CVSS the PR:N value is used because no local privileges are required on the victim once the attacker can supply a malicious OSD map from a monitor endpoint. The issue is network reachable only in the Ceph cluster control path and is typically limited to trusted storage or data center networks rather than the public Internet. Impact is at least denial of service and in the paranoid case may include confidentiality or integrity impact because the bug class is out-of-bounds access in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE OOB SIMPLEFIX NETWORK LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The malformed `CEPH_MSG_OSD_MAP` can be supplied over the Ceph control path by a malicious or compromised monitor endpoint.
* Memory corruption, Weak/indirect corruption candidate: +1
  The mismatch between `alg` and `b->alg` can cause a bucket allocated as one CRUSH type to be processed or destroyed as another type.
* Reliable kernel crash / strong DoS: +1
  Wrong-type bucket destruction can access invalid fields and pass invalid pointers to cleanup paths.
* Confidentiality impact plausible: +1
  OOB access in kernel memory gives at least a limited concern for unintended reads, although no direct leak primitive is shown.
* Integrity impact plausible: +1
  Wrong-type destruction and invalid frees create a plausible kernel memory corruption concern, although no controlled write primitive is demonstrated.

Paranoid delta:

* Upgrade weak corruption to stronger memory-corruption concern: +1
  The patch explicitly describes out-of-bounds access during processing and again during destruction, including freeing memory pointed to by fields selected from the wrong bucket layout.

## 3. Reachability analysis

The realistic attacker is not an arbitrary Internet host in most deployments, but a malicious or compromised Ceph monitor, or an attacker able to tamper with monitor traffic. No local privileges are required on the victim client once it accepts the malicious OSD map, so the practical CVSS PR interpretation is `PR:N`. This is network-triggerable within the Ceph cluster control plane, typically an isolated storage or data-center network. The affected code is relevant only for systems using Ceph/libceph clients, so exposure is deployment-dependent but security-relevant where Ceph is in use.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch fixes a kernel memory-safety issue in network-fed CRUSH map parsing, not just a validation warning or resource leak. The realistic impact is likely kernel crash or client-side DoS. The theoretical worst case includes limited or high C/I concern because the bug can cause wrong-structure interpretation and out-of-bounds access during processing and cleanup. Practical privilege escalation or reliable code execution is not demonstrated by the patch, so the conservative score remains Actionable Moderate, while the paranoid score reaches Strong Important candidate.

## 5. One-sentence report phrase

A malicious or compromised Ceph monitor can send a malformed OSD map with inconsistent CRUSH bucket algorithm fields, causing libceph to process or destroy a bucket using the wrong layout and potentially triggering out-of-bounds kernel memory access and DoS with broader memory-corruption concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because it is a network-fed kernel memory-safety bug involving OOB access and wrong-type cleanup in libceph CRUSH map parsing, even though practical exploitation beyond DoS is not proven.

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

Patch: libceph: Fix potential out-of-bounds access in crush_decode()
Commit: 6e70ef53e818c53eab28d7b0026b7fd03dddaba5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e70ef53e818c53eab28d7b0026b7fd03dddaba5

Commit description:

A message of type CEPH_MSG_OSD_MAP containing a crush map with at least
one bucket has two fields holding the bucket algorithm. If the values
in these two fields differ, an out-of-bounds access can occur. This is
the case because the first algorithm field (alg) is used to allocate
the correct amount of memory for a bucket of this type, while the second
algorithm field inside the bucket (b->alg) is used in the subsequent
processing.

This patch fixes the issue by adding a check that compares alg and
b->alg and aborts the processing in case they differ. Furthermore,
b->alg is set to 0 in this case, because the destruction of the crush
map also uses this field to determine the bucket type, which can again
result in an out-of-bounds access when trying to free the memory pointed
to by the fields of the bucket. To correctly free the memory allocated
for the bucket in such a case, the corresponding call to kfree is moved
from the algorithm-specific crush_destroy_bucket functions to the
generic crush_destroy_bucket().

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/crush/crush.c
  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=6e70ef53e818c53eab28d7b0026b7fd03dddaba5

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

The original Linux kernel CVE announcement for CVE-2026-52955 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52955

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:L/I:L/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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).