[CVE-2026-80557][MODERATE 7.0] libceph: fix OOB read in decode_watchers() via missing bounds check

[CVE-2026-80557][MODERATE 7.0] libceph: fix OOB read in decode_watchers() via missing bounds check

[AL-KERNEL]

CVE: CVE-2026-80557
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: libceph: fix OOB read in decode_watchers() via missing bounds check
Commit: 7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80557
Analysis date: Wed, 26 Aug 2026 13:49:02 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or compromised Ceph OSD can trigger a 4-byte OOB read in libceph `decode_watchers()` by returning a malformed watcher response with `struct_len=0`, potentially causing kernel client DoS through a bogus watcher count allocation path.

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

The original Linux kernel CVE announcement for CVE-2026-80557 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80557

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-2026-80557	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-126;*CWE-20;**CWE-400;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'An OOB read in libceph decode_watchers can occur when ceph_start_decoding accepts struct_len 0 and the next ceph_decode_32 reads 4 bytes past the validated response buffer. A malicious or compromised Ceph OSD can trigger this against a kernel client that issues CEPH_OSD_OP_LIST_WATCHERS, and the garbage value can be used as the watcher count for allocation. For the CVSS the PR:N is used because the attacker does not need privileges on the victim host, but does need control of or access to the OSD side of the Ceph storage relationship. The vector is treated as AV:A because Ceph OSD traffic is normally inside a storage or data center network rather than a public Internet service. Impact is at least denial of service via kernel crash or resource exhaustion. For the paranoid score, limited confidentiality impact is kept because this is a real kernel OOB read, but integrity impact is not raised without an OOB write, UAF, or arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE OOB NETWORK LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A malicious or compromised Ceph OSD can send a crafted `obj_list_watch_response_t` reply to a kernel client over the Ceph storage network.

* Reliable kernel crash / strong DoS: +1
  The 4-byte OOB read can produce a bogus watcher count that is passed to `kzalloc_objs()`, making kernel crash or resource-exhaustion DoS plausible.

* Important filesystem/storage path: +1
  The bug is in libceph OSD client response parsing, affecting kernel clients in Ceph storage deployments.

Paranoid additional signal:

* Confidentiality impact plausible: +1
  This is a real kernel OOB read past the validated response buffer. No direct disclosure path is shown, so this is limited and only used for the paranoid score.

Signals not awarded:

* Generic memory corruption: +0
  The patch shows OOB read, not OOB write, UAF, double free, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  No reclaim, overwrite, callback control, refcount abuse, or object replacement primitive is demonstrated.

* Integrity impact plausible: +0
  The garbage value influences allocation size, but there is no supported write or ownership-bypass primitive.

## 3. Reachability analysis

The realistic attacker is a malicious or compromised Ceph OSD, or an attacker able to tamper with OSD replies in the Ceph storage network. The victim is a Linux kernel Ceph client that issues `CEPH_OSD_OP_LIST_WATCHERS`.

This is not a generic public Internet remote bug. It is network-adjacent in typical Ceph deployments, usually inside storage or data-center networks. The attacker does not need privileges on the victim host, so victim-side PR is effectively none, but the attacker must control or influence the OSD side of an established Ceph relationship.

Namespaces and containers do not materially lower the trigger requirement unless the containerized workload can cause the host kernel Ceph client to issue the affected operation.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is network-adjacent protocol parsing in a kernel storage client and fixes a concrete missing bounds check before a kernel read. However, it does not reach a Strong Important candidate by itself because the primitive is a small OOB read with no shown disclosure channel, no write primitive, and no plausible LPE path from the patch context.

The conservative interpretation is Actionable Borderline to Moderate due to DoS. The paranoid interpretation is Actionable Moderate because limited confidentiality concern is defensible for a real kernel OOB read in externally influenced response parsing.

## 5. One-sentence report phrase

A malicious or compromised Ceph OSD can trigger a 4-byte OOB read in libceph `decode_watchers()` by returning a malformed watcher response with `struct_len=0`, potentially causing kernel client DoS through a bogus watcher count allocation path.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network-adjacent Ceph response parsing with a real kernel OOB read primitive, and the bogus value affects allocation behavior. Manual review should confirm whether any deployment-specific disclosure, crash reliability, or secondary allocation side effects exist.

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

Patch: libceph: fix OOB read in decode_watchers() via missing bounds check
Commit: 7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1

Commit description:

ceph_start_decoding() validates that struct_len bytes remain in the
buffer after the encoding header, but accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends an obj_list_watch_response_t reply with
struct_len=0, ceph_start_decoding() returns success with p == end,
leaving zero bytes guaranteed for subsequent reads.

The immediately following ceph_decode_32(p) in decode_watchers() has
no preceding bounds check. With p == end this is a 4-byte read past
the validated buffer boundary. The garbage value is then passed
directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants
(ceph_decode_copy_safe, ceph_decode_64_safe, ceph_decode_skip_32)
after its own ceph_start_decoding() call. decode_watchers() is the
only site that uses the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_watchers, bad), consistent with the established pattern.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment (e.g. cloud) can trigger this against any kernel client
that calls CEPH_OSD_OP_LIST_WATCHERS, without any further privileges
beyond OSD session establishment.

[ idryomov: trim changelog ]

Cc: stable@vger.kernel.org
Fixes: a4ed38d ("libceph: support for CEPH_OSD_OP_LIST_WATCHERS")
Signed-off-by: Pavitra Jha <jhapavitra98@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:
  net/ceph/osd_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=7130d94846dadbb97b6b7f4d78a3a7bba6e3daa1

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

The original Linux kernel CVE announcement for CVE-2026-80557 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80557

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: NO
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).