[CVE-2026-89649][MODERATE 7.0] ceph: bound xattr value length in __build_xattrs()

[CVE-2026-89649][MODERATE 7.0] ceph: bound xattr value length in __build_xattrs()

[AL-KERNEL]

CVE: CVE-2026-89649
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ceph: bound xattr value length in __build_xattrs()
Commit: 38be2f3c41ecaba58bb92dc16603e09ba9fd64f1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38be2f3c41ecaba58bb92dc16603e09ba9fd64f1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89649
Analysis date: Sat, 12 Sep 2026 00:29:31 -0400
ActionableScore: 6
ActionableScore lower bound: 5
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-2026-89649 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89649

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-2026-89649	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89649 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';CWE-125;CWE-126;*CWE-200;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A kernel heap out-of-bounds read in CephFS xattr decoding can occur because __build_xattrs() validates the encoded value length field but did not verify that the final value bytes remain inside the MDS supplied xattr blob. A malicious or compromised Ceph metadata server can set the last xattr value length larger than the bytes actually present, causing a later getxattr(2) call to copy adjacent kernel heap bytes to a user buffer. For the CVSS the PR:L is used because the disclosure is obtained through a local user context calling getxattr(2) on the affected CephFS mount, while the malformed metadata is supplied by the MDS. The issue is not a direct public Internet attack against an arbitrary host, but it is relevant in environments where CephFS clients trust storage servers or metadata servers reachable over an internal storage network. Impact is confidentiality loss through kernel memory disclosure, with no concrete write primitive or privilege escalation primitive shown by this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	WRITE OOB DISK IMPROVEONLY LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap was applied. The paranoid score is emitted as ActionableScore because it is the highest still-defensible interpretation supported by the established kernel heap disclosure primitive.

2. Signal breakdown

Conservative positive signals:

* +2 CLASS_A substantial kernel disclosure primitive. The MDS supplied xattr value length can exceed the actual kvmalloc backed blob, and getxattr(2) later copies bytes past the allocation into a user buffer.
* +1 Confidentiality plausible. The disclosure path is explicit and user-visible, not merely an internal kernel OOB read.
* +1 Practical exploit chain strongly implied. Malformed final xattr length from a malicious or compromised MDS is stored as xattr val_len and later used by getxattr(2) for the copy length.
* +1 Important filesystem or storage trust path. CephFS metadata supplied by the metadata server is decoded into client kernel xattr state and then exposed through a normal filesystem syscall.

Conservative negative signals:

* 0 No host-admin penalty. Host-admin-equivalent privilege on the victim is not required for the local getxattr(2) read once the affected CephFS mount exists.
* 0 No rare-hardware or physical-access penalty. This is a filesystem client parsing issue, not a hardware-only path.

Paranoid additional signal:

* +1 Memory layout invariant relevance. The bug breaks the invariant between the wire-length-sized kvmalloc allocation and the later xattr value copy length, making the disclosure primitive more actionable than a passive invalid read.

Important non-awarded signals:

* RealUAF not awarded. The primitive is an out-of-bounds read, not use-after-free.
* Integrity not awarded. The patch and supplied context establish read past allocation into userspace, not a write primitive.
* Strong LPE not awarded. No credential overwrite, arbitrary write, controlled reclaim, or privilege-escalation chain is established.
* Remote public Internet reachability not awarded. The attacker influence is through a malicious or compromised Ceph metadata server in the CephFS trust model, not arbitrary packet traffic to a generic host service.
* Extra Availability not awarded. The established impact is disclosure, not crash or persistent DoS.

3. Reachability analysis

The complete practical scenario requires an affected CephFS client that trusts and communicates with a malicious or compromised MDS. The malformed xattr blob is supplied by the MDS, while the disclosure is obtained when a local user calls getxattr(2) on the affected CephFS file.

The local syscall side is below host-admin and ordinary filesystem access can be sufficient, assuming the CephFS mount already exists and the user can access the relevant file. The prior CephFS mount and MDS trust relationship are deployment prerequisites, but they do not make the disclosure path host-admin-only.

This is not namespace-scoped in the relevant sense. No evidence shows that a user namespace or namespace-local capability is required or sufficient for the full trigger chain.

No special hardware, boot mode, or narrow race is involved. The vulnerable behavior is deterministic once a malformed final xattr value length is accepted and later exposed via getxattr(2).

4. Primitive and severity interpretation

The exact primitive is a kernel heap out-of-bounds read with a concrete disclosure path to userspace. The blob is allocated to the wire length, the final value length is not checked against the remaining blob size, and the oversized value length is later used as the memcpy length for getxattr(2).

This is not fixed stale storage, not an active-object UAF, and not a cleanup-only crash path. It is a direct bounds-validation failure in filesystem metadata decoding that creates a user-visible kernel heap disclosure.

PrimitiveClass is CLASS_A because substantial kernel disclosure is directly established. ImportantCandidate is YES because kernel heap disclosure can expose sensitive adjacent heap contents and should not be automatically treated as ordinary Moderate. AutoDowngradeAllowed is NO because the disclosure primitive is concrete and externally influenced through the CephFS metadata trust boundary.

Likely operational severity is Actionable Moderate, with Strong Important review justified depending on deployment sensitivity and whether the leaked heap contents can contain secrets or hardening bypass material.

5. One-sentence report phrase

A CephFS xattr decoding bug lets a malicious or compromised metadata server provide an oversized final xattr value length, causing getxattr(2) to copy adjacent kernel heap bytes to a local user buffer.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue has a concrete kernel heap out-of-bounds read with user-visible information disclosure, even though no write primitive or privilege escalation chain is established.

7. Call-site confidence

Call-site confidence: high

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

Patch: ceph: bound xattr value length in __build_xattrs()
Commit: 38be2f3c41ecaba58bb92dc16603e09ba9fd64f1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38be2f3c41ecaba58bb92dc16603e09ba9fd64f1

Commit description:

__build_xattrs() decodes the MDS-supplied xattr blob one attribute at a
time. For each attribute it reads a 32-bit name length, advances past the
name bytes, reads a 32-bit value length, records the value pointer, and
advances past the value bytes. The two length fields are read with
ceph_decode_32_safe(), but the value bytes themselves are advanced over
with a bare "p += len" and no ceph_decode_need() check that "len" bytes
remain in the blob.

For every attribute except the last, the next iteration's
ceph_decode_32_safe() on the following name length implicitly verifies
that the previous value did not run past the blob end. The final
attribute has no successor, so its decoded value length is never checked
against the blob bounds. A malicious or compromised metadata server can
set the last attribute's value length larger than the bytes actually
present in the blob.

The blob is a dedicated kvmalloc() allocation sized to the wire length
(ceph_buffer_new() in ceph_fill_inode()). __set_xattr() records the
oversized length in xattr->val_len verbatim, and a later getxattr(2) runs
memcpy(value, xattr->val, xattr->val_len) into a user-supplied buffer,
copying bytes past the end of the allocation back to user space.

Impact: a malicious metadata server discloses adjacent kernel heap bytes
to a local user via getxattr(2) on a CephFS file. Add the missing
ceph_decode_need() so an out-of-bounds value length on the final
attribute fails the decode and returns -EIO instead of being stored.

Cc: stable@vger.kernel.org
Fixes: 355da1e ("ceph: inode operations")
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/xattr.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=38be2f3c41ecaba58bb92dc16603e09ba9fd64f1

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

The original Linux kernel CVE announcement for CVE-2026-89649 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89649

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:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

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: 5
  Paranoid ActionableScore: 6

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).