[CVE-2026-99132][LOW] xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf

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

CVE: CVE-2026-99132
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf
Commit: e5456dec79c71c8ef8f9f9adb77d320f113b34db
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e5456dec79c71c8ef8f9f9adb77d320f113b34db
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99132
Analysis date: Wed, 28 Oct 2026 23:26:44 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A privileged or delegated XFS label operation can deadlock filesystem transaction processing when shutdown overlaps a synchronous CIL push.

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

The original Linux kernel CVE announcement for CVE-2026-99132 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99132

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: LOW
Manual review required: NO

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
======================================================================

CVE-2026-99132	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99132 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-833;CWE-667;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'xfs_sync_sb_buf could retain the superblock buffer locks while a synchronous transaction commit flushes the CIL push workqueue. If filesystem shutdown occurs during that push, xfs_buf_item_unpin can block while acquiring the same buffer and the committing task can block while waiting for the worker, producing a lock inversion and filesystem availability loss. For the CVSS the PR:L value in the worst case applies only where filesystem label management is delegated to a constrained service or reduced capability administrator. Typical direct exploitation requires administrative filesystem control and precise shutdown timing, and there is no supported memory corruption, information disclosure, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 1.3 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	DEADLOCK DISK LEAK LOG LINUS SYZBOT 	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1 (+1 strong filesystem availability loss, +1 important XFS transaction/writeback path, -1 precise shutdown/CIL timing).
* Conservative score: 1; no floor or cap applies.
* Raw paranoid score: 1; no technically supported stronger primitive exists.
* Paranoid score: 1.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable strong DoS: holding the superblock buffer lock while synchronously flushing the CIL worker creates a circular wait if `xfs_buf_item_unpin()` needs that same lock.
* +1 Important filesystem path: the deadlock affects XFS superblock transaction commit and on-disk writeback.
* -1 Hard timing: shutdown must occur during the synchronous CIL push while the set-label path retains the relevant lock.
* No memory corruption, UAF, OOB access, disclosure, authorization bypass, persistent metadata corruption, or privilege-escalation primitive is shown.
* No extra availability point: the filesystem deadlock is the single demonstrated consequence.
* NEW52/B5-R and NEW53/B3-PENDING do not apply; there is no write primitive or shared-backing violation.

3. Reachability analysis

The initiating operation is online filesystem label management. The evidence indicates this normally requires administrative filesystem control, although a constrained service or reduced-capability administrator could be delegated that operation. It does not establish one universal privilege model, so effective host-admin and namespace-scoped privilege remain unknown. The actor can invoke label synchronization but is not shown to directly force the coincident filesystem shutdown. No special hardware or rare filesystem feature is required beyond mounted XFS and the affected label path.

4. Primitive and severity reading

The exact primitive is a lock-order inversion: the committing task holds the superblock buffer lock while waiting for the CIL workqueue, whose worker can wait for the same lock during shutdown. This is CLASS_C availability behavior, not active-state corruption. The patch releases locks through normal transaction commit and reacquires buffers afterward. ImportantCandidate is NO because no exploit leverage past the deadlock is present; automatic downgrade is safe.

5. One-sentence report phrase

A privileged or delegated XFS label operation can deadlock filesystem transaction processing when shutdown overlaps a synchronous CIL push.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch-supported impact is limited to a timing-dependent filesystem availability failure without corruption or boundary bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: xfs: don't hold buffer locks across sync transaction commit in xfs_sync_sb_buf
Commit: e5456dec79c71c8ef8f9f9adb77d320f113b34db
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e5456dec79c71c8ef8f9f9adb77d320f113b34db

Commit description:

xfs_sync_sb_buf() holds sb/rtsb buffer locks across a synchronous
xfs_trans_commit(), which flushes the CIL push workqueue internally.
If shutdown occurs during the CIL push, xfs_buf_item_unpin() needs to
lock these buffers to fail them, causing a deadlock:

  setlabel:          holds buf lock -> flush_workqueue(xfs-cil)
  CIL push worker:   xfs_buf_item_unpin -> xfs_buf_lock(same buf)

Remove the xfs_trans_bhold() calls so that commit releases the buffer
locks normally.  After the sync commit, re-acquire the buffers via
mp->m_sb_bp / mp->m_rtsb_bp for the on-disk writeback.

Fixes: f7664b3 ("xfs: implement online get/set fs label")
Reported-by: syzbot+837bcd54843dd6262f2f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=837bcd54843dd6262f2f
Cc: stable@vger.kernel.org
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/libxfs/xfs_rtgroup.h
  fs/xfs/libxfs/xfs_sb.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=e5456dec79c71c8ef8f9f9adb77d320f113b34db

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

The original Linux kernel CVE announcement for CVE-2026-99132 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99132

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).