[CVE-2026-99131][IMPORTANT] xfs: fix capability check in xfs

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

CVE: CVE-2026-99131
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xfs: fix capability check in xfs
Commit: 19a1a22029ad3ae3604c763af6bd1195cb73648f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=19a1a22029ad3ae3604c763af6bd1195cb73648f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99131
Analysis date: Fri, 30 Oct 2026 10:33:34 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
An authenticated NFS user can exploit XFS’s incorrect credential capability check to bypass a target group’s enforced hard quota.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99131 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99131

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-2026-99131	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99131 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H';CWE-863;CWE-269;CWE-285;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '8.1';DESCR 'An incorrect XFS capability check used the current task real credentials instead of the effective subjective credentials applied for an NFS request. During NFS credential overriding this could test the privileged nfsd credentials and incorrectly enable forced quota reservation. An authenticated user could change a large file to another group the user belonged to and bypass the target group hard quota even when quota enforcement was enabled. For the CVSS the PR:L value is selected because the attacker needs an authenticated user identity with permission to modify the file and select an allowed group. The concrete primitive is an authorization and quota enforcement bypass rather than memory corruption. Impact is limited integrity and availability in the base case, while repeated abuse on a shared filesystem can potentially exhaust storage and cause a broader service outage. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 8.1 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and TEMP_KPANIC_MERGE_IMPORTANT_VETO and B4_CONFIRMED_AUTHORIZATION_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 5.4 and ActionableScore 4 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	MAYBE	DISK INIT LEAK HARDWARE LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_VETO B4_CONFIRMED_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=YES
B4PendingConfirmed=YES

1. ActionableScore

* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; speculative storage exhaustion, confidentiality, and privilege escalation were not scored.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 Concrete authorization decision bypass: `has_capability_noaudit(current, CAP_FOWNER)` tests the wrong credentials during NFS credential overriding, incorrectly enabling `XFS_QMOPT_FORCE_RES` instead of enforcing the quota hard limit.
* +2 Remote/network reachability: the reported complete path is an authenticated NFS client issuing an attribute/group change.
* +1 Integrity impact: the user can allocate beyond an enforced target-group hard quota.
* +1 Practical exploit chain: NFS credential override → wrong capability result → forced quota reservation → quota-limit bypass.
* +1 Important filesystem/storage path: XFS quota enforcement protects shared storage allocation.
* No availability point: filesystem exhaustion is possible through repetition but not established as a reliable host-wide outage.
* No confidentiality, LPE, memory-corruption, or arbitrary-write signal is shown.
* No separate security-state lifetime point: this is credential-selection semantics, not premature destruction of security state.

3. Reachability analysis

An authenticated, non-UID-0 NFS user who may modify the file and select another group they belong to can exercise the reported path. The attacker need not possess host `CAP_FOWNER`; the defect mistakenly observes privileged server credentials. The corrected check explicitly uses `init_user_ns`, so this is not a namespace-scoped capability path. No special hardware, race, boot option, or separate vulnerability is required.

4. Primitive and severity reading

The concrete DENY-to-ALLOW path is NFS `setattr`/group change → `xfs_setattr_nonsize()` or ioctl attribute transaction → wrong `CAP_FOWNER` result → `xfs_trans_alloc_ichange()` permits forced reservation despite the group hard limit. This is a CLASS_A policy-enforcement bypass with confirmed protected-resource impact, not memory corruption. It proves quota integrity bypass, but not protected-file overwrite, root escalation, confidentiality loss, or guaranteed filesystem exhaustion. B4-PENDING is confirmed because the caller, below-host-admin credentials, mandatory guard, and protected quota resource are all grounded.

5. One-sentence report phrase

An authenticated NFS user can exploit XFS’s incorrect credential capability check to bypass a target group’s enforced hard quota.

6. Manual review advice

MANUAL CHECK REQUIRED

The confirmed below-host-admin authorization bypass affects shared-filesystem quota enforcement and must not be automatically downgraded.

7. Call-site confidence

Call-site confidence: high

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UPSTREAM PATCH SUMMARY
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Patch: xfs: fix capability check in xfs
Commit: 19a1a22029ad3ae3604c763af6bd1195cb73648f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=19a1a22029ad3ae3604c763af6bd1195cb73648f

Commit description:

An user reported a bug where he managed to evade group's quota
by changing a file's gid to a different group id the same user
belonged to, even though quotas were enforced on both gids and the
file's size was big enough to exceed the quota's hardlimit.

Commit eba0549 replaced a capable() call by a
has_capability_noaudit() to prevent unnecessary selinux audit messages.
Turns out that both calls have slightly different semantics even though
their documentation seems similar. Where in a nutshell:

capable() - Tests the task's effective credentials
has_ns_capability_noaudit() - Tests the task's real credentials

This most of the time has no practical difference but in some cases like
changing attrs (specifically group id in this case) through a NFS client
this will allow the quota code to use XFS_QMOPT_FORCE_RES, effectively
bypassing quota accounting checks.

Using instead ns_capable_noaudit() should fix this issue and prevent
selinux audit messages.

This also fix the remaining calls to has_capability_noaudit()

Fixes: eba0549 ("xfs: don't generate selinux audit messages for capability testing")
Cc: stable@vger.kernel.org # v5.18
Reported-by: Dr. Thomas Orgis <thomas.orgis@uni-hamburg.de>
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Serge Hallyn <sergeh@kernel.org>
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/xfs_fsmap.c
  fs/xfs/xfs_ioctl.c
  fs/xfs/xfs_iops.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=19a1a22029ad3ae3604c763af6bd1195cb73648f

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

The original Linux kernel CVE announcement for CVE-2026-99131 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99131

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:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: unknown
  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: NO
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).