[CVE-2026-99051][IMPORTANT] fsverity: RCU-delay the freeing of struct fsverity_info

[CVE-2026-99051][IMPORTANT] fsverity: RCU-delay the freeing of struct fsverity_info

[AL-KERNEL]

CVE: CVE-2026-99051
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: fsverity: RCU-delay the freeing of struct fsverity_info
Commit: fa84f091a8e7fc46fd29834496fab9739dd4c8dc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fa84f091a8e7fc46fd29834496fab9739dd4c8dc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99051
Analysis date: Tue, 27 Oct 2026 02:31:24 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A local low-privilege actor can race fs-verity info removal against an RCU hash lookup, causing active traversal and inode comparison through freed slab storage.

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

The original Linux kernel CVE announcement for CVE-2026-99051 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99051

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

CVE-2026-99051	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-99051 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME (final reconciliation agrees; DECISION: both IMPORTANT)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in fsverity info removal can free a struct fsverity_info while a concurrent lookup for a different inode is still traversing the same rhashtable chain under RCU. The confirmed primitive is a kernel use-after-free read involving rhash_head and inode fields, with kernel crash and local denial of service as the directly supported impact. Slab reuse before the lookup finishes could make the traversal consume repurposed object fields, so the paranoid score treats broader memory safety impact as possible but not proven. For the CVSS the PR:L value reflects that a local user or process must drive fsverity inode lookup and cleanup activity. AC:H reflects the required race window and concurrent hash-chain operations. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because 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_HIGH_BASED_ON_GUESSCVSS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DISK INIT UAF LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 5. REAL UAF FLOOR 5 applies because the below-host-admin lookup path performs active hash-chain pointer traversal and inode comparison through the stale object, beyond cleanup-only dereference.
* Raw paranoid score: 3.
* Paranoid score: 5. No additional speculative reuse point was awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Real UAF: `fsverity_remove_info()` immediately frees `vi` while concurrent RCU-protected `__fsverity_get_info()` can still access its `rhash_head` and `inode`.
* +1 — Local unprivileged trigger: supplied evidence identifies a local low-privilege process driving fs-verity lookup and inode cleanup activity.
* +1 — Plausible kernel crash/DoS from dereferencing freed slab storage.
* -1 — Hard race: exploitation requires lookup to overlap the short interval after hash removal and before/while immediate freeing.
* Floor 5 — The stale object remains part of an active rhashtable traversal; stale `rhash_head` affects pointer traversal and stale `inode` affects key comparison.

Not awarded: controlled reclaim, attacker-shaped replacement contents, OOB write, arbitrary write, disclosure, authorization bypass, C/I impact, or LPE. The same crash was not counted again as Availability. NEW52 B5-R and B3/B4 pending profiles do not apply.

3. Reachability analysis

The supported actor is a local non-root process operating on accessible fs-verity inodes and inducing concurrent lookup and cleanup/eviction activity. No host-administrator capability, namespace-scoped capability, prior privileged setup, special hardware, or remote path is shown. The race is timing-sensitive, although repeated filesystem activity may permit retries.

4. Primitive and severity reading

This is a CLASS_B active-structured-object read UAF, not fixed stale storage and not a cleanup-only stale dereference. The later reader consumes `rhash_head` for hash-chain traversal and `inode` for comparison. A crash is directly plausible; reuse-driven pointer traversal or broader corruption is unresolved, with no controlled reclaim or write primitive proven.

ImportantCandidate is YES because a below-host-admin actor can race an active RCU lookup against premature slab freeing, and the stale pointer-bearing state is consumed by the kernel. Auto-downgrade is disallowed pending analysis of slab reuse and traversal behavior. Likely operational severity is actionable Moderate, with Important retention for review.

5. One-sentence report phrase

A local low-privilege actor can race fs-verity info removal against an RCU hash lookup, causing active traversal and inode comparison through freed slab storage.

6. Manual review advice

MANUAL CHECK REQUIRED

Determine whether repeatable inode churn permits useful slab replacement or whether all practical executions terminate as crash-only stale reads.

7. Call-site confidence

Call-site confidence: medium

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

Patch: fsverity: RCU-delay the freeing of struct fsverity_info
Commit: fa84f091a8e7fc46fd29834496fab9739dd4c8dc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fa84f091a8e7fc46fd29834496fab9739dd4c8dc

Commit description:

__fsverity_get_info() calls rhashtable_lookup_fast(), which just uses
rcu_read_lock() and doesn't directly synchronize with
fsverity_remove_info().

For the same inode this isn't a problem: its fsverity_info is removed
only at inode eviction time or upon failure to enable verity, when the
inode no longer needs its fsverity_info and it will no longer be
accessed via that inode.

However, this is broken and can cause a use-after-free for concurrent
__fsverity_get_info() for *different* inodes.  Those rely on following
fsverity_info::rhash_head in the rhashtable under rcu_read_lock() only.
They also use fsverity_info::inode to do the key comparison.

Fix this by RCU-delaying the freeing of 'struct fsverity_info' after
it's been removed from the rhashtable.

Fixes: f77f281 ("fsverity: use a hashtable to find the fsverity_info")
Cc: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Link: https://patch.msgid.link/20261001171349.81454-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/verity/fsverity_private.h
  fs/verity/open.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=fa84f091a8e7fc46fd29834496fab9739dd4c8dc

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

The original Linux kernel CVE announcement for CVE-2026-99051 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99051

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