[CVE-2026-99173][MODERATE 7.0] eventfs: Free the eventfs_inode after an rcu grace period too

[CVE-2026-99173][MODERATE 7.0] eventfs: Free the eventfs_inode after an rcu grace period too

[AL-KERNEL]

CVE: CVE-2026-99173
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: eventfs: Free the eventfs_inode after an rcu grace period too
Commit: 4586ef24ce390e8becb9f3e5852d5d3978b01c33
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4586ef24ce390e8becb9f3e5852d5d3978b01c33
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99173
Analysis date: Wed, 28 Oct 2026 22:58:10 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
An RCU pathwalker can read `eventfs_inode->is_freed` after SRCU-only reclamation, causing a one-byte slab UAF read and possible kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-99173 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99173

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-2026-99173	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99173 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/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: '5.8';DESCR 'A race in eventfs object lifetime management allows tracefs_d_revalidate to read is_freed from an eventfs_inode after the object was released. The reader is protected only by rcu_read_lock during RCU pathwalk, while the object was freed after an eventfs SRCU grace period that does not wait for ordinary RCU readers. The demonstrated primitive is a one byte slab use after free read that can produce a KASAN failure or kernel crash. Slab reuse may also affect path revalidation results, but the code provides no arbitrary write primitive or proven privilege escalation path. For the CVSS the PR:L value reflects that a local process and access to the relevant tracefs path are required, while reliable control of concurrent eventfs removal may require additional system activity or tracing management. AC:H reflects the need to win the pathwalk and dentry removal race. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. 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. Exact triggered rule markers were V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 2 with primitive class CLASS_B 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 MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	READ DISK TRACE LEAK UAF LINUS  KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 4. Applied the Real-UAF floor: 3 -> 4.
* Raw paranoid score: 3.
* Paranoid score: 4. The same floor applies; no additional exploit primitive is supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 — Real UAF: `tracefs_d_revalidate()` reads `eventfs_inode->is_freed` after `release_ei()` can free that object.
* +1 — Concrete RCU referenced-object lifetime mismatch: an RCU-visible dentry retains `d_fsdata` pointing to the inode, but SRCU reclamation does not wait for the ordinary RCU pathwalker. Chaining `call_rcu()` restores the missing lifetime guarantee.
* +1 — Plausible kernel crash/DoS, supported by the demonstrated KASAN slab-UAF failure.
* -1 — Hard concurrent timing: pathwalk must remain in `d_revalidate()` while the dentry is killed and SRCU reclamation completes.
* No extra Availability point: it would duplicate the same UAF crash.
* No structured-stale leverage: only a one-byte flag read is shown; attacker-controlled reclaim, replacement contents, pointers, callbacks, or a later write are not shown.
* No confidentiality, integrity, arbitrary-write, authorization-bypass, or B5-R retry-accumulated OOB signal is established.

3. Reachability analysis

A local process can perform the path lookup, but the evidence does not establish whether that same below-host-admin actor can cause the required eventfs dentry removal and reclamation. The complete trigger may depend on tracing-management activity by another actor. No namespace-scoped capability chain, prior delegation, special hardware, or remote path is shown. Therefore both effective host-admin and namespace privilege requirements remain unknown.

4. Primitive and severity reading

This is an active structured-object read UAF, not fixed stale storage: the dentry remains valid for RCU pathwalk while its referenced `eventfs_inode` is reclaimed under an incompatible SRCU lifetime. The shown stale operation is a one-byte read affecting dentry revalidation and capable of faulting. Slab reuse could change the revalidation result, but no controlled reuse or protected-resource consequence is demonstrated. It is CLASS_B, but constrained enough that Important candidacy is not presently supported and automatic Moderate handling remains permissible after review.

5. One-sentence report phrase

An RCU pathwalker can read `eventfs_inode->is_freed` after SRCU-only reclamation, causing a one-byte slab UAF read and possible kernel crash.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether an unprivileged actor can drive both lookup and eventfs removal and whether stale flag values can produce a material path-resolution consequence beyond crashing.

7. Call-site confidence

Call-site confidence: high

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

Patch: eventfs: Free the eventfs_inode after an rcu grace period too
Commit: 4586ef24ce390e8becb9f3e5852d5d3978b01c33
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4586ef24ce390e8becb9f3e5852d5d3978b01c33

Commit description:

tracefs_d_revalidate() reads ->is_freed of the eventfs_inode that
dentry->d_fsdata points to, and ->d_revalidate() is called in rcu
pathwalk mode with nothing but rcu_read_lock() held. The dentry itself
stays around for such a walker, but tracefs_d_release() drops its
reference to the eventfs_inode when the dentry is killed, and
release_ei() then frees it after a grace period of eventfs_srcu. An
SRCU grace period doesn't wait for rcu_read_lock() sections, so the
walker can read an eventfs_inode that is already gone.

With a pathwalk held up in tracefs_d_revalidate() while the dentry goes
away:

  BUG: KASAN: slab-use-after-free in tracefs_d_revalidate+0x76/0x90
  Read of size 1 at addr ff1100010152d8d0 by task walker/97
  Call Trace:
   <TASK>
   tracefs_d_revalidate+0x76/0x90
   lookup_fast+0x320/0x640
   path_lookupat+0x160/0xb00
   filename_lookup+0x1f1/0x5b0
   vfs_statx+0xd7/0x3b0
   vfs_fstatat+0x4d/0x90
   __do_sys_newfstatat+0xa0/0x100
   do_syscall_64+0x10e/0x5e0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  [...]
  Allocated by task 1:
   __kmalloc_cache_noprof+0x249/0x560
   eventfs_create_dir+0x7a/0x590
   event_create_dir+0x2fb/0xcf0
   trace_add_event_call+0xbf/0x150
  [...]
  Freed by task 11:
   kfree+0x1db/0x5b0
   srcu_invoke_callbacks+0x3a3/0x620
   process_one_work+0x937/0x1810
   worker_thread+0x4f1/0xd60

Before commit 8e55643 ("eventfs: Use SRCU for freeing
eventfs_inodes") release_ei() used call_rcu(), which covered the walker,
while eventfs_iterate() needs the SRCU grace period. Wait for both and
chain call_rcu() from the SRCU callback.

Cc: stable@vger.kernel.org
Fixes: 8e55643 ("eventfs: Use SRCU for freeing eventfs_inodes")
Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-4-7446d2d335e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/tracefs/event_inode.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=4586ef24ce390e8becb9f3e5852d5d3978b01c33

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

The original Linux kernel CVE announcement for CVE-2026-99173 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99173

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4

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).