[CVE-2026-74606][MODERATE 7.0] eventfs: Fix use-after-free in eventfs_remove_rec()

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

CVE: CVE-2026-74606
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: eventfs: Fix use-after-free in eventfs_remove_rec()
Commit: b77581b25e213e83b79ce11eb30024e55ceeb3e9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b77581b25e213e83b79ce11eb30024e55ceeb3e9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74606
Analysis date: Sat, 22 Aug 2026 12:08:38 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A UAF in `eventfs_remove_rec()` can occur when recursive eventfs teardown removes the current child and the non-safe list iterator later reads `list.next` from the freed object, causing at least a local kernel crash and requiring manual review due to kernel lifetime corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74606 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74606

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74606	MODERATE	CHECK WITH IMPACT FROM ORIG NN 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-672;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in eventfs_remove_rec() because the recursive removal path deletes the current child while list_for_each_entry later advances by reading list.next from that removed child. If free_ei() drops the final reference, release_ei() can reuse the list/rcu storage for an SRCU callback and the child may be freed before the iterator reads the next pointer. For the CVSS the PR:L is used for the paranoid score because a local user or less privileged tracing service may be able to trigger eventfs teardown when tracefs access is delegated, while default systems often restrict this path to administrators. The issue is not network reachable and requires local interaction with tracefs or tracing event lifetime management. Impact is at least local denial of service via kernel crash. In the worst defensible case, the kernel UAF may allow confidentiality or integrity impact and possible privilege escalation, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-74606 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB DISK TRACE UAF LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

**Conservative signals**

* **Memory corruption, weak primitive: +1**
  This is a real UAF condition, but the demonstrated access is iterator advancement reading `list.next` from a removed child. No controlled reclaim, overwrite, callback control, or type confusion is shown.

* **Real lifetime corruption: +1**
  `free_ei()` may drop the final reference, and `release_ei()` may reuse the `list/rcu` union for an SRCU callback while the caller still depends on the old list node.

* **Reliable kernel crash / strong DoS: +1**
  Reading list pointers from a freed `eventfs_inode` can plausibly cause an Oops or kernel crash.

* **Availability impact realistic: +1**
  If triggered, the impact is host kernel instability or crash, not just a local task failure.

* **Hard or timing-dependent lifetime window: -1**
  The issue depends on recursive removal, final reference dropping, and SRCU/free timing.

* **Requires admin/root/tracing control in typical deployments: -2**
  Tracefs/eventfs teardown paths are normally controlled by root or tracing administrators.

**Paranoid additional interpretation**

* **Local unprivileged or reduced-privilege trigger possible: +1**
  In environments where tracefs access is delegated to a tracing service, container root, or reduced-privilege monitoring component, the practical PR may be closer to PR:L than PR:H.

* **Broad/common kernel facility exposure: +1**
  Tracefs/ftrace/eventfs are common kernel facilities on many systems, even if access is usually restricted.

**Not awarded**

* **Privilege escalation plausible: +0**
  The patch shows UAF, but not attacker-controlled reclaim, writable stale access, object replacement, type confusion, refcount takeover, or callback dispatch.

* **Strong memory corruption primitive: +0**
  This is downgraded under the race-UAF rule because the demonstrated primitive is stale iterator access, not a controlled write or reclaim-based UAF.

* **Remote reachable: +0**
  No network-triggerable path is shown.

## 3. Reachability analysis

The affected path is local and tied to tracefs/eventfs removal. In normal deployments, manipulating tracing infrastructure usually requires root, CAP_SYS_ADMIN, or equivalent tracing administration rights, so the conservative interpretation treats this as privileged local reachability.

Namespaces and containers may matter if tracefs or tracing controls are delegated into a container, service account, or reduced-capability monitoring environment. In that case, the attacker may not be equivalent to full host root, and the practical privilege requirement can be closer to PR:L. The bug is not network reachable and does not appear to be triggerable by packet traffic or remote input alone.

Call-site confidence: **high**, because the recursive removal loop and the unsafe iterator pattern are directly visible in the patch context.

## 4. Severity interpretation

This behaves as a **kernel UAF with realistic DoS impact** and **manual-review-worthy memory lifetime corruption**, but not as a demonstrated Important-class LPE.

The theoretical concern is that a freed `eventfs_inode` can be accessed after its lifetime ends, and kernel UAFs can sometimes become stronger primitives. The realistic evidence in this patch supports stale list-pointer access and crash risk, not controlled memory reuse or privilege escalation. Therefore the conservative score is low because of privileged tracefs reachability, while the paranoid score reaches Actionable Moderate because it is a real lifetime bug in kernel object teardown and reduced-privilege tracing deployments are plausible.

## 5. One-sentence report phrase

A UAF in `eventfs_remove_rec()` can occur when recursive eventfs teardown removes the current child and the non-safe list iterator later reads `list.next` from the freed object, causing at least a local kernel crash and requiring manual review due to kernel lifetime corruption.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a real kernel UAF in tracefs/eventfs teardown. Even though the demonstrated impact is mainly DoS and the usual trigger is privileged, lifetime bugs involving freed kernel list objects should not be auto-closed without manual validation of reachability and reclaim potential.

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

Patch: eventfs: Fix use-after-free in eventfs_remove_rec()
Commit: b77581b25e213e83b79ce11eb30024e55ceeb3e9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b77581b25e213e83b79ce11eb30024e55ceeb3e9

Commit description:

eventfs_remove_rec() recursively removes the child at the current loop
position. After the recursive call returns, list_for_each_entry() advances
by reading list.next from the removed child.

If free_ei() drops the final reference, release_ei() reuses the list/rcu
union to queue an SRCU callback. The child may be freed before that read.
The eventfs_mutex serializes list updates, but it does not keep the removed
child alive or prevent the SRCU callback from running.

Use list_for_each_entry_safe() to save the next sibling before recursively
removing the current child.

Cc: stable@vger.kernel.org
Fixes: 43aa6f9 ("eventfs: Get rid of dentry pointers without refcounts")
Link: https://patch.msgid.link/20260806022719.375354-1-shuangpeng.kernel@gmail.com
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@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=b77581b25e213e83b79ce11eb30024e55ceeb3e9

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

The original Linux kernel CVE announcement for CVE-2026-74606 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74606

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: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: 2
  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: MODERATE
KPANIC detected for this report: NO
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).