[CVE-2026-46242][IMPORTANT] eventpoll: fix ep_remove struct eventpoll / struct file UAF [ Upstream

[CVE-2026-46242][IMPORTANT] eventpoll: fix ep_remove struct eventpoll / struct file UAF [ Upstream

[AL-KERNEL]

CVE: CVE-2026-46242
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: eventpoll: fix ep_remove struct eventpoll / struct file UAF [ Upstream
Commit: ef4ca02e95363e78977ca04340d44fe3b4b2b81f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ef4ca02e95363e78977ca04340d44fe3b4b2b81f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46242
Analysis date: Sat, 30 May 2026 08:51:15 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged epoll race in ep_remove() can cause struct file and struct eventpoll use-after-free with writes into freed kmalloc memory, making kernel crash likely and local privilege escalation plausibly possible.

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

The original Linux kernel CVE announcement for CVE-2026-46242 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46242

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.

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

CVE-2026-46242	IMPORTANT	CHECK	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-667;*CWE-787;BEST CVSS score: '7';DESCR 'A race in eventpoll ep_remove() can let a concurrent file release observe a transient cleared file f_ep state and skip the normal eventpoll_release_file cleanup path. In the epoll watches epoll case this can free the watched struct eventpoll while ep_remove() still uses list pointers that refer into that freed object, so hlist_del_rcu() can write into freed kmalloc memory. This is a use-after-free memory corruption issue rather than a pure crash or resource leak. For the CVSS the PR:L is selected because a local unprivileged process can use epoll interfaces and local code execution is required, but full administrative privileges are not required. AC:H is selected because reliable triggering depends on a race window between ep_remove() and __fput(). The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality, integrity, and availability compromise due to kernel heap memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	LOCK DISK INIT UAF LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	guess

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

ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  epoll is reachable by ordinary local processes without administrative privileges.

* Privilege escalation plausible: +2
  The commit describes UAF with write into freed kmalloc memory and an attacker-controllable kmem_cache_free against the wrong slab cache. This is a plausible LPE class, not just a crash.

* Generic memory corruption, strong corruption primitive: +2
  The issue includes real UAF and freed slab memory scribble via hlist_del_rcu().

* Real lifetime corruption: +1
  The bug is a struct file / struct eventpoll lifetime race involving __fput(), eventpoll_release(), ep_free(), and stale use in ep_remove().

* Reliable kernel crash / strong DoS: +1
  UAF and wrong-cache free are sufficient for likely kernel crash even if LPE is not achieved.

* Broad/default/common subsystem: +1
  eventpoll/epoll is a common core Linux local API.

* Confidentiality impact plausible: +1
  Kernel heap UAF corruption can plausibly become read or object-confusion based disclosure in worst-case exploitation.

* Integrity impact plausible: +1
  Freed slab overwrite and wrong-cache free can plausibly corrupt kernel objects.

* Hard or unreliable race / special timing required: -1
  Triggering depends on a race between ep_remove() and __fput(), so reliable exploitation likely requires timing control.

Conservative total: 8.
Paranoid total: 9 if counting the described wrong-slab-cache free as especially strong LPE evidence.

## 3. Reachability analysis

A local unprivileged user can use epoll and create epoll-watches-epoll object graphs, so PR is effectively low rather than admin-only. The path is not network reachable. Containers and namespaces do not remove concern because epoll is normally available inside containers, and a kernel UAF reached from a container process may still affect the host kernel depending on isolation and kernel sharing. The main constraint is the race window, but the affected objects and lifetimes are under substantial local process control.

## 4. Severity interpretation

This behaves like an Important-class kernel vulnerability candidate rather than ordinary Moderate. The patch text supports real memory corruption: UAF, freed kmalloc memory write, and possible wrong-slab-cache free. Realistic exploitation is not proven from the patch alone, and the race raises complexity, but the primitive is strong enough that auto-closing as a low or ordinary Moderate issue would be unsafe.

## 5. One-sentence report phrase

A local unprivileged epoll race in ep_remove() can cause struct file and struct eventpoll use-after-free with writes into freed kmalloc memory, making kernel crash likely and local privilege escalation plausibly possible.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a local unprivileged UAF with freed slab overwrite and wrong-cache-free implications in a common kernel subsystem.

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

Patch: eventpoll: fix ep_remove struct eventpoll / struct file UAF [ Upstream
Commit: ef4ca02e95363e78977ca04340d44fe3b4b2b81f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ef4ca02e95363e78977ca04340d44fe3b4b2b81f

Changed files:
  fs/eventpoll.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=ef4ca02e95363e78977ca04340d44fe3b4b2b81f

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

The original Linux kernel CVE announcement for CVE-2026-46242 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46242

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: Not available
  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: 8
  Paranoid ActionableScore: 9

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: YES
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).