[CVE-2026-74363][MODERATE 7.0] bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs [ Upstream

[CVE-2026-74363][MODERATE 7.0] bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74363
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs [ Upstream
Commit: ea1c243c39e32b7fc1c2edfe32081ff7e30a877c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea1c243c39e32b7fc1c2edfe32081ff7e30a877c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74363
Analysis date: Sun, 16 Aug 2026 04:58:42 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A bpffs inode lifetime bug can free inode and i_link before an RCU grace period while VFS RCU pathwalk may still read them, allowing a local race to trigger a use-after-free and kernel crash with limited but review-worthy memory corruption concern.

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

The original Linux kernel CVE announcement for CVE-2026-74363 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74363

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-2026-74363	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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 use-after-free in bpffs inode lifetime handling can occur because destroy_inode() freed the inode and cached symlink body immediately while RCU pathwalk may still access them. A local attacker who can race unlinkat() against path lookup on bpffs paths may cause RCU pathwalk to read freed inode state such as inode i_opflags, producing a kernel crash and potentially exposing a memory corruption primitive if the freed inode is reused. For the CVSS the PR:L is used for the paranoid score because triggering requires local execution and access to relevant bpffs operations or paths, but does not necessarily require full host administrator privileges in delegated or containerized environments. The issue is not network reachable and requires a timing race, therefore AC:H is appropriate. Impact is at least local denial of service via kernel crash, and in the paranoid case may include confidentiality and integrity impact due to use-after-free on kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-74363 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 4f375ade6aa9f37fd72d7a78682f639772089eed YES  NO NO unknown 	MAYBE	READ LOCK INIT BPF UAF IMPROVEONLY LINUS SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  Local execution is required. Triggering depends on access to bpffs paths and concurrent `unlinkat()` plus path lookup behavior. This is not remote.
* Memory corruption, weak primitive: +1
  This is a real UAF, but the demonstrated access is an RCU pathwalk read of freed inode state such as `inode->i_opflags`. No write-after-free, callback control, type confusion, or controlled reclaim is shown.
* Real lifetime corruption: +1
  The patch restores the required RCU-delayed freeing of inode and `i_link`, so this is clearly an inode lifetime / RCU lifetime bug.
* Reliable kernel crash / strong DoS: +1
  KASAN reports slab-use-after-free in `is_mgtime()`, and this can plausibly crash the kernel.
* Hard or unreliable race / special timing required: -1
  The trigger requires a race between inode final reference drop and RCU pathwalk.

Paranoid additional signals:

* Broad/common kernel path exposure: +1
  Although bpffs access may be restricted, the unsafe access occurs through common VFS RCU pathwalk semantics.
* Availability impact realistic: +1
  In the paranoid interpretation, the UAF can produce a host-wide kernel crash, not merely a local operation failure.

Not awarded:

* Strong LPE plausibility: +0
  No attacker-controlled reclaim, stale object replacement, writable stale access, callback dispatch, refcount takeover, or arbitrary write is shown.
* Local unprivileged high-control BPF API: +0
  This is bpffs inode lifetime handling, not an unprivileged BPF program loading primitive. In many evaluated product configurations unprivileged BPF is disabled or gated.
* Remote reachable: +0
  No network trigger is indicated.

## 3. Reachability analysis

The bug is locally reachable through bpffs filesystem operations and VFS path lookup behavior. A plausible trigger requires one task to drop the last inode reference through `unlinkat()` while another task is still walking the same path in RCU mode.

In typical hardened deployments, creating or managing bpffs entries may require privileged BPF capabilities, root-controlled bpffs permissions, or administrative setup. However, delegated bpffs directories, containers, service accounts, or existing writable bpffs paths can make the practical privilege level lower than full host root. Namespaces may matter if bpffs is exposed or delegated into a container, but the patch alone does not prove a container escape.

The path is not network reachable. Exploitation requires a race and local filesystem activity.

Call-site confidence: high. The commit message and patch directly identify `destroy_inode()`, `free_inode()`, RCU pathwalk, `i_link`, and the KASAN UAF read site.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness bug because it is a real RCU lifetime UAF in VFS-visible inode handling. The realistic demonstrated impact is local DoS via kernel crash.

Theoretical memory corruption potential exists because a freed inode may be reused before an RCU pathwalk completes. However, the supplied evidence shows a stale read, not a controlled reclaim, write primitive, object replacement, callback hijack, or reliable privilege escalation path. Therefore the conservative score stays below Important, while the paranoid score reaches Actionable Moderate and should not be auto-closed without review.

## 5. One-sentence report phrase

A bpffs inode lifetime bug can free inode and i_link before an RCU grace period while VFS RCU pathwalk may still read them, allowing a local race to trigger a use-after-free and kernel crash with limited but review-worthy memory corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed UAF in inode lifetime handling with RCU pathwalk involvement. Even though the demonstrated primitive is a stale read and DoS, kernel lifetime bugs in VFS-facing objects deserve manual review for allocator reuse, bpffs permission exposure, and possible container or delegated-access reachability.

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

Patch: bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs [ Upstream
Commit: ea1c243c39e32b7fc1c2edfe32081ff7e30a877c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea1c243c39e32b7fc1c2edfe32081ff7e30a877c

Changed files:
  include/linux/fs.h
  kernel/bpf/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=ea1c243c39e32b7fc1c2edfe32081ff7e30a877c

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

The original Linux kernel CVE announcement for CVE-2026-74363 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74363

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: 3
  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).