[CVE-2026-53381][MODERATE 7.0] virtiofs: fix UAF on submount umount [ Upstream

[CVE-2026-53381][MODERATE 7.0] virtiofs: fix UAF on submount umount [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53381
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: virtiofs: fix UAF on submount umount [ Upstream
Commit: 607a1d4c42f649e6197567c0448fd9ebb316cd42
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=607a1d4c42f649e6197567c0448fd9ebb316cd42
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53381
Analysis date: Sun, 19 Jul 2026 09:33:24 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in virtiofs auto_submount release handling can let fuse_release_end() call iput() after the related superblock has been destroyed, causing an explicit UAF and likely local guest kernel crash, with privilege escalation remaining theoretical but worth manual review.

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

The original Linux kernel CVE announcement for CVE-2026-53381 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53381

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-2026-53381	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-664;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 virtiofs submount teardown can occur when fuse_release_end calls iput after the related superblock has already been destroyed. The unsafe window exists because outstanding release requests are tracked per FUSE connection rather than per superblock, so waiting for num_waiting does not protect earlier submount instances during shutdown. A local process in a guest using virtiofs auto_submounts may be able to trigger this by keeping file releases pending while a submount is unmounted or torn down, causing a kernel Oops or crash. For the CVSS the PR:L is used because reliable triggering requires local guest execution and access to the virtiofs mount behavior, but not necessarily full host privileges. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact because this is an explicit UAF in kernel object lifetime handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-53381 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 26e5c67deb2e1f42a951f022fdf5b9f7eb747b01 YES  NO NO unknown 	MAYBE	DISK INIT UAF KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process in a guest using virtiofs auto_submounts may influence open, close, release, and submount teardown behavior.
* Memory corruption, weak primitive: +1. The commit explicitly describes UAF, but the patch does not show attacker-controlled reclaim, object replacement, write-after-free, callback control, or type confusion.
* Real lifetime corruption: +1. This is an async release versus superblock teardown lifetime bug involving fuse_release_end(), iput(), and destroyed superblock state.
* Reliable kernel crash / strong DoS: +1. The commit states iput() can Oops after the superblock has already been destroyed.
* Important filesystem/storage path: +1. The affected path is FUSE/virtiofs file release and submount teardown, which is filesystem object lifetime handling.
* Hard or unreliable race / special teardown timing: -1. Triggering depends on pending release requests overlapping submount shutdown.

Paranoid adjustment:

* Paranoid score treats the explicit UAF in inode/superblock lifetime handling as a higher manual-review memory-corruption concern, but still capped at 5 because the patch does not demonstrate reclaim, overwrite, callback dispatch, or a practical LPE primitive.

Call-site confidence: medium. The changed call is visible, and the commit explains the fuse_release_end() to iput() consequence, but deeper reclaim behavior and exact teardown call paths are not shown in the provided patch.

## 3. Reachability analysis

The likely attacker is a local process inside a guest or workload environment where virtiofs with auto_submounts is enabled. The issue is not directly network reachable. It requires the virtiofs auto_submounts case and a submount unmount or teardown while asynchronous release work is still pending.

Namespaces and containers can matter in practice. In Kata Containers or VM-backed container setups, a container workload may be able to exercise guest filesystem behavior without having full host privileges. However, the patch does not show a host escape or host-kernel corruption path. The realistic impact is guest kernel Oops or crash. Full privilege escalation remains theoretical without evidence of controlled reclaim or reuse.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is an explicit kernel UAF in filesystem lifetime handling, not just a warning or resource leak. Realistically, the demonstrated effect is local DoS through guest kernel Oops during virtiofs submount teardown. Theoretical confidentiality or integrity impact is possible in the broad sense that UAF bugs can sometimes become exploitable, but this patch does not show a concrete primitive for LPE.

Overall this should be treated as Actionable Moderate at minimum, with manual review required. It is not a strong Important candidate from the patch alone because there is no demonstrated attacker-controlled reclaim, write-after-free, function pointer influence, refcount takeover, or host escape.

## 5. One-sentence report phrase

A race in virtiofs auto_submount release handling can let fuse_release_end() call iput() after the related superblock has been destroyed, causing an explicit UAF and likely local guest kernel crash, with privilege escalation remaining theoretical but worth manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes a UAF in kernel filesystem object lifetime handling. Even though the demonstrated impact is an Oops/DoS and the race appears timing-dependent, UAF issues in inode/superblock teardown paths should not be auto-closed without checking whether controlled object reuse or a guest-to-host relevant scenario exists.

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

Patch: virtiofs: fix UAF on submount umount [ Upstream
Commit: 607a1d4c42f649e6197567c0448fd9ebb316cd42
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=607a1d4c42f649e6197567c0448fd9ebb316cd42

Changed files:
  fs/fuse/file.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=607a1d4c42f649e6197567c0448fd9ebb316cd42

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

The original Linux kernel CVE announcement for CVE-2026-53381 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53381

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: 4
  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: 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).