[CVE-2026-53388][MODERATE 7.0] fuse: re-lock request before replacing page cache folio [ Upstream

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

CVE: CVE-2026-53388
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fuse: re-lock request before replacing page cache folio [ Upstream
Commit: 7c18691e0cfda29672f79bafde8abdb7710674f6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7c18691e0cfda29672f79bafde8abdb7710674f6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53388
Analysis date: Sun, 19 Jul 2026 10:33:31 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in FUSE folio movement can allow `fuse_chan_abort()` to free `fuse_io_args` while later copy-chain logic still accesses it, causing a local kernel use-after-free with reliable DoS impact and possible but unproven privilege-escalation relevance.

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

The original Linux kernel CVE announcement for CVE-2026-53388 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53388

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
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CVE-2026-53388	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-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in FUSE request handling can lead to use after free because fuse_try_move_folio() can continue into later copy logic after fuse_chan_abort() has ended the request and freed fuse_io_args. The bug is triggered locally through FUSE page and splice style paths and requires hitting a timing window between request abort and the success path after page cache folio replacement. For the CVSS the PR:L is used because a local user or local FUSE process is required to drive the vulnerable interface, while full administrator privileges are not necessarily required on systems that allow user FUSE mounts. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because this is a kernel use after free rather than a pure warning or resource leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	LOCK DANGER DISK INIT UAF LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* **Local unprivileged trigger: +1**
  FUSE is commonly reachable by local users on systems allowing user FUSE mounts. Full host-root is not necessarily required.

* **Memory corruption, weak race-UAF primitive: +1**
  The commit explicitly describes `fuse_chan_abort()` freeing `fuse_io_args` while later copy-chain logic may still access it. However, the patch does not show controlled reclaim, type confusion, callback control, or write-after-free, so this is scored as weak race-UAF rather than strong corruption.

* **Real lifetime corruption: +1**
  This is a request lifetime bug where a concurrent abort can end the request and free associated I/O arguments while another path still uses them.

* **Reliable kernel crash / strong DoS: +1**
  A kernel UAF in filesystem/page-cache copy logic is a credible crash condition.

* **Important filesystem/page-cache path: +1, paranoid only**
  The bug sits in FUSE page-cache folio replacement and splice-style page movement. This raises triage priority, but the conservative score is capped by the race-UAF downgrade rule.

Subtracted signals:

* **Hard or unreliable race / special timing required: -1**
  Exploitation requires racing the success path of `fuse_try_move_folio()` against `fuse_chan_abort()`.

Applied cap:

* **Race-UAF downgrade cap**
  Because the patch prevents a stale pointer/lifetime race but does not show attacker-controlled reclaim, write-after-free, type confusion, callback dispatch, or refcount takeover, the conservative score is capped at **4** and paranoid score at **5**.

## 3. Reachability analysis

The bug is locally reachable through FUSE request handling, especially paths involving splice/page movement and page-cache folio replacement. On systems where unprivileged users can mount or interact with FUSE filesystems, PR should be treated as local low privilege rather than administrator-only. Namespaces and container setups may make this more relevant if FUSE access is delegated to non-root or container users.

The path is not network-reachable by itself. Remote triggering would require an additional application-specific layer exposing FUSE operations, which is not implied by the patch. Realistic exploitation requires local code execution and a timing window between request abort and later copy-chain use.

Call-site confidence: **medium-high**. The commit message directly names the conflicting functions and the freed object, and the patch changes locking around `replace_page_cache_folio()` in the affected path.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a kernel UAF in a filesystem/page-cache path reachable from local FUSE activity. It should not be auto-closed as a simple DoS.

At the same time, the patch does not demonstrate a strong LPE primitive such as controlled reclaim, object replacement, callback hijack, arbitrary write, or refcount takeover. Realistic demonstrated impact is local kernel crash / DoS. The paranoid interpretation keeps it actionable because local kernel UAFs in FUSE/page-cache paths historically deserve manual review for possible privilege-escalation potential.

## 5. One-sentence report phrase

A race in FUSE folio movement can allow `fuse_chan_abort()` to free `fuse_io_args` while later copy-chain logic still accesses it, causing a local kernel use-after-free with reliable DoS impact and possible but unproven privilege-escalation relevance.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is an explicit race-based kernel UAF in a local filesystem/page-cache path. Even though the current patch does not prove controlled exploitation beyond DoS, the lifetime primitive is strong enough to require manual security review.

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

Patch: fuse: re-lock request before replacing page cache folio [ Upstream
Commit: 7c18691e0cfda29672f79bafde8abdb7710674f6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7c18691e0cfda29672f79bafde8abdb7710674f6

Changed files:
  fs/fuse/dev.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=7c18691e0cfda29672f79bafde8abdb7710674f6

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

The original Linux kernel CVE announcement for CVE-2026-53388 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53388

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:L/I:L/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).