[CVE-2026-80855][LOW] fuse: fix invalidate lock leak on open O_TRUNC DAX failure

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

CVE: CVE-2026-80855
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fuse: fix invalidate lock leak on open O_TRUNC DAX failure
Commit: 1b04d80a27d317064cce2307472f5bef9975bc50
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1b04d80a27d317064cce2307472f5bef9975bc50
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80855
Analysis date: Fri, 04 Sep 2026 14:42:39 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80855 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80855

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: LOW
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-2026-80855	LOW	https://www.kernelcve.org/list/?q=CVE-2026-80855 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:N/I:N/A:H';CWE-667;*CWE-772;CWE-404;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'FUSE DAX open with O_TRUNC can leak filemap_invalidate_lock() when fuse_dax_break_layouts() fails after an interruptible wait for busy DAX pages returns -ERESTARTSYS. The leaked rwsem remains held on the file mapping, so later page faults or truncate operations on the same file can stall indefinitely and cause a local denial of service. For the CVSS the PR:L is used because a local user or process with access to the affected FUSE DAX file can trigger the open path and interrupt its own operation without administrative privileges. The issue is not network reachable and does not show a memory corruption primitive. Impact is availability loss only, mainly stalled file operations or hung tasks rather than privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED (with actual score 2)	YES	DEADLOCK DISK LEAK ERRORPATH 	NO	NO	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local process with access to an affected FUSE DAX file can enter the O_TRUNC open path and interrupt its own wait.
* Reliable local DoS / hung task condition: +1. The leaked filemap invalidate rwsem can indefinitely stall later faults or truncate operations on the same file.
* Availability impact beyond the immediate failing syscall: +1. The stale lock persists after the interrupted open path and affects subsequent operations.
* Important filesystem/storage path: +1. The bug is in FUSE DAX truncate and page invalidation handling.
* Hard or special timing required: -1. Triggering requires fuse_dax_break_layouts() to fail through an interruptible wait, typically involving busy DAX pages and a signal.
* Rare/non-default configuration: -1. FUSE DAX is not a default generic filesystem path and requires a specific deployment setup.
* No memory corruption, UAF, OOB write, arbitrary write, or page-cache ownership bypass signal is supported.

3. Reachability analysis
A local user or process can plausibly trigger this if it has access to a FUSE DAX file and can open it with O_TRUNC. The vulnerable path is not network reachable. Containers or namespaces matter only if the affected FUSE DAX mount is exposed inside them. The path is configuration dependent because FUSE DAX must be in use, and reliable triggering needs an interruptible DAX layout break failure.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like local DoS, not an Important-class issue. The realistic impact is stalled file operations or hung tasks due to a leaked rwsem. There is no evidence of memory corruption, privilege escalation, information disclosure, or cross-file/page-cache ownership corruption.

5. One-sentence report phrase
A local denial of service in FUSE DAX O_TRUNC handling can leak filemap_invalidate_lock() when an interruptible DAX layout break fails, causing later faults or truncates on the same file to stall indefinitely.

6. Manual review recommendation
NO MANUAL CHECK NEEDED. The issue is a configuration-dependent lock leak with availability-only impact and no supported memory corruption or privilege escalation primitive.

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

Patch: fuse: fix invalidate lock leak on open O_TRUNC DAX failure
Commit: 1b04d80a27d317064cce2307472f5bef9975bc50
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1b04d80a27d317064cce2307472f5bef9975bc50

Commit description:

fuse_open() takes filemap_invalidate_lock() for a DAX truncate
(dax_truncate = true) and releases it before the out_inode_unlock
label.  But when fuse_dax_break_layouts() fails, the goto
out_inode_unlock skips the unlock and leaks the rwsem, so any later
fault or truncate on the file stalls on the stale lock.

fuse_dax_break_layouts() can fail with -ERESTARTSYS when a signal
interrupts the wait for busy DAX pages to drain:

  open("file", O_RDWR | O_TRUNC)
  └─ fuse_open()
     ├─ filemap_invalidate_lock()        # dax_truncate
     └─ fuse_dax_break_layouts()
        └─ dax_break_layout()
           └─ wait_page_idle()           # TASK_INTERRUPTIBLE
              └─ fuse_wait_dax_page()    # unlock, schedule, re-lock
                 └─ signal → -ERESTARTSYS
     goto out_inode_unlock               # <- lock leaked

Fix this by moving filemap_invalidate_unlock() below the label so
that all error paths release the lock, and rename the label to
out_unlock as it now covers more than just the inode lock.

Fixes: 2fdbb8d ("fuse: fix deadlock between atomic O_TRUNC and page invalidation")
Cc: stable@vger.kernel.org # v6.0+
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=1b04d80a27d317064cce2307472f5bef9975bc50

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

The original Linux kernel CVE announcement for CVE-2026-80855 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80855

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).