[CVE-2026-80808][MODERATE REGULAR] ext4: stop retrying saturated xattr cache entries

[CVE-2026-80808][MODERATE REGULAR] ext4: stop retrying saturated xattr cache entries

[AL-KERNEL]

CVE: CVE-2026-80808
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: stop retrying saturated xattr cache entries
Commit: 119a2f053242ed75bdd2ebc95baf3ae7db6ccacf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=119a2f053242ed75bdd2ebc95baf3ae7db6ccacf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80808
Analysis date: Fri, 04 Sep 2026 12:25:13 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A saturated or corrupted ext4 xattr refcount can leave an unusable mbcache entry reusable, causing `ext4_xattr_block_set()` to retry indefinitely while holding directory locking and resulting in local filesystem DoS.

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

The original Linux kernel CVE announcement for CVE-2026-80808 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80808

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 REGULAR
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-2026-80808	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80808 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-835;**CWE-400;CWE-190;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An ext4 xattr cache entry with a saturated or corrupted on disk reference count can remain marked reusable and be selected again by ext4_xattr_block_set. The retry path can loop indefinitely while holding the parent directory i_rwsem, which leaves concurrent rmdir callers blocked and can produce hung tasks. For the CVSS the PR:L is used in the paranoid score because an untrusted local user may trigger operations on an already mounted crafted or corrupted ext4 filesystem, while creating or mounting that filesystem normally requires administrator control. The issue is not network reachable. Impact is denial of service through a local filesystem hang and blocked VFS operations. No evidence from the code path indicates kernel memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	YES	DANGER DISK RACE LINUS SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel hang / strong DoS: +1. The commit reports repeated hung-task behavior, with 6 of 8 unpatched trials hanging for 420 seconds and NMI traces showing spinning in `ext4_xattr_block_set()`.
* Important filesystem/storage path: +1. The affected code is ext4 xattr metadata/cache handling, and the hang can block VFS operations such as `rmdir`.
* Requires admin/root/CAP_* in typical deployments: -2. Creating or mounting the corrupted ext4 filesystem that violates the xattr refcount invariant normally requires administrator control.
* Availability impact realistic: +1. The hang is reproducible and can block directory removal and related filesystem operations.
* Rare/difficult precondition: -1. The issue depends on a corrupted filesystem/xattr state rather than a normal clean ext4 filesystem.

Conservative total: 2.

Paranoid additional interpretation:

* Local unprivileged trigger: +1. If the corrupted filesystem is already mounted and writable by an untrusted local user, ordinary filesystem operations may reach the affected xattr path.
* Broad/default/common subsystem: +1. ext4 is a broadly deployed filesystem, even though this specific trigger depends on corrupted metadata.

Paranoid total: 4.

Not awarded:

* No generic memory corruption bonus. The patch prevents refcount overflow and an endless retry loop, but does not show UAF, OOB write, double free, type confusion, arbitrary write, or attacker-controlled reclaim.
* No privilege escalation plausible bonus. The demonstrated consequence is a filesystem hang, not LPE.
* No confidentiality or integrity impact. The provided code path supports DoS through retry-loop behavior, not disclosure or unauthorized data modification.

## 3. Reachability analysis

The realistic trigger requires a corrupted ext4 filesystem where an xattr cache entry remains marked reusable despite having a saturated on-disk reference count. In normal deployments, preparing and mounting such a filesystem is usually an administrative action, so the conservative privilege assumption is high privilege. A lower-privilege scenario is possible if an already mounted corrupted filesystem is exposed to untrusted local users and their filesystem operations can trigger xattr block reuse. Namespaces or containers may matter only if the container has access to such a mounted ext4 filesystem and can perform the relevant operations, but this does not appear to be a network-reachable issue.

## 4. Severity interpretation

This behaves like a Moderate local DoS issue rather than an Important-class vulnerability. The DoS evidence is strong because the hang was reproduced repeatedly and the blocked path can hold `i_rwsem`. However, the exploitability is limited by the corrupted-filesystem precondition, and the patch does not establish a memory corruption primitive that would justify LPE, confidentiality impact, or integrity impact.

## 5. One-sentence report phrase

A saturated or corrupted ext4 xattr refcount can leave an unusable mbcache entry reusable, causing `ext4_xattr_block_set()` to retry indefinitely while holding directory locking and resulting in local filesystem DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

Reason: the bug is not an Important candidate based on the current evidence, but the reproducible ext4 hang and PR:L paranoid scenario make it borderline enough that it should not be silently treated as a trivial cleanup without at least a lightweight filesystem-impact review.

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

Patch: ext4: stop retrying saturated xattr cache entries
Commit: 119a2f053242ed75bdd2ebc95baf3ae7db6ccacf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=119a2f053242ed75bdd2ebc95baf3ae7db6ccacf

Commit description:

ext4_xattr_block_set() retries when a cache entry selected for reuse
has a saturated reference count after taking the buffer lock. The retry
returns to the mbcache lookup without making that entry ineligible, so
it can select the same unusable entry indefinitely. A task spinning
there can hold the parent directory's i_rwsem and leave concurrent
rmdir callers blocked.

Normally a reusable entry has a reference count below
EXT4_XATTR_REFCOUNT_MAX because the count and MBE_REUSABLE_B are
updated under the same buffer lock. A corrupted filesystem can violate
that invariant. The syzbot reproducer reports allocator and xattr
corruption before triggering this retry loop.

Check the untrusted on-disk count before incrementing it, avoiding
overflow, and clear MBE_REUSABLE_B when it is already saturated. The
next lookup then skips the entry that was just proven unusable. This
mirrors the normal transition at EXT4_XATTR_REFCOUNT_MAX; the release
path marks the entry reusable again on the exact 1024-to-1023
transition.

Using the same QEMU harness and guest parameters, current unpatched
Linux hung in 6 of 8 420-second trials with the do_rmdir signature;
representative NMI backtraces caught the owner spinning in
ext4_xattr_block_set(). The patched kernel completed 28 of 28 trials
without a hung-task report; the final twelve trials exercised the
reviewed overflow-safe form of the change. syzbot's patch testing also
completed without reproducing the hang.

Reported-and-tested-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e68dbebd9617a9250e8d
Fixes: 65f8b80 ("ext4: fix race when reusing xattr blocks")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Tested-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20260802065941.1726052-1-matthias.goergens@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/ext4/xattr.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=119a2f053242ed75bdd2ebc95baf3ae7db6ccacf

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

The original Linux kernel CVE announcement for CVE-2026-80808 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80808

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 4

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 REGULAR

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).