[CVE-2026-63804][MODERATE 7.0] gfs2: fix use-after-free in gfs2_qd_dealloc

[CVE-2026-63804][MODERATE 7.0] gfs2: fix use-after-free in gfs2_qd_dealloc

[AL-KERNEL]

CVE: CVE-2026-63804
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: gfs2: fix use-after-free in gfs2_qd_dealloc
Commit: 4fe388218826df8607ae41a6305df67db08a9093
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fe388218826df8607ae41a6305df67db08a9093
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63804
Analysis date: Sun, 19 Jul 2026 08:58:46 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A GFS2 RCU lifetime bug can let pending quota callbacks access a freed superblock during unmount, causing a local kernel UAF with realistic DoS impact and weak but nonzero privilege escalation concern in delegated mount scenarios.

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

The original Linux kernel CVE announcement for CVE-2026-63804 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63804

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-2026-63804	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-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in the GFS2 unmount path because gfs2_qd_dealloc runs as an RCU callback and can still reference qd_sbd after the superblock object has been freed. The vulnerable window appears during quota cleanup and superblock teardown when pending quota callbacks can outlive the wait path and later access freed superblock state. For the CVSS the PR:L is used for the paranoid score because a local user or delegated mount context may be able to drive mount/unmount and quota cleanup paths without full host administrative control in some deployments. The issue is not network reachable and requires local filesystem lifecycle activity. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel use after free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	DANGER DISK UAF DEBUGFS LINUS SYZBOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

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

## 2. Signal breakdown

Paranoid scoring signals:

* Memory corruption, strong corruption primitive: +2
  This is a real UAF in an RCU teardown path. The callback can access `qd->qd_sbd` after the GFS2 superblock `sdp` has been freed.
* Real lifetime corruption: +1
  The fix is `rcu_barrier()` before `free_sbd()`, which directly addresses pending RCU callbacks outliving the freed object.
* Reliable kernel crash / strong DoS: +1
  The bug was found and reproduced by syzbot, and stale callback access to freed superblock state is a credible crash path.
* Weak LPE concern: +1
  The callback does not merely read stale memory. It decrements `sd_quota_count` and wakes `sd_kill_wait`, so there is stale freed-object access with at least some write-side behavior. No controlled reclaim primitive is shown, so this is weak rather than strong LPE plausibility.
* Important filesystem/storage path: +1
  GFS2 unmount and quota cleanup are filesystem lifecycle paths affecting kernel-owned filesystem state.
* Hard or unreliable race / special timing required: -1
  Triggering depends on RCU callback timing, unmount cleanup, timeout behavior, or additional `qd_put()` scheduling.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -1 paranoid, -2 conservative
  Mounting and unmounting GFS2 normally requires administrative privileges. The paranoid score uses a reduced penalty because delegated mount contexts, containerized service accounts, or reduced-capability root-like environments may make this less than full host-root in practical risk terms.

Conservative score is lower because typical real-world triggering requires privileged filesystem lifecycle control and no attacker-controlled reclaim, object replacement, type confusion, or arbitrary write primitive is demonstrated.

## 3. Reachability analysis

The bug is locally reachable through GFS2 filesystem teardown, especially unmount paths involving quota cleanup and pending RCU callbacks. In ordinary deployments, mounting or unmounting GFS2 requires CAP_SYS_ADMIN or equivalent administrative control, so this is not a normal unprivileged user trigger. Namespaces or containers may matter only if a reduced-privilege actor can control GFS2 mount lifecycle or trigger unmount cleanup without full host-root authority. The path is not network reachable by itself. Realistic exploitation requires local filesystem lifecycle activity plus a timing window in RCU callback completion.

Call-site confidence: high, because the patch and commit message explicitly identify `gfs2_qd_dealloc()`, `gfs2_qd_dispose()`, `gfs2_quota_cleanup()`, `gfs2_put_super()`, `free_sbd()`, and the missing `rcu_barrier()` ordering.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is a real kernel UAF in an RCU lifetime path, not merely a warning or resource leak. Realistic impact is most clearly local DoS via crash during GFS2 unmount or quota cleanup. Privilege escalation is plausible only in a weak paranoid sense because stale freed-object access includes write-side behavior, but the patch does not show controlled reclaim or a reliable corruption primitive. Therefore it is best treated as Actionable Moderate at minimum, with manual review required before downgrading.

## 5. One-sentence report phrase

A GFS2 RCU lifetime bug can let pending quota callbacks access a freed superblock during unmount, causing a local kernel UAF with realistic DoS impact and weak but nonzero privilege escalation concern in delegated mount scenarios.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real UAF in filesystem teardown with RCU lifetime semantics and stale freed-object access. Even though typical triggering requires mount or unmount privileges and no controlled reclaim primitive is shown, the memory lifetime class is high enough risk to avoid auto-closure.

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

Patch: gfs2: fix use-after-free in gfs2_qd_dealloc
Commit: 4fe388218826df8607ae41a6305df67db08a9093
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4fe388218826df8607ae41a6305df67db08a9093

Commit description:

gfs2_qd_dealloc(), called as an RCU callback from gfs2_qd_dispose(),
accesses the superblock object sdp through qd->qd_sbd after freeing qd.
It does so to decrement sd_quota_count and wake up sd_kill_wait.

However, by the time the RCU callback runs, gfs2_put_super() may have
already freed sdp via free_sbd().  This can happen when
gfs2_quota_cleanup() is called during unmount: it disposes of quota
objects via call_rcu() and then waits on sd_kill_wait with a 60-second
timeout.  If the timeout expires, or if gfs2_gl_hash_clear() triggers
additional qd_put() calls that schedule more RCU callbacks after the
wait completes, gfs2_put_super() will proceed to free the superblock
while RCU callbacks referencing it are still pending.

Add an rcu_barrier() before free_sbd() in gfs2_put_super() to ensure
all pending RCU callbacks (including gfs2_qd_dealloc) have completed
before the superblock is freed.

Fixes: a475c5d ("gfs2: Free quota data objects synchronously")
Reported-by: syzbot+42a37bf8045847d8f9d2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=42a37bf8045847d8f9d2
Tested-by: syzbot+42a37bf8045847d8f9d2@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/gfs2/super.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=4fe388218826df8607ae41a6305df67db08a9093

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

The original Linux kernel CVE announcement for CVE-2026-63804 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63804

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:H/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).