[CVE-2026-63800][MODERATE 7.0] pNFS: Fix use-after-free in pnfs_update_layout()

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

CVE: CVE-2026-63800
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: pNFS: Fix use-after-free in pnfs_update_layout()
Commit: 4ad8b9a85dbf57ca532ee9e65ad7e6498bfbbf98
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ad8b9a85dbf57ca532ee9e65ad7e6498bfbbf98
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63800
Analysis date: Sun, 19 Jul 2026 09:23:14 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A use-after-free in the pNFS layout retry path can pass a freed layout header to trace_pnfs_update_layout(), allowing a local user with access to an affected pNFS mount to trigger a kernel crash and possibly limited stale-data exposure when tracing is active.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63800 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63800

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-2026-63800	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-416;*CWE-672;CWE-825;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A use-after-free can occur in the pNFS layout update retry path because pnfs_update_layout may drop the layout header reference with pnfs_put_layout_hdr(lo) before calling trace_pnfs_update_layout with the same lo pointer. If the reference drop frees the layout header, the tracepoint can later read fields from freed kernel memory and cause a crash or expose stale data through tracing. For the CVSS the PR:L is used because a local user or process with access to the affected NFS mount may be able to drive the pNFS client path, while reliable observation or triggering also depends on pNFS state and tracing configuration. The issue is not treated as directly network reachable because the vulnerable dereference occurs in the client control path and normally requires local activity on the mounted filesystem. Impact is at least local denial of service via kernel crash. In the paranoid score, limited confidentiality and integrity impact are kept because this is a real UAF dereference, but the patch does not show a clear arbitrary write primitive or a reliable privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK UAF NETWORK LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	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
  A local process with access to the affected NFS pNFS mount can plausibly drive the pnfs_update_layout() path through filesystem activity.

* Generic memory corruption, weak primitive: +1
  This is a real use-after-free, but the stale pointer is passed to a tracepoint and the patch does not show attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover.

* Real lifetime corruption: +1
  pnfs_put_layout_hdr(lo) can drop the last reference before trace_pnfs_update_layout() still uses lo.

* Reliable kernel crash / strong DoS: +1
  When the tracepoint actually dereferences freed memory, the realistic consequence is KASAN splat, Oops, or kernel crash.

Paranoid-only signal:

* Confidentiality impact plausible: +1
  In a worst-case interpretation, a tracepoint reading freed object fields could expose stale kernel object data to tracing consumers. This is gated by tracing configuration and permissions, so it is not counted in the conservative score.

Negative / limiting signals:

* Hard or configuration-dependent trigger: -1
  The vulnerable dereference appears dependent on the NFS_LAYOUT_RETURN retry path and on tracepoint execution.

* Rare or difficult-to-reach configuration: -1
  pNFS plus the specific layout retry condition plus relevant tracing makes this less broadly exposed than ordinary local syscall UAF paths.

Call-site confidence: high.
The patch directly shows the lifetime ordering bug at the call site: pnfs_put_layout_hdr(lo) before trace_pnfs_update_layout(... lo ...).

## 3. Reachability analysis

The bug is local from the client perspective. A user normally needs access to an affected NFS pNFS mount and must cause activity that reaches the NFS_LAYOUT_RETURN retry branch in pnfs_update_layout(). It is not directly network-triggerable as a packet parser or server-side remote attack path.

Namespaces or containers may matter if an untrusted workload can access a host-mounted NFS pNFS filesystem. In typical deployments, the NFS mount itself is configured by an administrator, but ordinary local users may be able to perform file operations on it.

The path is not default-enabled on all systems because it depends on NFS pNFS use and the relevant tracepoint behavior. Realistic exploitation is more likely as DoS or diagnostic leakage than as privilege escalation.

## 4. Severity interpretation

This behaves like an actionable Moderate kernel lifetime bug, not a clear Important-class LPE.

The theoretical concern is higher than ordinary DoS because the patch fixes a genuine UAF on a kernel object. However, the realistic primitive appears to be a stale read through tracing rather than attacker-controlled reuse, write-after-free, callback hijack, or object confusion. No strong privilege escalation primitive is demonstrated by the patch.

Conservative handling should keep it near borderline/manual-review territory. Paranoid handling reaches Actionable Moderate because kernel UAFs are historically under-triaged and this one may expose stale data or crash the client under specific pNFS tracing conditions.

## 5. One-sentence report phrase

A use-after-free in the pNFS layout retry path can pass a freed layout header to trace_pnfs_update_layout(), allowing a local user with access to an affected pNFS mount to trigger a kernel crash and possibly limited stale-data exposure when tracing is active.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is a real kernel UAF, but its practical severity depends on pNFS deployment, tracepoint activation, object lifetime timing, and whether stale layout memory can be observed or influenced.

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

Patch: pNFS: Fix use-after-free in pnfs_update_layout()
Commit: 4ad8b9a85dbf57ca532ee9e65ad7e6498bfbbf98
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4ad8b9a85dbf57ca532ee9e65ad7e6498bfbbf98

Commit description:

When hitting the NFS_LAYOUT_RETURN branch in pnfs_update_layout(),
the code calls pnfs_prepare_to_retry_layoutget(lo). If it succeeds,
pnfs_put_layout_hdr(lo) is called before trace_pnfs_update_layout(),
which still references 'lo'. This results in a use-after-free when the
tracepoint accesses lo's fields.

Fix this by moving the tracepoint call before pnfs_put_layout_hdr(lo).

Fixes: 2c8d5fc ("pNFS: Stricter ordering of layoutget and layoutreturn")
Cc: stable@vger.kernel.org
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/pnfs.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=4ad8b9a85dbf57ca532ee9e65ad7e6498bfbbf98

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

The original Linux kernel CVE announcement for CVE-2026-63800 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63800

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).