[CVE-2026-63795][IMPORTANT] 9p: avoid putting oldfid in p9_client_walk() error path

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

CVE: CVE-2026-63795
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: 9p: avoid putting oldfid in p9_client_walk() error path
Commit: 99c379ca1e221c3d75c7c804ebbf4e5ee37a3070
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99c379ca1e221c3d75c7c804ebbf4e5ee37a3070
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63795
Analysis date: Sun, 19 Jul 2026 08:51:49 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A 9p client reference-counting bug in p9_client_walk() can incorrectly drop the caller-owned oldfid on an error path, allowing a later use or put to trigger use-after-free or refcount underflow, with realistic DoS impact and a manually reviewable worst-case memory-corruption concern.

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

The original Linux kernel CVE announcement for CVE-2026-63795 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63795

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: IMPORTANT
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-2026-63795	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H';*CWE-416;CWE-911;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'A reference counting bug in the 9p client p9_client_walk error path can drop a reference to oldfid even when fid aliases oldfid and ownership remains with the caller. If the failed walk is reached after the request is sent, oldfid may be clunked and destroyed while later client code still expects it to be valid, creating a use after free or refcount underflow condition. For the CVSS the PR:N value is used in the paranoid network adjacent interpretation because a malicious or compromised 9p server side endpoint can trigger failure responses without local privileges on the victim once the victim has a mounted 9p filesystem and performs a path walk. The issue is usually not public Internet exposed and is more realistic in VM host guest, container, development, or internal file sharing environments using 9p. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-63795 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB ERRORPATH UAF LINUS  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  Once a 9p filesystem is mounted, ordinary local filesystem operations can plausibly trigger path walks without requiring the user to mount the filesystem.

* Remote reachable / network-triggerable: +2 in paranoid interpretation
  A malicious or compromised 9p server endpoint can influence walk failure responses after the request is sent. This is not typical public Internet exposure, but it is realistic across VM host guest, container, development, or internal 9p file sharing boundaries.

* Generic memory corruption, strong corruption primitive: +2
  The commit explicitly describes use-after-free and refcount underflow caused by dropping the caller-owned oldfid reference.

* Real lifetime corruption: +1
  This is a concrete reference lifetime bug where p9_fid_put() can destroy an object still owned and expected to remain valid by the caller.

* Weak LPE concern: +1 in paranoid interpretation
  The bug is a UAF/refcount-underflow class issue, but no attacker-controlled reclaim, object replacement, callback control, or write primitive is demonstrated. This supports manual-review sensitivity but not a strong LPE claim.

* Reliable kernel crash / strong DoS: +1
  Later use or put of oldfid can trigger UAF or refcount underflow, and warning logs/reproducer are mentioned.

* Important filesystem/storage path: +1
  The affected code is in the 9p filesystem client path and impacts filesystem object lifetime handling.

Subtracted signals:

* Rare / configuration-dependent exposure: -1
  9p is not a default general-purpose filesystem on most production systems and usually requires an existing 9p mount or VM/container/internal sharing setup.

## 3. Reachability analysis

The local trigger requires a system where a 9p filesystem is already mounted. In that case, an unprivileged local user may be able to trigger multi-component path walks through normal filesystem access, while the failure condition depends on the 9p server behavior and the split non-cloning walk path.

The paranoid attacker model is stronger: a malicious or compromised 9p server can influence walk failure responses after the request has been sent. This gives a network-adjacent or cross-boundary angle, especially in VM host guest, container, development sandbox, or internal file sharing deployments.

Mounting 9p itself is usually privileged, but the trigger after mount may not require full administrative privileges. Namespaces or containerized environments can reduce the practical privilege barrier if 9p is exposed into the guest or container by orchestration.

Call-site confidence: high. The patch and commit message directly describe the aliasing condition, the wrong p9_fid_put(), and the later UAF/refcount-underflow consequence.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup. It is an actionable lifetime bug in filesystem client code with an explicit UAF/refcount-underflow consequence.

Realistically, the most likely impact is denial of service through refcount warning, kernel crash, or object lifetime corruption. Practical privilege escalation is not demonstrated because the patch does not show attacker-controlled reclaim, arbitrary write, callback hijack, or type confusion. However, because this is a real reference-counting lifetime bug reachable through filesystem operations and potentially influenced by a server endpoint, it should not be auto-closed as a routine Moderate.

Conservative interpretation: Actionable Moderate.
Paranoid interpretation: Strong Important candidate for manual review.

## 5. One-sentence report phrase

A 9p client reference-counting bug in p9_client_walk() can incorrectly drop the caller-owned oldfid on an error path, allowing a later use or put to trigger use-after-free or refcount underflow, with realistic DoS impact and a manually reviewable worst-case memory-corruption concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes a concrete UAF/refcount lifetime bug, the affected path can be reached through mounted 9p filesystem operations, and a malicious 9p server may influence the failing walk path. The realistic impact is likely DoS, but the memory lifetime primitive is strong enough to require manual triage.

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

Patch: 9p: avoid putting oldfid in p9_client_walk() error path
Commit: 99c379ca1e221c3d75c7c804ebbf4e5ee37a3070
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99c379ca1e221c3d75c7c804ebbf4e5ee37a3070

Commit description:

When p9_client_walk() is called with clone set to false, fid aliases
oldfid. If the walk subsequently fails after the request has been sent,
the error path jumps to clunk_fid, which currently calls p9_fid_put(fid)
unconditionally.

This drops a reference to oldfid even though ownership of oldfid remains
with the caller. If this is the last reference, oldfid can be clunked and
destroyed while the caller still expects it to be valid. A later use or
put of oldfid can then trigger a use-after-free or refcount underflow.

Fix this by only putting fid in the clunk_fid error path when it does not
alias oldfid, matching the existing guard in the error path below.

This can be triggered when a multi-component walk is split into multiple
p9_client_walk() calls and a later non-cloning walk fails. A reproducer
and refcount warning logs are available on request.

Fixes: b48dbb9 ("9p fid refcount: add p9_fid_get/put wrappers")
Cc: stable@vger.kernel.org
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Assisted-by: GLM 5.1
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Message-ID: <20260528053918.53550-1-zhaoyz24@mails.tsinghua.edu.cn>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/9p/client.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=99c379ca1e221c3d75c7c804ebbf4e5ee37a3070

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

The original Linux kernel CVE announcement for CVE-2026-63795 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63795

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:A/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.1

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).