[CVE-2026-74506][MODERATE 7.0] afs: Fix UAF when sending a message

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

CVE: CVE-2026-74506
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: afs: Fix UAF when sending a message
Commit: c0d3b81f703b2a9e37fe1347610a50cdf0078c27
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0d3b81f703b2a9e37fe1347610a50cdf0078c27
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74506
Analysis date: Sat, 15 Aug 2026 12:36:42 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum. Strong Important candidate only if manual review finds controllable reclaim or write-after-free
Manual review required: YES

Summary:
A race in the AFS client can allow async rxrpc completion or abort handling to free an afs_call while afs_make_call() still accesses it, causing a kernel use-after-free that is locally triggerable through AFS filesystem IO and should be reviewed for DoS and possible LPE impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74506 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74506

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-2026-74506	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	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:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free race in the AFS client can occur when afs_make_call() continues to access an asynchronous afs_call after rxrpc completion or abort handling has already dropped the last reference. A local user can trigger the vulnerable path by performing AFS IO on a mounted AFS filesystem, while a malicious or compromised AFS server may influence timing by completing or aborting the request after the client initiated operation. For the CVSS the PR:L is used because reliable triggering normally requires a local user or process with access to the AFS mount, not full administrative privileges. The issue is not directly network reachable as an unsolicited server side attack against a listening service. Impact is at least a local denial of service via kernel crash and in worst case may allow privilege escalation due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum. Strong Important candidate only if manual review finds controllable reclaim or write-after-free (with actual score 5)  SKIP CVE-2026-74506 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	READ DANGER DISK RACE UAF SKIP HARDWARE LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	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. Strong Important candidate only if manual review finds controllable reclaim or write-after-free**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user with access to an AFS mount can trigger async AFS IO such as FetchData.
* Memory corruption, weak primitive: +1. The commit explicitly reports a KASAN slab-use-after-free in afs_make_call(), but the shown access is a stale read and no controlled reclaim or write-after-free primitive is demonstrated.
* Real lifetime corruption: +1. The bug is a reference ownership error where async rxrpc completion or abort can drop the last afs_call reference while afs_make_call() still uses it.
* Reliable kernel crash / strong DoS: +1. KASAN reports slab-use-after-free, and non-KASAN kernels may plausibly crash or hit corrupted state.
* Hard or unreliable race: 0. Although this is a race, async network completion can naturally happen after the last request packet is queued, so I would not subtract in the main score.
* Rare configuration: 0. AFS is not universal, but if an AFS mount is present the path is ordinary filesystem IO, so I would not heavily penalize reachability.

Paranoid additional signal:

* Weak LPE concern: +1. Kernel UAF lifetime bugs can sometimes become privilege escalation candidates, but here this remains weak because the patch does not show attacker-controlled reclaim, stale callback dispatch, object replacement, type confusion, or write-after-free.

## 3. Reachability analysis

The realistic trigger is local access to an AFS-mounted filesystem plus filesystem activity that issues asynchronous AFS calls. A malicious or compromised AFS server can influence timing by replying quickly or aborting the request, but this is not a generic unsolicited remote attack against a listening service on the victim. Namespaces do not automatically make this unprivileged unless the AFS mount is exposed inside the namespace or container. The bug is not default-exposed on systems without AFS usage, but it is reachable through normal file IO once AFS is mounted.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a real kernel use-after-free in an async filesystem/network-client lifetime path. Conservative handling should treat it as borderline requiring review, not auto-close. The realistic demonstrated impact is local kernel crash or instability. Theoretical privilege escalation is plausible enough for paranoid triage, but not proven by the patch because there is no demonstrated reclaim, controlled overwrite, callback control, or arbitrary memory primitive.

## 5. One-sentence report phrase

A race in the AFS client can allow async rxrpc completion or abort handling to free an afs_call while afs_make_call() still accesses it, causing a kernel use-after-free that is locally triggerable through AFS filesystem IO and should be reviewed for DoS and possible LPE impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed kernel UAF race with KASAN evidence and a plausible but unproven privilege escalation concern. Manual review should check whether local users can reliably trigger the race on affected deployments and whether freed afs_call memory can be reclaimed or influenced before the stale access.

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

Patch: afs: Fix UAF when sending a message
Commit: c0d3b81f703b2a9e37fe1347610a50cdf0078c27
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0d3b81f703b2a9e37fe1347610a50cdf0078c27

Commit description:

In afs_make_call(), there's a race with async call reception and
destruction.  If a call is dispatched that doesn't have call->write_iter
set (used to specify the data content for FS.StoreData), then the first
rxrpc_kernel_send_data() will not set MSG_MORE in the msghdr.

Once rxrpc_send_data() queues the last request packet, the response could
come in at any time and cause the call to be completed and put.  However,
afs_make_call() will look at the call again to see it ->write_iter should
be handled - something it's only allowed to do if it has its own ref on the
call.  Whilst this is the case for synchronous calls, it isn't true for
async calls such as FS.FetchData.

There's also a potential UAF in afs_make_call() in the event that an
asynchronous call is being sent, but the call fails in some way (e.g. it
gets aborted from the server).  The problem there is that afs_make_call()
tries to abort a call if the rxrpc send fails, but the asynchronous
notification from rxrpc may have caused the afs_call to be torn down.

generic/650 plays games with randomly taking CPUs offline, and can
interject a significant delay such that the call is deallocated before
afs_make_call() gets to check call->write_iter - and a UAF ensues (caught
by KASAN).

   BUG: KASAN: slab-use-after-free in afs_make_call+0x1c90/0x2210 [kafs]
   Read of size 8 at addr ffff888035e050e8 by task fsstress/1409

Fix this by making afs_make_op_call() give the op->call its own ref rather
than transferring the caller's ref to it and then dropping the ref when
afs_make_call() returns.

This also means that the afs_make_call() func never loses its ref on the
call now.

Fixes: eddf51f ("afs: Make {Y,}FS.FetchData an asynchronous operation")
Fixes: e49c7b2 ("afs: Build an abstraction around an "operation" concept")
Link: https://sashiko.dev/#/patchset/20260702144919.172295-1-dhowells%40redhat.com
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260723113452.566619-4-dhowells@redhat.com
cc: Jeffrey Altman <jaltman@auristor.com>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/afs/internal.h

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=c0d3b81f703b2a9e37fe1347610a50cdf0078c27

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

The original Linux kernel CVE announcement for CVE-2026-74506 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74506

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:L/I:L/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: 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).