[CVE-2026-74493][MODERATE 7.0] net/smc: fix socket use-after-free during link group termination

[CVE-2026-74493][MODERATE 7.0] net/smc: fix socket use-after-free during link group termination

[AL-KERNEL]

CVE: CVE-2026-74493
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/smc: fix socket use-after-free during link group termination
Commit: 5a42f162b857019a4c10ff687dc3bcdf51831865
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a42f162b857019a4c10ff687dc3bcdf51831865
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74493
Analysis date: Sat, 15 Aug 2026 14:49:20 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in net/smc link group termination can free an embedded socket before `sock_hold()` is taken, causing a kernel use-after-free with refcount misuse and at least a local DoS, with limited but nonzero LPE concern.

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

The original Linux kernel CVE announcement for CVE-2026-74493 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74493

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74493	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-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in net/smc link group termination can lead to a socket use after free because __smc_lgr_terminate released conns_lock after finding a connection but before taking a socket reference with sock_hold(). A concurrent close can unregister the connection, drop the last reference, and free the embedded smc_sock before the termination worker uses it. For the CVSS the PR:L is used because a local user or process normally needs to create and close AF_SMC sockets to race link group termination, while no administrator privileges are required in the plausible triggering model. The attack complexity is high because reliable exploitation depends on a timing window between close and termination work. Impact is at least local denial of service via kernel crash, and in the paranoid score may include confidentiality and integrity impact because this is a real UAF with refcount misuse in kernel memory. For the paranoid score, choose the highest still-defensible interpretation supported by the bug class and patch context, with preference for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	LOCK DANGER RACE UAF TEST  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  AF_SMC socket creation and close can plausibly be driven by a local user or local process without administrator privileges.

* Memory corruption, strong corruption primitive: +2
  This is a real socket UAF. KASAN reports slab-use-after-free, write of size 4, and refcount_t addition on 0.

* Real lifetime corruption: +1
  The bug is a lifetime race where the socket can be freed before `sock_hold()` takes a reference.

* Reliable kernel crash / strong DoS: +1
  Local stress testing reproduced the issue and KASAN reported a concrete UAF crash path.

* Hard or unreliable race / special timing required: -1
  Triggering requires overlap between close and link group termination.

* Paranoid only, Weak LPE concern: +1
  The UAF includes write-after-free/refcount misuse in kernel memory. No attacker-controlled reclaim or object replacement is shown, so this is not a strong LPE primitive, but it deserves manual review.

## 3. Reachability analysis

A local user or process that can create and close AF_SMC sockets may be able to race socket close with SMC link group termination. No CAP_SYS_ADMIN or CAP_NET_ADMIN requirement is evident from the patch context for the basic socket-level trigger. The issue is not clearly remote-network-triggerable by itself, although remote peer or link state may influence termination timing. SMC is not as universally exposed as TCP or UDP, but it is a normal kernel networking subsystem and the bug was reproduced by local stress testing.

Call-site confidence: high. The affected lifetime sequence and the moved `sock_hold()` are directly visible in the patch.

## 4. Severity interpretation

This is above an ordinary Moderate because it is a confirmed kernel UAF with a write/refcount misuse, not just a NULL dereference or warning. Realistically demonstrated impact is local DoS via kernel crash. Privilege escalation remains theoretical because the patch does not show controlled reclaim, object replacement, callback control, or arbitrary write. The practical bucket is Actionable Moderate, with manual review required due to the UAF/refcount nature.

## 5. One-sentence report phrase

A race in net/smc link group termination can free an embedded socket before `sock_hold()` is taken, causing a kernel use-after-free with refcount misuse and at least a local DoS, with limited but nonzero LPE concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a confirmed UAF race in kernel networking code with write-after-free/refcount misuse, so it should not be auto-closed even though the realistic demonstrated impact is currently DoS.

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

Patch: net/smc: fix socket use-after-free during link group termination
Commit: 5a42f162b857019a4c10ff687dc3bcdf51831865
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5a42f162b857019a4c10ff687dc3bcdf51831865

Commit description:

__smc_lgr_terminate() drops conns_lock after finding a connection in
lgr->conns_all, but before taking a reference on its socket. The connection
is embedded in the socket, and its registration reference protects it only
while the connection remains in the tree.

A concurrent close can unregister the connection and drop that reference,
freeing the socket before the termination worker reaches sock_hold().

The race is reachable when close overlaps link group termination.
Local stress testing reproduced the use-after-free and KASAN reported:

  BUG: KASAN: slab-use-after-free in __smc_lgr_terminate.part.0 [smc]
  Write of size 4 by task kworker/3:3
  Workqueue: events smc_lgr_terminate_work [smc]
  __smc_lgr_terminate.part.0 [smc]

The socket was allocated by smc_create(), freed through
slab_free_after_rcu_debug(), and was followed by:

  refcount_t: addition on 0; use-after-free.
  __smc_lgr_terminate.part.0 [smc]

Take the socket reference while conns_lock still protects the tree entry.
The unregister path then cannot drop the last reference until termination
has finished using the socket.

Fixes: 69318b5 ("net/smc: improve abnormal termination locking")
Cc: stable@vger.kernel.org
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Link: https://patch.msgid.link/20260723105454.87016-1-xuanqiang.luo@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/smc/smc_core.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=5a42f162b857019a4c10ff687dc3bcdf51831865

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

The original Linux kernel CVE announcement for CVE-2026-74493 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74493

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: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).