[CVE-2026-74631][MODERATE 7.0] net: smc: fix splice entry lifetime imbalance in smc_rx_splice

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

CVE: CVE-2026-74631
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: smc: fix splice entry lifetime imbalance in smc_rx_splice
Commit: c841789e456ec6751342fa800639ce8e82ff0e6b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c841789e456ec6751342fa800639ce8e82ff0e6b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74631
Analysis date: Sat, 22 Aug 2026 18:11:59 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local use-after-free in the SMC splice receive path can occur because pages are passed to splice_to_pipe() before matching page and socket references are taken, allowing page refcount underflow and at least kernel crash DoS, with plausible privilege-escalation concern due to kernel lifetime corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74631 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74631

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-2026-74631	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:H/I:H/A:H';*CWE-416;CWE-911;*CWE-664;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 can occur in the SMC splice receive path because smc_rx_splice() passes pages to splice_to_pipe() before taking the page and socket references that cover each splice entry lifetime. If splice_to_pipe() drops unqueued entries through smc_rx_spd_release() while queued entries are released later by pipe buffer callbacks, the old accounting can underflow page refcounts and leave stale page lifetime state. For the CVSS the PR:L value is used because a local process must call splice on an SMC socket, but administrative privileges are not normally required for socket and splice use. The issue is not directly network reachable in CVSS terms because remote traffic alone does not invoke the vulnerable local splice path, although a remote SMC peer may provide the data consumed by a local application. Impact is at least local denial of service via kernel crash and in the paranoid case may include privilege escalation due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB UAF LINUS KPANIC MEMORY  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1
  A local process can plausibly create or use an SMC socket and call splice(). This is not normally an admin-only operation once the protocol is available.

* Memory corruption, strong corruption primitive: +2
  The commit explicitly states page refcount underflow and use-after-free. This is stronger than a pure NULL dereference or bookkeeping-only bug.

* Real lifetime corruption: +1
  The fix changes page and socket reference lifetime handling around splice_to_pipe(), smc_rx_spd_release(), get_page(), put_page(), sock_hold(), and sock_put().

* Reliable kernel crash / strong DoS: +1
  Page refcount underflow and UAF in a kernel networking and pipe path can realistically crash the kernel.

* Local high-control kernel data-plane API: +1
  splice(), pipe buffers, pages, and socket receive paths give local users relatively fine-grained influence over buffer lifetime and queueing behavior.

* Weak LPE concern: +1
  Refcount underflow plus UAF creates plausible privilege-escalation concern, but the patch does not demonstrate controlled reclaim, object replacement, or an arbitrary write.

* Rare AND difficult-to-reach subsystem/configuration: -1
  SMC is less commonly deployed than TCP/IP core paths, and the risky path depends on SMC splice usage and the VM-backed RMB path.

Conservative total: 6

Paranoid adjustment:

* Privilege escalation plausibility upgraded from weak +1 to strong +2 in the paranoid view.
  This is based on page refcount underflow in a pipe/splice-backed lifetime path, which is historically a sensitive kernel exploitation area even without a demonstrated exploit.

Paranoid total: 7

Call-site confidence: high. The relevant call to splice_to_pipe() and the release callback smc_rx_spd_release() are visible in the provided patch context.

## 3. Reachability analysis

The bug is locally triggerable by a process using SMC sockets and splice(). It is not directly network-triggerable in CVSS or ActionableScore terms because remote traffic alone does not invoke the vulnerable local splice path. A remote SMC peer may influence received data, but a local process or service must perform the splice operation.

No full root or CAP_SYS_ADMIN requirement is apparent from the provided patch. Container or namespace impact depends on whether AF_SMC is available inside the environment and whether the container can establish the required SMC communication path. The path is not as broadly exposed as TCP/IP core processing, but it is reachable through ordinary local socket and splice interfaces when SMC is enabled and usable.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or warning-only issue. The patch describes a real page and socket lifetime imbalance that can underflow page refcounts and trigger use-after-free. Realistic demonstrated impact is at least local kernel crash or host DoS.

Privilege escalation is not proven by the patch, because there is no demonstrated controlled reclaim, overwrite, callback hijack, or object replacement primitive. However, page refcount underflow and UAF in a splice and pipe lifetime path are serious enough to require manual review and should not be auto-closed as a low-priority Moderate.

Overall this behaves as Actionable Moderate at minimum, with a defensible Strong Important candidate paranoid score.

## 5. One-sentence report phrase

A local use-after-free in the SMC splice receive path can occur because pages are passed to splice_to_pipe() before matching page and socket references are taken, allowing page refcount underflow and at least kernel crash DoS, with plausible privilege-escalation concern due to kernel lifetime corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue explicitly involves page refcount underflow and use-after-free in a splice, pipe, and networking lifetime path. Even though remote-only exploitation and practical LPE are not demonstrated, this class is too sensitive for automatic closure.

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

Patch: net: smc: fix splice entry lifetime imbalance in smc_rx_splice
Commit: c841789e456ec6751342fa800639ce8e82ff0e6b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c841789e456ec6751342fa800639ce8e82ff0e6b

Commit description:

smc_rx_splice() passes pages to splice_to_pipe() before taking the
references that cover the lifetime of each splice entry. In the
VM-backed RMB path, splice_to_pipe() may drop unqueued entries through
smc_rx_spd_release(), while queued entries are released later via the
pipe buffer callback.

The old post-splice accounting also derives the number of queued VM pages
from an offset mutated while building the descriptor, and a multi-page
splice pairs one sock_hold() with multiple sock_put() calls.

Take the page and socket references for every candidate entry before
splice_to_pipe(), and drop the matching private state, page reference,
and socket reference from smc_rx_spd_release() for entries that never
get queued. This fixes a refcount imbalance that can underflow page
refcounts and trigger a use-after-free.

Fixes: 9014db2 ("smc: add support for splice()")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Co-developed-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Daming Li <d4n.for.sec@gmail.com>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
Reviewed-by: Sidraya Jayagond <sidraya@linux.ibm.com>
Link: https://patch.msgid.link/20260730145552.360287-2-enjou1224z@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/smc/smc_rx.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=c841789e456ec6751342fa800639ce8e82ff0e6b

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

The original Linux kernel CVE announcement for CVE-2026-74631 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74631

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: 6
  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: 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).