[CVE-2026-72221][IMPORTANT] sunrpc: wait for in-flight TLS handshake callback when cancel loses race

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

CVE: CVE-2026-72221
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sunrpc: wait for in-flight TLS handshake callback when cancel loses race
Commit: 0d8ceb39884148dc7a2fdf71e1cac5961ed1d2b9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0d8ceb39884148dc7a2fdf71e1cac5961ed1d2b9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72221
Analysis date: Sat, 15 Aug 2026 21:04:29 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-reachable race in the SUNRPC TLS server handshake can let a remote client trigger concurrent completion reinitialization and callback completion, corrupting an embedded swait_queue and causing at least a kernel crash DoS with memory corruption concerns.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72221 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72221

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-72221	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A race in the SUNRPC/TLS server handshake can occur when svc_tcp_handshake() times out or is interrupted and tls_handshake_cancel() loses to handshake_complete(). The callback may have marked the request complete but not yet committed XPT_TLS_SESSION, XPT_HANDSHAKE, and sk_handshake_done side effects. A second handshake path can then reinitialize sk_handshake_done while the original callback calls complete_all(), corrupting the embedded swait_queue and potentially crashing the kernel. For the CVSS the PR:N is used because a remote client generally only needs network reachability to the RPC TLS listener and no local user context on the target. The issue is network reachable when an NFS or SUNRPC service with server side TLS handshakes is exposed. Impact is at least remote denial of service via kernel crash. For the paranoid score, the waitqueue corruption is treated as a memory corruption candidate, so confidentiality and integrity impact are included for manual review sensitivity.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE RACE NETWORK KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score signals:

* Remote reachable / network-triggerable: +2. The affected path is server-side SUNRPC TLS handshake handling and can be reached by a remote RPC or NFS client when RPC TLS is enabled and exposed.
* Memory corruption, weak/indirect corruption candidate: +1. The commit explicitly describes corruption of the embedded swait_queue due to concurrent init_completion() and complete_all().
* Real lifetime / async completion mismatch: +1. The bug is a cancellation race where an in-flight callback has not committed its side effects before the caller proceeds.
* Reliable kernel crash / strong DoS: +1. Corrupting a completion waitqueue is a credible kernel crash or hang condition.
* Broad/common subsystem exposure: +1. SUNRPC/NFS is a common server subsystem, although TLS for SUNRPC is deployment-dependent.
* Hard or unreliable race / special timing required: -1. Triggering requires winning a specific timeout or signal and cancellation race.
* Non-default runtime feature: -1. Server-side SUNRPC TLS must be enabled and reachable.
* Highly constrained corruption / no attacker-controlled payload shown: -1. The patch shows waitqueue corruption, but not controlled overwrite, reclaim, type confusion, or arbitrary write.

Paranoid score signals:

* Remote reachable / network-triggerable: +2.
* Generic memory corruption: +2. For paranoid triage, explicit swait_queue corruption in kernel synchronization state is treated as a concrete memory corruption candidate.
* Real lifetime / async completion mismatch: +1.
* Reliable kernel crash / strong DoS: +1.
* Weak LPE concern: +1. The corruption target is not attacker-controlled, but kernel waitqueue/list corruption is serious enough to require manual review.
* Confidentiality impact plausible: +1. Only in the paranoid interpretation, because memory corruption could theoretically be shaped further.
* Integrity impact plausible: +1. Only in the paranoid interpretation for the same reason.
* Broad/common subsystem exposure: +1.
* Hard race: -1.
* Non-default SUNRPC TLS exposure: -1.
* Constrained primitive: -1.

## 3. Reachability analysis

A remote client can plausibly trigger the affected path by interacting with an exposed SUNRPC/NFS service that uses server-side TLS handshakes. No local account on the target is required for the network-facing interpretation, so PR:N is reasonable for CVSS-style paranoid triage.

The path is not universally default-enabled because it depends on SUNRPC TLS support and service configuration. If NFS/RPC TLS is not enabled or is restricted to a trusted storage network, exposure is much lower. Containers and namespaces do not materially reduce the remote requirement, because the relevant trigger is network access to the host service.

Call-site confidence: high. The commit message gives the direct race sequence and names the affected callback, flags, completion object, and second handshake path.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. The realistic demonstrated impact is remote denial of service through a race that can corrupt a kernel completion waitqueue and likely crash or destabilize the server.

Theoretical exploitation beyond DoS is not proven. There is no demonstrated attacker-controlled reclaim, arbitrary write, type confusion, or function pointer control. However, because the patch explicitly describes corruption of an embedded swait_queue in a network-reachable server path, the paranoid score should preserve Important-candidate handling for manual review.

## 5. One-sentence report phrase

A network-reachable race in the SUNRPC TLS server handshake can let a remote client trigger concurrent completion reinitialization and callback completion, corrupting an embedded swait_queue and causing at least a kernel crash DoS with memory corruption concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: the issue is potentially network reachable and the commit explicitly describes kernel waitqueue corruption, so it should not be auto-closed even though practical exploitation beyond DoS is not demonstrated.

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

Patch: sunrpc: wait for in-flight TLS handshake callback when cancel loses race
Commit: 0d8ceb39884148dc7a2fdf71e1cac5961ed1d2b9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0d8ceb39884148dc7a2fdf71e1cac5961ed1d2b9

Commit description:

When wait_for_completion_interruptible_timeout() in
svc_tcp_handshake() returns 0 (timeout) or -ERESTARTSYS (signal) and
tls_handshake_cancel() then returns false, handshake_complete() has
won the cancellation race: it has set HANDSHAKE_F_REQ_COMPLETED and
is about to invoke svc_tcp_handshake_done(), but the callback's
side effects on xpt_flags and on svsk->sk_handshake_done have not
yet committed.

The current code reads xpt_flags immediately to decide whether the
session succeeded. Two races result.

If the callback has executed set_bit(XPT_TLS_SESSION) but not yet
clear_bit(XPT_HANDSHAKE), svc_tcp_handshake() sees a session,
enqueues the transport, and returns. svc_xprt_received() then
clears XPT_BUSY, a worker thread picks the transport up, the
dispatcher in svc_handle_xprt() observes XPT_HANDSHAKE still set,
and xpo_handshake is invoked a second time. That svc_tcp_handshake()
calls init_completion(&svsk->sk_handshake_done) while the original
callback concurrently calls complete_all() on it, corrupting the
embedded swait_queue.

If the callback has set HANDSHAKE_F_REQ_COMPLETED but not yet
entered svc_tcp_handshake_done(), svc_tcp_handshake() reads
XPT_TLS_SESSION as clear and tears the connection down even though
the handshake is about to succeed.

Wait for the callback to commit before inspecting xpt_flags. The
completion is guaranteed to fire because handshake_complete()
invokes svc_tcp_handshake_done() unconditionally once it has set
HANDSHAKE_F_REQ_COMPLETED.

Fixes: b3cbf98 ("SUNRPC: Support TLS handshake in the server-side TCP socket code")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/svcsock.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=0d8ceb39884148dc7a2fdf71e1cac5961ed1d2b9

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

The original Linux kernel CVE announcement for CVE-2026-72221 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72221

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 4
  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).