[CVE-2026-72222][IMPORTANT] sunrpc: pin svc_xprt across the asynchronous TLS handshake callback

[CVE-2026-72222][IMPORTANT] sunrpc: pin svc_xprt across the asynchronous TLS handshake callback

[AL-KERNEL]

CVE: CVE-2026-72222
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sunrpc: pin svc_xprt across the asynchronous TLS handshake callback
Commit: f3b55945dd99f29d83e1965d0141040a35262346
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3b55945dd99f29d83e1965d0141040a35262346
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72222
Analysis date: Sat, 15 Aug 2026 12:10:58 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote client that can reach a TLS-enabled NFS server may trigger an async SUNRPC TLS handshake lifetime race, causing a use-after-free with writes into freed `svc_sock` state and potential kernel crash or stronger memory-corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-72222 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72222

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-2026-72222	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use after free can occur in the SUNRPC server TLS handshake path because svc_tcp_handshake stores an svc_xprt pointer for an asynchronous callback without holding a reference to the embedding svc_sock lifetime. A remote client that can reach a TLS-enabled NFS server may trigger the race by closing or interrupting the connection while the TLS handshake downcall callback is still in flight. The callback can then update xpt_flags and complete sk_handshake_done after the svc_sock has been freed, which makes this a slab corruption candidate rather than a benign invalid read. For the CVSS the PR:N is used because the attacker does not need local privileges on the victim and typically only needs network reachability to the TLS-enabled NFS endpoint. AC:H is used because reliable triggering depends on a close/cancel race with asynchronous callback delivery. Impact is at least remote denial of service via kernel crash. 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. NFS over TLS servers are usually deployed on internal storage or data center networks and are often not exposed to the public Internet.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE DANGER RACE UAF NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +2 Remote reachable / network-triggerable: the affected path is reachable by a remote client connecting to a TLS-enabled NFS/SUNRPC server.
* +2 Memory corruption, strong corruption primitive: the callback can write through a freed `svc_sock`, including `xpt_flags` bit operations and `complete_all()` on an embedded waitqueue.
* +1 Real lifetime corruption: this is an async callback lifetime mismatch fixed by adding `svc_xprt_get()` / `svc_xprt_put()`.
* +1 Reliable kernel crash / strong DoS: freed slab writes and waitqueue traversal can plausibly crash the kernel.
* +1 Broad deployed server subsystem: SUNRPC/NFS server code is a security-relevant network service path, even if TLS mode is configuration-dependent.
* -1 Hard or unreliable race: exploitation depends on a close/cancel race with asynchronous TLS handshake completion.

Paranoid elevation:

* +2 Privilege escalation plausible: not proven, but writable UAF into freed slab state and waitqueue structures is strong enough for Important-class manual review.
* Paranoid score keeps AC/race difficulty in mind and does not assume arbitrary code execution.

Call-site confidence: high. The commit message and diff directly show the async callback, missing reference, free race, and added lifetime pinning.

## 3. Reachability analysis

A remote client that can reach an NFS server with server-side TLS enabled can potentially trigger the bug by causing connection close or cancellation to overlap with the asynchronous TLS handshake callback. No local account on the victim is required for the network-facing scenario.

The main limiting factor is deployment exposure: NFS over TLS is usually used on internal storage or data-center networks and is typically not exposed to the public Internet. Containers and namespaces are not the primary issue here, because the vulnerable object lifetime is in the host kernel SUNRPC server path.

## 4. Severity interpretation

This behaves more like an Important candidate than an ordinary Moderate. The realistic minimum impact is remote DoS through kernel crash. The theoretical higher impact is memory corruption from write-after-free into slab-backed kernel objects, but there is no demonstrated controlled reclaim or reliable code-execution primitive in the provided material.

Conservative handling should still avoid auto-close because this is not a benign NULL dereference or warning. It is a network-reachable lifetime bug with concrete freed-object writes.

## 5. One-sentence report phrase

A remote client that can reach a TLS-enabled NFS server may trigger an async SUNRPC TLS handshake lifetime race, causing a use-after-free with writes into freed `svc_sock` state and potential kernel crash or stronger memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable UAF with freed-object writes and callback lifetime corruption is high-risk enough for manual review, even though reliable privilege escalation is not demonstrated.

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

Patch: sunrpc: pin svc_xprt across the asynchronous TLS handshake callback
Commit: f3b55945dd99f29d83e1965d0141040a35262346
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3b55945dd99f29d83e1965d0141040a35262346

Commit description:

svc_tcp_handshake() stores the raw svc_xprt pointer in
tls_handshake_args.ta_data and submits the request through
tls_server_hello_x509(). The handshake core takes only
sock_hold(req->hr_sk); nothing references the embedding struct
svc_sock that svc_tcp_handshake_done() reaches via container_of().

Two close races leave the in-flight callback writing through a freed
svc_sock. svc_sock_free() calls tls_handshake_cancel() and discards
its return value: a false return means handshake_complete() has
already set HANDSHAKE_F_REQ_COMPLETED but hp_done() may not have
finished, yet svc_sock_free() proceeds to kfree(svsk). The
cancel-loser fall-through inside svc_tcp_handshake() itself produces
the same window: when wait_for_completion_interruptible_timeout()
returns <= 0 (timeout or signal) and tls_handshake_cancel() returns
false, the function does not drain, returns, and svc_handle_xprt()
calls svc_xprt_received(), which clears XPT_BUSY and can drop the
last reference. A concurrent close then runs svc_sock_free() while
svc_tcp_handshake_done() is still updating xpt_flags and walking
svsk->sk_handshake_done.

The corruption surfaces as set_bit/clear_bit RMW into the freed
xpt_flags slab slot and as complete_all() walking and writing the
freed wait_queue_head_t list embedded in sk_handshake_done -- a
slab-corruption primitive, not a benign read. The path is reachable
on any TLS-enabled NFS server whenever a connection close overlaps
the tlshd downcall delivery window; the interruptible wait means
signal delivery suffices, not just SVC_HANDSHAKE_TO expiry.

Take svc_xprt_get(xprt) immediately before tls_server_hello_x509()
so the in-flight callback owns its own reference. Release it on the
two edges where the callback is guaranteed not to fire -- submission
failure from tls_server_hello_x509() and a successful
tls_handshake_cancel() -- and at the tail of
svc_tcp_handshake_done() after complete_all().

Fixes: b3cbf98 ("SUNRPC: Support TLS handshake in the server-side TCP socket code")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <clm@meta.com>
Assisted-by: kres (claude-opus-4-7)
[cel: rewrote commit message to describe the actual change]
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=f3b55945dd99f29d83e1965d0141040a35262346

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

The original Linux kernel CVE announcement for CVE-2026-72222 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72222

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:L/I:L/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: 6
  Paranoid ActionableScore: 8

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).