[CVE-2026-89536][IMPORTANT] SUNRPC: wait for in-flight client TLS handshake callback

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

CVE: CVE-2026-89536
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: SUNRPC: wait for in-flight client TLS handshake callback
Commit: 15431820f448e09f8029b670d5c82aa5917d4625
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15431820f448e09f8029b670d5c82aa5917d4625
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89536
Analysis date: Sat, 12 Sep 2026 01:22:36 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89536 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89536

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-2026-89536	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89536 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-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'SUNRPC/TLS client handshake handling has a race where xs_tls_handshake_sync can time out or be interrupted while xs_tls_handshake_done is still running after handshake completion. If cancellation loses to completion, the waiter can drop the callback owned reference to the lower transport before the callback stores xprt_err or releases its own reference, creating a use-after-free candidate and likely kernel crash. For the CVSS the PR:N is used for the paranoid score because a malicious or compromised RPC TLS server, or a network attacker able to interfere with the handshake, does not need an account on the victim once the client attempts the TLS transport. The issue is network influenced but typically limited to environments where SUNRPC over TLS is configured, such as NFS or RPC deployments using TCP with TLS. Impact is at least denial of service and in the paranoid interpretation may include confidentiality or integrity impact because the bug class is a race based use-after-free in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE SIMPLEFIX NETWORK LINUS KPANIC 	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative floor applied: 4 -> 5 because this is a concrete lifetime race where the waiter can drop the callback-owned transport reference while the callback is still in flight, and the callback may then use and release the same potentially freed transport.

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation: the lower SUNRPC transport can be freed while xs_tls_handshake_done() is still running and still expects the transport to be alive.
* +1 Concrete repeated release / refcount operation: the synchronous waiter can drop the callback-owned xprt reference after cancellation loses to completion, while the callback still later uses the same object and drops its own reference.
* +1 Reliable kernel crash / strong DoS: post-lifetime access to the lower transport before storing xprt_err or dropping the reference can realistically fault or corrupt lifetime state.
* +1 Network-influenced reachability: a malicious or compromised RPC TLS peer, or a network attacker able to influence the handshake, can affect the completion / timeout ordering once the client attempts SUNRPC over TLS.
* -1 Hard timing race: the vulnerable path requires cancellation or timeout to lose against handshake completion while the callback is still in flight.

Paranoid additional signal:

* +1 Paranoid repeated-refcount leverage: the vulnerable stale operation is not only a stale read. It includes lifetime/refcount mutation through storage whose original object lifetime may already have ended.

Non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Callback or function pointer control not demonstrated.
* Confidentiality impact not awarded because no disclosure path is established.
* Integrity impact as a separate score signal not awarded because the stale xprt_err write and refcount mutation are already represented as Class B lifetime corruption, not as a controlled write primitive.
* Extra Availability not awarded because the same crash is already counted once.
* Structured stale-object leverage not separately awarded because the strongest established extra fact is the concrete repeated lifetime/refcount mutation.
* Weak LPE not separately awarded for the same repeated-release fact.
* CLASS_A not assigned because there is no demonstrated controlled reclaim, directed kernel write, privilege escalation, or security-boundary bypass.

Repeated-release checks:

* First release can end lifetime: yes, the waiter drops the callback-owned reference and this can free the lower transport.
* Exact same reference/object survives: yes, the callback is still in flight and still refers to the lower transport.
* Second operation is lifetime/refcount mutation: yes, the callback later drops its reference.
* +1 Concrete repeated release/refcount awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes.
* FLOOR 5 applied: yes, for the conservative score.

3. Reachability analysis

The evaluated attacker is a remote or network-adjacent actor influencing the SUNRPC TLS handshake endpoint after the victim client has attempted a TCP-with-TLS RPC transport. This is not a generic inbound remote kernel attack against all hosts. It depends on SUNRPC over TLS being configured and used, for example in NFS/RPC deployments.

Host administrator privilege is not positively established as required for the attacker, but prior victim-side configuration of SUNRPC TLS may be needed. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

No namespace-scoped capability path is relevant from the supplied patch. NamespaceScopedPrivilege is NO.

The race is timing-sensitive because cancellation or timeout must lose to handshake completion while the completion callback is still running. The timing may be influenced by a malicious peer or network conditions and may be repeatable across connection attempts, but the exact winning interleaving remains a real constraint.

No special hardware is required.

4. Primitive and severity interpretation

The exact primitive is a race-based lifetime violation in the SUNRPC client TLS handshake path. xs_tls_handshake_sync() gives xs_tls_handshake_done() a reference to the lower transport. If the synchronous wait times out or is interrupted and tls_handshake_cancel() loses to handshake completion, the callback may still be executing. The old code could then drop the callback-owned reference from the waiter side, potentially freeing the lower transport before the callback stores xprt_err or releases its reference.

This is an active structured object lifetime bug rather than fixed stale storage. It is not merely an ordinary stale cleanup dereference because the callback can continue active processing and then perform another lifetime/refcount operation on the same potentially freed transport.

PrimitiveClass is CLASS_B. The demonstrated consequence is kernel crash or DoS. The speculative consequence is stronger post-free corruption if the freed transport storage is reused before callback completion, but controlled reclaim or arbitrary read/write is not established.

ImportantCandidate is YES because a real UAF plus concrete repeated lifetime/refcount mutation leaves meaningful exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would discard a concrete post-lifetime refcount/lifetime corruption primitive.

Likely operational severity is actionable Moderate at minimum, with Strong Important review warranted due to the network-influenced active lifetime race.

5. One-sentence report phrase

A race in the SUNRPC TLS client handshake can let the waiter free the lower transport while the completion callback is still running, causing a use-after-free with at least kernel crash impact and unresolved post-lifetime refcount corruption risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete network-influenced Class B UAF with repeated lifetime/refcount mutation on a potentially freed transport object, and exploitation leverage beyond DoS is unresolved.

7. Call-site confidence

Call-site confidence: medium

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

Patch: SUNRPC: wait for in-flight client TLS handshake callback
Commit: 15431820f448e09f8029b670d5c82aa5917d4625
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15431820f448e09f8029b670d5c82aa5917d4625

Commit description:

xs_tls_handshake_sync() gives xs_tls_handshake_done() a reference to the
lower transport before submitting the handshake request. On timeout or
signal, the synchronous waiter drops that reference after calling
tls_handshake_cancel().

handshake_req_cancel() returns false when handshake_complete() has
already marked the request complete. In that case the completion callback
can still be running, so dropping the callback-owned reference in the
waiter can free the lower transport before xs_tls_handshake_done() stores
xprt_err or drops its own reference.

If cancellation loses to completion, wait until xs_tls_handshake_done()
signals handshake_done and let the callback release its reference. This
mirrors the server-side handshake lifetime handling and keeps the timeout
or signal return value unchanged.

Fixes: 75eb6af ("SUNRPC: Add a TCP-with-TLS RPC transport class")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Reviewed-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/xprtsock.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=15431820f448e09f8029b670d5c82aa5917d4625

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

The original Linux kernel CVE announcement for CVE-2026-89536 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89536

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:R/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: 5
  Paranoid ActionableScore: 6

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).