[CVE-2026-64281][MODERATE REGULAR] svcrdma: wake sq waiters when the transport closes

[CVE-2026-64281][MODERATE REGULAR] svcrdma: wake sq waiters when the transport closes

[AL-KERNEL]

CVE: CVE-2026-64281
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: svcrdma: wake sq waiters when the transport closes
Commit: 40eedc4253dbda0b29b7961200534dfcecb48ace
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40eedc4253dbda0b29b7961200534dfcecb48ace
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64281
Analysis date: Sat, 25 Jul 2026 10:19:40 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended, not Important
Manual review required: YES

Summary:
An NFS/RDMA remote disconnect or completion error can leave SQ waiters stuck in svc_rdma_sq_wait, pinning transport references and blocking teardown, resulting in a persistent denial of service without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64281 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64281

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 REGULAR
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-2026-64281	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-833;CWE-667;**CWE-400;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'An NFS/RDMA transport close can leave threads parked in svc_rdma_sq_wait because the close path sets XPT_CLOSE but does not wake sc_sq_ticket_wait or sc_send_wait. A remote RDMA disconnect or RDMA completion error can therefore leave workers in TASK_UNINTERRUPTIBLE state, pin svc_xprt references, and block svc_rdma_free from completing. For the CVSS the PR:N is used because an attacker does not need local user access on the victim when the NFS/RDMA endpoint is reachable from the RDMA fabric. The issue is adjacent network reachable in typical storage or data center RDMA deployments rather than public Internet reachable. Impact is denial of service through stuck kernel worker threads and blocked transport teardown. 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. The patch context does not support memory corruption or privilege escalation, so confidentiality and integrity remain none.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended, not Important (with actual score 4)  SKIP CVE-2026-64281 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: ccc89b9d1ed233349cfe8d87b842e7351b74d8de YES  NO NO unknown 	MAYBE	LOCK ERRORPATH HARDWARE  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended, not Important**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  A remote RDMA disconnect reaches `svc_rdma_cma_handler()` and previously called `svc_xprt_deferred_close()` without waking RDMA SQ waiters.

* Reliable kernel crash / strong DoS: +1
  This is not a crash, but it is a strong service DoS: worker threads can remain in `TASK_UNINTERRUPTIBLE`, pin `svc_xprt` references, and block `svc_rdma_free()`.

* Availability impact realistic: +1 in paranoid score
  The impact can persist beyond a transient failure because references remain pinned and teardown can block indefinitely, especially if all workers are parked.

Subtracted or limited signals:

* Rare / configuration-dependent exposure: -1 in conservative score
  NFS/RDMA requires RDMA-capable deployment and is typically limited to storage or data-center fabrics, not public Internet exposure.

Not applied:

* No memory corruption signal
  The patch fixes missing wakeups and stuck waiters. It does not show UAF, double-free, OOB access, type confusion, arbitrary write, or stale callback execution.

* No privilege escalation signal
  The retained references prevent teardown rather than creating attacker-controlled reuse or corruption.

* No confidentiality or integrity impact
  The patch context supports availability impact only.

Call-site confidence: high. The patch shows all converted close/error paths and the added wakeups.

## 3. Reachability analysis

A remote peer on the RDMA fabric can trigger relevant close paths through disconnects or RDMA completion errors. No local user access on the victim is required once the NFS/RDMA endpoint is reachable. This is adjacent-network rather than general Internet exposure.

Namespaces and containers do not materially lower or raise the attacker privilege requirement for the remote case. Local administrative setup is required to run NFS/RDMA, but that is deployment configuration, not attacker privilege.

The path is not default-enabled on ordinary Linux systems because it requires NFS/RDMA and RDMA infrastructure. In environments where NFS/RDMA is used, the bug is reachable through normal transport teardown and error handling.

## 4. Severity interpretation

This behaves like an actionable Moderate DoS issue, not an Important-class memory-corruption vulnerability. The theoretical risk is bounded by the observed primitive: missing wakeups cause stuck waiters and reference pinning. There is no evidence of attacker-controlled reclaim, writable stale object access, callback corruption, refcount takeover, or privilege escalation.

The conservative score is lower because NFS/RDMA is deployment-specific. The paranoid score is higher because the trigger can be remote within an RDMA fabric and the hang can persist across transport teardown.

## 5. One-sentence report phrase

An NFS/RDMA remote disconnect or completion error can leave SQ waiters stuck in svc_rdma_sq_wait, pinning transport references and blocking teardown, resulting in a persistent denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is remotely reachable in NFS/RDMA deployments and can cause persistent service degradation or teardown hangs, but it should be reviewed as DoS-only unless additional evidence shows a memory lifetime or corruption primitive.

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

Patch: svcrdma: wake sq waiters when the transport closes
Commit: 40eedc4253dbda0b29b7961200534dfcecb48ace
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40eedc4253dbda0b29b7961200534dfcecb48ace

Commit description:

Threads parked in svc_rdma_sq_wait() on sc_sq_ticket_wait or
sc_send_wait can hang indefinitely in TASK_UNINTERRUPTIBLE state
across transport teardown, pinning svc_xprt references and
blocking svc_rdma_free().

The close path sets XPT_CLOSE before invoking xpo_detach and both
wait_event predicates include an XPT_CLOSE term, but the
predicates are re-evaluated only on wakeup. sc_sq_ticket_wait has
no completion-driven wake path; it is advanced solely by the
chained ticket handoff inside svc_rdma_sq_wait() itself. Without
an explicit wake at close, parked threads never observe
XPT_CLOSE, hold their svc_xprt_get reference forever, and
svc_rdma_free() blocks on xpt_ref dropping to zero.

Two close entry points reach this transport. Local teardown runs
svc_rdma_detach() from svc_handle_xprt() -> svc_delete_xprt() ->
xpo_detach() on a worker thread. A remote disconnect arrives at
svc_rdma_cma_handler(), which calls svc_xprt_deferred_close():
that sets XPT_CLOSE and enqueues the transport but does not
access either RDMA waitqueue, so a worker already parked in
svc_rdma_sq_wait() never re-evaluates its predicate. With every
worker parked on this transport, no thread is available to run
the local teardown either, and the wake site there is
unreachable.

Introduce svc_rdma_xprt_deferred_close(), a thin svcrdma wrapper
that calls svc_xprt_deferred_close() and then wakes both
sc_sq_ticket_wait and sc_send_wait. Convert the svcrdma producers
that called svc_xprt_deferred_close() directly:
svc_rdma_cma_handler(), qp_event_handler(),
svc_rdma_post_send_err(), svc_rdma_wc_send(), the sendto drop
path, the rw completion error paths, and the recvfrom flush and
read-list error paths.

Wake both waitqueues from svc_rdma_detach() as well. The
synchronous svc_xprt_close() path (backchannel ENOTCONN, device
removal via svc_rdma_xprt_done) reaches detach without flowing
through svc_xprt_deferred_close() and therefore does not invoke
the new helper.

Fixes: ccc89b9 ("svcrdma: Add fair queuing for Send Queue access")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
[ cel: add svc_rdma_xprt_deferred_close() to complete the fix ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/linux/sunrpc/svc_rdma.h
  net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
  net/sunrpc/xprtrdma/svc_rdma_rw.c
  net/sunrpc/xprtrdma/svc_rdma_sendto.c
  net/sunrpc/xprtrdma/svc_rdma_transport.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=40eedc4253dbda0b29b7961200534dfcecb48ace

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

The original Linux kernel CVE announcement for CVE-2026-64281 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64281

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).