[CVE-2026-52946][MODERATE REGULAR] fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling

[CVE-2026-52946][MODERATE REGULAR] fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling

[AL-KERNEL]

CVE: CVE-2026-52946
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling
Commit: 54626335ea4174ab2d9a183b511d825f6765e47b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54626335ea4174ab2d9a183b511d825f6765e47b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52946
Analysis date: Fri, 26 Jun 2026 10:46:15 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A SOFTIRQ-unsafe lock order in fasync process-group signaling can deadlock `send_sigurg()` or `send_sigio()`, allowing a potential remote denial of service via TCP urgent data when the target service uses the affected FASYNC process-group configuration.

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

The original Linux kernel CVE announcement for CVE-2026-52946 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52946

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-52946	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-667;*CWE-833;CWE-362;BEST CVSS score: '5.9';DESCR 'A SOFTIRQ unsafe lock order in fasync process group signaling can deadlock when send_sigio or send_sigurg takes tasklist_lock from softirq context while another CPU is waiting for the tasklist_lock writer side. The TCP urgent data path can reach send_sigurg from NET_RX_SOFTIRQ, so a remote peer may contribute to triggering the issue when the target has a reachable TCP service with FASYNC ownership configured for a process group. For the CVSS the PR:N is used because the network peer does not need local privileges on the victim for the TCP URG based trigger path, although the vulnerable configuration must already exist on the target. AC:H is used because reliable triggering depends on timing, softirq interruption, and concurrent tasklist_lock writer pressure from fork or exit paths. Impact is denial of service through CPU or system lockup, with no clear memory corruption or privilege escalation primitive indicated by the code change.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4) 	MAYBE	LOCK DEADLOCK DISK WARNONLY HARDWARE LINUS PACKET  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The commit explicitly identifies a potential remote DoS vector through TCP URG packets reaching `send_sigurg()` from `NET_RX_SOFTIRQ`.

* Reliable kernel crash / strong DoS: +1
  The issue is a lock-order deadlock that can cause CPU soft lockup or system hang.

* Hard or unreliable race / special timing required: -1
  Reliable triggering requires a specific interleaving involving `tasklist_lock`, a waiting writer from fork or exit, and a softirq interrupt path.

* Availability impact realistic: +1
  The expected impact is availability loss through deadlock or CPU lockup.

Conservative total: 3

Paranoid additional signal:

* Common networking core path or packet-processing path: +1
  The TCP urgent data receive path is part of normal kernel networking receive processing, even though the vulnerable FASYNC process-group configuration is not typical for all services.

Paranoid total: 4

Not counted:

* Generic memory corruption: +0
  The patch shows a locking deadlock, not UAF, OOB write, double free, or object lifetime corruption.

* Privilege escalation plausible: +0
  No realistic LPE primitive is implied by the patch.

* Confidentiality / integrity impact: +0
  The issue is DoS-only based on the shown code change.

## 3. Reachability analysis

A remote peer may contribute to triggering the bug by sending TCP urgent data to a service that has FASYNC configured for a process group. No local privileges on the victim are needed for that network-side trigger, so the remote interpretation is defensible. However, the vulnerable state requires a specific local socket ownership/signaling setup and a timing window involving softirq execution and `tasklist_lock` writer pressure from fork or exit paths. Namespaces do not materially improve exploitation unless a containerized workload can both expose the relevant TCP service and configure the affected FASYNC process-group behavior.

## 4. Severity interpretation

This behaves like a borderline actionable Moderate rather than an Important-class vulnerability. The security impact is a plausible remote or externally influenced denial of service, but the bug class is lock-order deadlock only. There is no evidence of memory corruption, privilege escalation, data disclosure, or integrity violation. The main reason not to auto-close is the explicit TCP URG remote DoS angle in a core networking receive path.

## 5. One-sentence report phrase

A SOFTIRQ-unsafe lock order in fasync process-group signaling can deadlock `send_sigurg()` or `send_sigio()`, allowing a potential remote denial of service via TCP urgent data when the target service uses the affected FASYNC process-group configuration.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the patch explicitly mentions a potential remote denial of service vector via TCP URG packets, even though the underlying bug is DoS-only and does not show memory corruption or privilege escalation.

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

Patch: fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling
Commit: 54626335ea4174ab2d9a183b511d825f6765e47b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=54626335ea4174ab2d9a183b511d825f6765e47b

Commit description:

A SOFTIRQ-safe to SOFTIRQ-unsafe lock order deadlock can occur in
send_sigio() and send_sigurg() when a process group receives a signal.

When FASYNC is configured for a process group (PIDTYPE_PGID), both
functions use read_lock(&tasklist_lock) to traverse the task list.
However, they are frequently called from softirq context:
- send_sigio() via input_inject_event -> kill_fasync
- send_sigurg() via tcp_check_urg -> sk_send_sigurg (NET_RX_SOFTIRQ)

The deadlock is caused by the rwlock writer fairness mechanism:
1. CPU 0 (process context) holds read_lock(&tasklist_lock) in do_wait().
2. CPU 1 (process context) attempts write_lock(&tasklist_lock) in
   fork() or exit() and spins, which blocks all new readers.
3. CPU 0 is interrupted by a softirq (e.g., TCP URG packet reception).
4. The softirq calls send_sigurg() and attempts to acquire
   read_lock(&tasklist_lock), deadlocking because CPU 1 is waiting.

Since PID hashing and do_each_pid_task() traversals are already
RCU-protected, the read_lock on tasklist_lock is no longer strictly
required for safe traversal. Fix this by replacing tasklist_lock with
rcu_read_lock(), aligning the process group signaling path with the
single-PID path. This also mitigates a potential remote denial of
service vector via TCP URG packets.

Lockdep splat:
=====================================================
WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
[...]
Chain exists of:
  &dev->event_lock --> &f_owner->lock --> tasklist_lock

Possible interrupt unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(tasklist_lock);
                           local_irq_disable();
                           lock(&dev->event_lock);
                           lock(&f_owner->lock);
  <Interrupt>
    lock(&dev->event_lock);

*** DEADLOCK ***

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
Link: https://patch.msgid.link/20260523135210.590928-1-w15303746062@163.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/fcntl.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=54626335ea4174ab2d9a183b511d825f6765e47b

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

The original Linux kernel CVE announcement for CVE-2026-52946 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52946

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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).