[CVE-2026-72166][LOW] net/9p: fix infinite loop in p9_client_rpc on fatal signal

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

CVE: CVE-2026-72166
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/9p: fix infinite loop in p9_client_rpc on fatal signal
Commit: 378481cc60a937ef8ea4ef6e4f95f0dbc4e21414
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=378481cc60a937ef8ea4ef6e4f95f0dbc4e21414
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72166
Analysis date: Sat, 15 Aug 2026 04:22:29 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local 9p client signal-handling bug can cause `p9_client_rpc()` to loop indefinitely during `P9_TFLUSH` on an fd transport without a live peer, leading to stuck tasks or stalled coredump/shutdown handling without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72166 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72166

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-72166	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-835;CWE-703;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An infinite loop in the 9p client RPC path can occur when p9_client_rpc processes P9_TFLUSH on an fd transport without a live peer and a fatal signal is delivered to the task. The retry path clears TIF_SIGPENDING and then waits again, so the task can repeatedly lose the pending fatal signal state and remain stuck instead of returning to the caller. A practical trigger described by syzkaller is a multithreaded process where coredump_wait sends SIGKILL to sibling threads and waits for mm_release, while one thread is blocked in the 9p RPC path. For the CVSS the PR:L is used for the paranoid score because a reduced capability root, container root, or delegated mount administration context may be sufficient to create or control the 9p fd transport in some deployments. The issue is local and not directly network reachable. Impact is denial of service through a stuck task, stalled coredump completion, zombie processes, or shutdown delay, with no evidence of memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-72166 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	WARNONLY HARDWARE DEBUGFS 	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Reliable kernel crash / strong DoS: +1
  The bug can leave a task stuck indefinitely in the `P9_TFLUSH` retry loop and can stall `coredump_wait()` or shutdown paths.

* Availability impact realistic: +1 in paranoid scoring
  The impact can persist as a stuck task, zombie process, stalled coredump, or shutdown delay. This is stronger than a transient warning, but still DoS-only.

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2 in conservative scoring
  Creating or controlling a problematic 9p fd transport is usually tied to mount setup or privileged/container management. Ordinary unprivileged users normally cannot set up this path on a default host.

* Rare or configuration-dependent path: -1 conservative consideration
  The issue requires 9p client usage with an fd transport lacking a live peer plus fatal-signal interaction. This is not a broad default kernel data path.

Not triggered:

* No generic memory corruption.
  The patch fixes signal-state handling and an infinite loop, not UAF, OOB, double-free, refcount abuse, or type confusion.

* No LPE signal.
  There is no supported path to privilege escalation or attacker-controlled kernel memory corruption.

* No confidentiality or integrity signal.
  The bug does not expose data or modify protected objects.

* No network reachability.
  This is not remotely packet-triggerable.

Call-site confidence: high.
The relevant loop and fixed condition are directly shown in `p9_client_rpc()`.

## 3. Reachability analysis

The realistic trigger is local and requires a task blocked in the 9p client RPC path using `P9_TFLUSH` over an fd transport without a live peer, followed by a fatal signal. The syzkaller scenario uses coredump handling where `coredump_wait()` sends `SIGKILL` to sibling threads and waits for `mm_release()`.

In typical deployments, setting up the problematic 9p transport is not available to a fully unprivileged user and usually requires mount or container-management privileges. In a paranoid interpretation, a reduced-capability root, container root, or delegated mount environment might make the setup reachable with less than full host-root authority. The path is not default-exposed to the network.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like DoS issue, not an Important-class vulnerability. The theoretical impact is a stuck task or stalled coredump/shutdown path. There is no evidence of memory corruption, file descriptor theft, credential exposure, namespace escape, or privilege escalation.

The paranoid interpretation raises practical concern slightly for delegated/containerized environments, but still keeps the issue below actionable Moderate because the primitive is only an infinite loop / signal-handling DoS.

## 5. One-sentence report phrase

A local 9p client signal-handling bug can cause `p9_client_rpc()` to loop indefinitely during `P9_TFLUSH` on an fd transport without a live peer, leading to stuck tasks or stalled coredump/shutdown handling without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a DoS-only infinite-loop bug with no memory corruption primitive and no realistic remote or privilege-escalation path. Manual review is only useful if the product heavily delegates 9p mount or fd-transport creation to untrusted containers.

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

Patch: net/9p: fix infinite loop in p9_client_rpc on fatal signal
Commit: 378481cc60a937ef8ea4ef6e4f95f0dbc4e21414
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=378481cc60a937ef8ea4ef6e4f95f0dbc4e21414

Commit description:

When p9_client_rpc() is called with type P9_TFLUSH and the transport
has no peer (e.g. fd transport backed by pipes with no 9p server),
a fatal signal causes an infinite loop:

  again:
	err = io_wait_event_killable(req->wq, ...)
	/* SIGKILL wakes the task, returns -ERESTARTSYS */

	if (err == -ERESTARTSYS && c->status == Connected &&
		type == P9_TFLUSH) {
		sigpending = 1;
		clear_thread_flag(TIF_SIGPENDING);
		goto again;
	}

clear_thread_flag() clears TIF_SIGPENDING before jumping back to
io_wait_event_killable(). signal_pending_state() checks TIF_SIGPENDING,
finds it zero, and the task goes to sleep again. The task can only wake
on the next signal delivery that calls signal_wake_up() and sets
TIF_SIGPENDING again. When that happens the loop repeats, clears
TIF_SIGPENDING, and sleeps again indefinitely.

This is triggered in practice by coredump_wait(): when a thread in a
multi-threaded process causes a coredump (e.g. via SIGSYS from Syscall
User Dispatch), coredump_wait() sends SIGKILL to all other threads and
waits for them to call mm_release(). If one of those threads is blocked
in p9_client_rpc() over an fd transport with no peer, it enters the
P9_TFLUSH loop and never calls mm_release(), so coredump_wait() stalls
forever:

INFO: task syz.0.18:676 blocked for more than 143 seconds.
      Not tainted 6.12.77+ #1
task:syz.0.18 state:D stack:27600 pid:676 tgid:673 ppid:630 flags:0x00000004
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5344 [inline]
 __schedule+0xcb4/0x5d50 kernel/sched/core.c:6724
 __schedule_loop kernel/sched/core.c:6801 [inline]
 schedule+0xe5/0x350 kernel/sched/core.c:6816
 schedule_timeout+0x253/0x290 kernel/time/timer.c:2593
 do_wait_for_common kernel/sched/completion.c:95 [inline]
 __wait_for_common+0x409/0x600 kernel/sched/completion.c:116
 wait_for_common kernel/sched/completion.c:127 [inline]
 wait_for_completion_state+0x1d/0x40 kernel/sched/completion.c:264
 coredump_wait fs/coredump.c:448 [inline]
 do_coredump+0x854/0x4350 fs/coredump.c:629
 get_signal+0x1425/0x2730 kernel/signal.c:2903
 arch_do_signal_or_restart+0x81/0x880 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
 syscall_exit_to_user_mode+0xf9/0x160 kernel/entry/common.c:218
 do_syscall_64+0x102/0x220 arch/x86/entry/common.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

Fix: check fatal_signal_pending() before clearing TIF_SIGPENDING in the
P9_TFLUSH retry loop. At that point TIF_SIGPENDING is still set, so
fatal_signal_pending() works correctly. If a fatal signal is pending,
jump to recalc_sigpending to restore TIF_SIGPENDING and return
-ERESTARTSYS to the caller.

The same defect is present in stable kernels back to 5.4. On those
kernels the infinite loop is broken earlier by a second SIGKILL from
the parent process (e.g. kill_and_wait() retrying after a timeout),
resulting in a zombie process and a shutdown delay rather than a
permanent D-state hang, but the underlying flaw is the same.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 91b8534 ("9p: make rpc code common and rework flush code")
Closes: https://syzkaller.appspot.com/bug?extid=3ce7863f8fc836a427e7
Cc: stable@vger.kernel.org
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Message-ID: <20260415155237.182891-1-kovalev@altlinux.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/sched/core.c
  kernel/time/timer.c
  kernel/sched/completion.c
  fs/coredump.c
  kernel/signal.c
  arch/x86/kernel/signal.c
  kernel/entry/common.c
  include/linux/entry-common.h
  arch/x86/entry/common.c
  net/9p/client.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=378481cc60a937ef8ea4ef6e4f95f0dbc4e21414

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

The original Linux kernel CVE announcement for CVE-2026-72166 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72166

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:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).