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

> A local attacker can cause a system hang by triggering an infinite loop in the 9p client RPC handler during coredump operations.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 23 Aug 2026
- **Also known as:** BELL-CVE-2026-72166, DEBIAN-CVE-2026-72166, GHSA-m993-rm7r-56rw, UBUNTU-CVE-2026-72166

## What is CVE-2026-72166?

CVE-2026-72166 is a denial of service vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

net/9p: fix infinite loop in p9_client_rpc on fatal signal

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.

## Which versions are affected by CVE-2026-72166?


## How do I fix CVE-2026-72166?

Apply the vendor's patch.

## Is CVE-2026-72166 being exploited?

There is no known exploitation of CVE-2026-72166 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72166 and fix it if it is affected.

What it is: CVE-2026-72166 is a denial of service vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-72166 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-72166/ (Markdown: https://insights.sbomify.com/CVE-2026-72166/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72166
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72166
- OSV: https://osv.dev/vulnerability/CVE-2026-72166

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72166/
Summaries are generated and may contain mistakes; the official description above is the record.