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.

https://git.kernel.org/stable/c/378481cc60a937ef8ea4ef6e4f95f0dbc4e21414

https://git.kernel.org/stable/c/4f621ae3a2d99b0bac50e8d66cbf7f68323c01e8

https://git.kernel.org/stable/c/6b4f48728faa8bb514368f7eacda05565dea8696

https://git.kernel.org/stable/c/823886a1b089b49bcd349bc8bd3417b7910cd1ac

https://git.kernel.org/stable/c/a8874c34c4a973f9922908a4b8be1d1278f01e42

https://git.kernel.org/stable/c/dc892cbb1e4341d427b1f940ebd6abd69bf8e479

https://git.kernel.org/stable/c/f62a1f245a71680033260a6f6d74011cc3acb3cd

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