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

fs/fcntl: fix SOFTIRQ-unsafe lock order in fasync signaling

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 ***

https://git.kernel.org/stable/c/1bee417678f1135e35b25a37734db46aa94258d2

https://git.kernel.org/stable/c/20a93e397abe850c49b6fa0e8cc827b5f634a8f5

https://git.kernel.org/stable/c/32dbd5ce4be3a3ed7e00f8af18795cc84fc50a33

https://git.kernel.org/stable/c/36c1b57b2ecf3c61ac93f5f07bd29b6f21e226ed

https://git.kernel.org/stable/c/54626335ea4174ab2d9a183b511d825f6765e47b

https://git.kernel.org/stable/c/897d6a7247739fb1528f98c575df4f2e5de7f994

https://git.kernel.org/stable/c/b5fa9e32fb6718f70c986ee14dd5d01b4846f331

https://git.kernel.org/stable/c/bfcc8e8d8a495bb34cae9e620adfb75fb13a3954

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