CVE-2026-52946

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 ***