From: Oleg Nesterov oleg@redhat.com
[ Upstream commit fb3bbcfe344e64a46574a638b051ffd78762c12d ]
A task can block a signal, accumulate up to RLIMIT_SIGPENDING sigqueues, and exit. In this case __exit_signal()->flush_sigqueue() called with irqs disabled can trigger a hard lockup, see https://lore.kernel.org/all/20190322114917.GC28876@redhat.com/
Fortunately, after the recent posixtimer changes sys_timer_delete() paths no longer try to clear SIGQUEUE_PREALLOC and/or free tmr->sigq, and after the exiting task passes __exit_signal() lock_task_sighand() can't succeed and pid_task(tmr->it_pid) will return NULL.
This means that after __exit_signal(tsk) nobody can play with tsk->pending or (if group_dead) with tsk->signal->shared_pending, so release_task() can safely call flush_sigqueue() after write_unlock_irq(&tasklist_lock).
TODO: - we can probably shift posix_cpu_timers_exit() as well - do_sigaction() can hit the similar problem
Signed-off-by: Oleg Nesterov oleg@redhat.com Link: https://lore.kernel.org/r/20250206152314.GA14620@redhat.com Reviewed-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/exit.c | 19 +++++++++++-------- 1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c index a9960dd6d0aa0..a345aa75eb7ff 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -200,20 +200,13 @@ static void __exit_signal(struct task_struct *tsk) __unhash_process(tsk, group_dead); write_sequnlock(&sig->stats_lock);
- /* - * Do this under ->siglock, we can race with another thread - * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals. - */ - flush_sigqueue(&tsk->pending); tsk->sighand = NULL; spin_unlock(&sighand->siglock);
__cleanup_sighand(sighand); clear_tsk_thread_flag(tsk, TIF_SIGPENDING); - if (group_dead) { - flush_sigqueue(&sig->shared_pending); + if (group_dead) tty_kref_put(tty); - } }
static void delayed_put_task_struct(struct rcu_head *rhp) @@ -282,6 +275,16 @@ void release_task(struct task_struct *p) proc_flush_pid(thread_pid); put_pid(thread_pid); release_thread(p); + /* + * This task was already removed from the process/thread/pid lists + * and lock_task_sighand(p) can't succeed. Nobody else can touch + * ->pending or, if group dead, signal->shared_pending. We can call + * flush_sigqueue() lockless. + */ + flush_sigqueue(&p->pending); + if (thread_group_leader(p)) + flush_sigqueue(&p->signal->shared_pending); + put_task_struct_rcu_user(p);
p = leader;