From: Oleg Nesterov Date: Thu, 6 Feb 2025 15:23:14 +0000 (+0100) Subject: exit: change the release_task() paths to call flush_sigqueue() lockless X-Git-Tag: v6.15-rc1~236^2~1^2~1 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fb3bbcfe344e64a46574a638b051ffd78762c12d;p=thirdparty%2Fkernel%2Flinux.git exit: change the release_task() paths to call flush_sigqueue() lockless 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 Link: https://lore.kernel.org/r/20250206152314.GA14620@redhat.com Reviewed-by: Frederic Weisbecker Signed-off-by: Christian Brauner --- diff --git a/kernel/exit.c b/kernel/exit.c index 3485e5fc499e4..2d7444da743dc 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) @@ -279,6 +272,16 @@ repeat: 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;