linux-kernel - [PATCH AUTOSEL 5.15 133/153] exit: change the release_task() paths to call flush

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Message-Id: <20250505231320.2695319-133-sashal@kernel.org>
Date: Mon,  5 May 2025 19:13:00 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	akpm@...ux-foundation.org,
	mhocko@...e.com,
	Liam.Howlett@...cle.com,
	mjguzik@...il.com,
	pasha.tatashin@...een.com,
	alexjlzheng@...cent.com
Subject: [PATCH AUTOSEL 5.15 133/153] exit: change the release_task() paths to call flush_sigqueue() lockless

From: Oleg Nesterov <oleg@...hat.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@...hat.com>
Link: https://lore.kernel.org/r/20250206152314.GA14620@redhat.com
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/exit.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 04fc65f2b690d..78bfd3aa2bdf8 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -195,20 +195,13 @@ 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)
@@ -272,6 +265,16 @@ 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;
-- 
2.39.5