linux-kernel - Re: [PATCH] futex: Use correct exit on failure from futex_hash_allocate

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Message-ID: <20250918113022.1b97d977@gandalf.local.home>
Date: Thu, 18 Sep 2025 11:30:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>, Thomas Gleixner <tglx@...utronix.de>,
 Peter Zijlstra <peterz@...radead.org>, syzbot
 <syzbot+80cb3cc5c14fad191a10@...kaller.appspotmail.com>,
 Liam.Howlett@...cle.com, akpm@...ux-foundation.org, bsegall@...gle.com,
 david@...hat.com, dietmar.eggemann@....com, juri.lelli@...hat.com,
 kees@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, mgorman@...e.de, mhocko@...e.com,
 mingo@...hat.com, rppt@...nel.org, surenb@...gle.com,
 syzkaller-bugs@...glegroups.com, vincent.guittot@...aro.org,
 vschneid@...hat.com
Subject: Re: [PATCH] futex: Use correct exit on failure from
 futex_hash_allocate_default()

On Thu, 18 Sep 2025 15:09:45 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> copy_process() uses the wrong error exit path from
> futex_hash_allocate_default().
> After exiting from futex_hash_allocate_default(), neither tasklist_lock
> nor siglock has been acquired. The exit label bad_fork_core_free unlocks
> both of these locks which is wrong.
> 
> The previous label, bad_fork_cancel_cgroup, is the correct exit.
> sched_cgroup_fork() did not allocate any resources that need to freed.
> 
> Use bad_fork_cancel_cgroup on error exit from
> futex_hash_allocate_default().

	if (need_futex_hash_allocate_default(clone_flags)) {
		retval = futex_hash_allocate_default();
		if (retval)
			goto bad_fork_core_free;
		[..]
	}
	[..]
	write_lock_irq(&tasklist_lock);
	[..]
	klp_copy_process(p);

	sched_core_fork(p);

	spin_lock(&current->sighand->siglock);

	[..]

 bad_fork_core_free:
	sched_core_free(p);
	spin_unlock(&current->sighand->siglock);
	write_unlock_irq(&tasklist_lock);
 bad_fork_cancel_cgroup:
	cgroup_cancel_fork(p, args);

Yep, looks bad to me!

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve


> 
> Fixes: 7c4f75a21f636 ("futex: Allow automatic allocation of process wide futex hash")
> Reported-by: syzbot+80cb3cc5c14fad191a10@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68cb1cbd.050a0220.2ff435.0599.GAE@google.com
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>