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

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Message-ID: <20250918130945.-7SIRk8Z@linutronix.de>
Date: Thu, 18 Sep 2025 15:09:45 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Cc: 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, peterz@...radead.org, rostedt@...dmis.org,
	rppt@...nel.org, surenb@...gle.com, syzkaller-bugs@...glegroups.com,
	vincent.guittot@...aro.org, vschneid@...hat.com
Subject: [PATCH] futex: Use correct exit on failure from
 futex_hash_allocate_default()

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().

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>
---

That private-futex code was marked BROKEN in v6.16 and re-enabled in
v6.17. It could use
  56180dd20c19e ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t")

as Fixes: instead to avoid backporting to v6.16.

 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index c4ada32598bd5..6ca8689a83b5b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2295,7 +2295,7 @@ __latent_entropy struct task_struct *copy_process(
 	if (need_futex_hash_allocate_default(clone_flags)) {
 		retval = futex_hash_allocate_default();
 		if (retval)
-			goto bad_fork_core_free;
+			goto bad_fork_cancel_cgroup;
 		/*
 		 * If we fail beyond this point we don't free the allocated
 		 * futex hash map. We assume that another thread will be created
-- 
2.51.0