posix-timers: Plug potential memory leak in do_timer_create()

When posix timer creation is set to allocate a given timer ID and the
access to the user space value faults, the function terminates without
freeing the already allocated posix timer structure.

Move the allocation after the user space access to cure that.

[ tglx: Massaged change log ]

Fixes: ec2d0c0 ("posix-timers: Provide a mechanism to allocate a given timer ID")
Reported-by: syzbot+9c47ad18f978d4394986@syzkaller.appspotmail.com
Suggested-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Eslam Khafagy <eslam.medhat1993@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251114122739.994326-1-eslam.medhat1993@gmail.com
Closes: https://lore.kernel.org/all/69155df4.a70a0220.3124cb.0017.GAE@google.com/T/

Author: eslammedhat

kernel/time/posix-timers.c

@@ -475,12 +475,6 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 	if (!kc->timer_create)
 		return -EOPNOTSUPP;
 
-	new_timer = alloc_posix_timer();
-	if (unlikely(!new_timer))
-		return -EAGAIN;
-
-	spin_lock_init(&new_timer->it_lock);
-
 	/* Special case for CRIU to restore timers with a given timer ID. */
 	if (unlikely(current->signal->timer_create_restore_ids)) {
 		if (copy_from_user(&req_id, created_timer_id, sizeof(req_id)))
@@ -490,6 +484,12 @@ static int do_timer_create(clockid_t which_clock, struct sigevent *event,
 			return -EINVAL;
 	}
 
+	new_timer = alloc_posix_timer();
+	if (unlikely(!new_timer))
+		return -EAGAIN;
+
+	spin_lock_init(&new_timer->it_lock);
+
 	/*
 	 * Add the timer to the hash table. The timer is not yet valid
 	 * after insertion, but has a unique ID allocated.