nilfs2: fix potential UAF of struct nilfs_sc_info in nilfs_segctor_thread()

The finalization of nilfs_segctor_thread() can race with
nilfs_segctor_kill_thread() which terminates that thread, potentially
causing a use-after-free BUG as KASAN detected.

At the end of nilfs_segctor_thread(), it assigns NULL to "sc_task" member
of "struct nilfs_sc_info" to indicate the thread has finished, and then
notifies nilfs_segctor_kill_thread() of this using waitqueue
"sc_wait_task" on the struct nilfs_sc_info.

However, here, immediately after the NULL assignment to "sc_task", it is
possible that nilfs_segctor_kill_thread() will detect it and return to
continue the deallocation, freeing the nilfs_sc_info structure before the
thread does the notification.

This fixes the issue by protecting the NULL assignment to "sc_task" and
its notification, with spinlock "sc_state_lock" of the struct
nilfs_sc_info.  Since nilfs_segctor_kill_thread() does a final check to
see if "sc_task" is NULL with "sc_state_lock" locked, this can eliminate
the race.

Link: https://lkml.kernel.org/r/20230327175318.8060-1-konishi.ryusuke@gmail.com
Reported-by: syzbot+b08ebcc22f8f3e6be43a@syzkaller.appspotmail.com
Link: https://lkml.kernel.org/r/00000000000000660d05f7dfa877@google.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/segment.c

@@ -2609,11 +2609,10 @@ static int nilfs_segctor_thread(void *arg)
 	goto loop;
 
  end_thread:
-	spin_unlock(&sci->sc_state_lock);
-
 	/* end sync. */
 	sci->sc_task = NULL;
 	wake_up(&sci->sc_wait_task); /* for nilfs_segctor_kill_thread() */
+	spin_unlock(&sci->sc_state_lock);
 	return 0;
 }