Patch "af_unix: Suppress false-positive lockdep splat for spin_lock() in __unix_gc()." has been added to the 5.4-stable tree

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This is a note to let you know that I've just added the patch titled

    af_unix: Suppress false-positive lockdep splat for spin_lock() in __unix_gc().

to the 5.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     af_unix-suppress-false-positive-lockdep-splat-for-sp.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f165bae1a9d0b8b0f64a0c9e17f41093b177406e
Author: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Date:   Wed Apr 24 10:04:43 2024 -0700

    af_unix: Suppress false-positive lockdep splat for spin_lock() in __unix_gc().
    
    [ Upstream commit 1971d13ffa84a551d29a81fdf5b5ec5be166ac83 ]
    
    syzbot reported a lockdep splat regarding unix_gc_lock and
    unix_state_lock().
    
    One is called from recvmsg() for a connected socket, and another
    is called from GC for TCP_LISTEN socket.
    
    So, the splat is false-positive.
    
    Let's add a dedicated lock class for the latter to suppress the splat.
    
    Note that this change is not necessary for net-next.git as the issue
    is only applied to the old GC impl.
    
    [0]:
    WARNING: possible circular locking dependency detected
    6.9.0-rc5-syzkaller-00007-g4d2008430ce8 #0 Not tainted
     -----------------------------------------------------
    kworker/u8:1/11 is trying to acquire lock:
    ffff88807cea4e70 (&u->lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
    ffff88807cea4e70 (&u->lock){+.+.}-{2:2}, at: __unix_gc+0x40e/0xf70 net/unix/garbage.c:302
    
    but task is already holding lock:
    ffffffff8f6ab638 (unix_gc_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
    ffffffff8f6ab638 (unix_gc_lock){+.+.}-{2:2}, at: __unix_gc+0x117/0xf70 net/unix/garbage.c:261
    
    which lock already depends on the new lock.
    
    the existing dependency chain (in reverse order) is:
    
     -> #1 (unix_gc_lock){+.+.}-{2:2}:
           lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
           __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
           _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
           spin_lock include/linux/spinlock.h:351 [inline]
           unix_notinflight+0x13d/0x390 net/unix/garbage.c:140
           unix_detach_fds net/unix/af_unix.c:1819 [inline]
           unix_destruct_scm+0x221/0x350 net/unix/af_unix.c:1876
           skb_release_head_state+0x100/0x250 net/core/skbuff.c:1188
           skb_release_all net/core/skbuff.c:1200 [inline]
           __kfree_skb net/core/skbuff.c:1216 [inline]
           kfree_skb_reason+0x16d/0x3b0 net/core/skbuff.c:1252
           kfree_skb include/linux/skbuff.h:1262 [inline]
           manage_oob net/unix/af_unix.c:2672 [inline]
           unix_stream_read_generic+0x1125/0x2700 net/unix/af_unix.c:2749
           unix_stream_splice_read+0x239/0x320 net/unix/af_unix.c:2981
           do_splice_read fs/splice.c:985 [inline]
           splice_file_to_pipe+0x299/0x500 fs/splice.c:1295
           do_splice+0xf2d/0x1880 fs/splice.c:1379
           __do_splice fs/splice.c:1436 [inline]
           __do_sys_splice fs/splice.c:1652 [inline]
           __se_sys_splice+0x331/0x4a0 fs/splice.c:1634
           do_syscall_x64 arch/x86/entry/common.c:52 [inline]
           do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
           entry_SYSCALL_64_after_hwframe+0x77/0x7f
    
     -> #0 (&u->lock){+.+.}-{2:2}:
           check_prev_add kernel/locking/lockdep.c:3134 [inline]
           check_prevs_add kernel/locking/lockdep.c:3253 [inline]
           validate_chain+0x18cb/0x58e0 kernel/locking/lockdep.c:3869
           __lock_acquire+0x1346/0x1fd0 kernel/locking/lockdep.c:5137
           lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
           __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
           _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
           spin_lock include/linux/spinlock.h:351 [inline]
           __unix_gc+0x40e/0xf70 net/unix/garbage.c:302
           process_one_work kernel/workqueue.c:3254 [inline]
           process_scheduled_works+0xa10/0x17c0 kernel/workqueue.c:3335
           worker_thread+0x86d/0xd70 kernel/workqueue.c:3416
           kthread+0x2f0/0x390 kernel/kthread.c:388
           ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
           ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
    
    other info that might help us debug this:
    
     Possible unsafe locking scenario:
    
           CPU0                    CPU1
           ----                    ----
      lock(unix_gc_lock);
                                   lock(&u->lock);
                                   lock(unix_gc_lock);
      lock(&u->lock);
    
     *** DEADLOCK ***
    
    3 locks held by kworker/u8:1/11:
     #0: ffff888015089148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3229 [inline]
     #0: ffff888015089148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_scheduled_works+0x8e0/0x17c0 kernel/workqueue.c:3335
     #1: ffffc90000107d00 (unix_gc_work){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3230 [inline]
     #1: ffffc90000107d00 (unix_gc_work){+.+.}-{0:0}, at: process_scheduled_works+0x91b/0x17c0 kernel/workqueue.c:3335
     #2: ffffffff8f6ab638 (unix_gc_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
     #2: ffffffff8f6ab638 (unix_gc_lock){+.+.}-{2:2}, at: __unix_gc+0x117/0xf70 net/unix/garbage.c:261
    
    stack backtrace:
    CPU: 0 PID: 11 Comm: kworker/u8:1 Not tainted 6.9.0-rc5-syzkaller-00007-g4d2008430ce8 #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
    Workqueue: events_unbound __unix_gc
    Call Trace:
     <TASK>
     __dump_stack lib/dump_stack.c:88 [inline]
     dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
     check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2187
     check_prev_add kernel/locking/lockdep.c:3134 [inline]
     check_prevs_add kernel/locking/lockdep.c:3253 [inline]
     validate_chain+0x18cb/0x58e0 kernel/locking/lockdep.c:3869
     __lock_acquire+0x1346/0x1fd0 kernel/locking/lockdep.c:5137
     lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
     __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
     _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
     spin_lock include/linux/spinlock.h:351 [inline]
     __unix_gc+0x40e/0xf70 net/unix/garbage.c:302
     process_one_work kernel/workqueue.c:3254 [inline]
     process_scheduled_works+0xa10/0x17c0 kernel/workqueue.c:3335
     worker_thread+0x86d/0xd70 kernel/workqueue.c:3416
     kthread+0x2f0/0x390 kernel/kthread.c:388
     ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
     ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
     </TASK>
    
    Fixes: 47d8ac011fe1 ("af_unix: Fix garbage collector racing against connect()")
    Reported-and-tested-by: syzbot+fa379358c28cc87cc307@xxxxxxxxxxxxxxxxxxxxxxxxx
    Closes: https://syzkaller.appspot.com/bug?extid=fa379358c28cc87cc307
    Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240424170443.9832-1-kuniyu@xxxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index e7612295b2626..96765349d6f11 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -72,6 +72,9 @@ enum unix_socket_lock_class {
 	U_LOCK_NORMAL,
 	U_LOCK_SECOND,	/* for double locking, see unix_state_double_lock(). */
 	U_LOCK_DIAG, /* used while dumping icons, see sk_diag_dump_icons(). */
+	U_LOCK_GC_LISTENER, /* used for listening socket while determining gc
+			     * candidates to close a small race window.
+			     */
 };
 
 static inline void unix_state_lock_nested(struct sock *sk,
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 58525311e903a..133ba5be4b580 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -260,7 +260,7 @@ void unix_gc(void)
 			__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
 
 			if (sk->sk_state == TCP_LISTEN) {
-				unix_state_lock(sk);
+				unix_state_lock_nested(sk, U_LOCK_GC_LISTENER);
 				unix_state_unlock(sk);
 			}
 		}




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