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Date: Wed, 24 Apr 2024 14:35:19 +0200
From: Michal Luczaj <mhal@...x.co>
To: Kuniyuki Iwashima <kuniyu@...zon.com>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
syzbot+fa379358c28cc87cc307@...kaller.appspotmail.com
Subject: Re: [PATCH v2 net] af_unix: Suppress false-positive lockdep splat for
spin_lock() in __unix_gc().
On 4/24/24 04:23, Kuniyuki Iwashima wrote:
> syzbot reported a lockdep splat regarding unix_gc_lock() and
> unix_state_lock().
Just a nit: probably unix_gc_lock, without brackets?
Thanks!
Michal
> 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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=fa379358c28cc87cc307
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
> include/net/af_unix.h | 3 +++
> net/unix/garbage.c | 2 +-
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index 627ea8e2d915..3dee0b2721aa 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -85,6 +85,9 @@ 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 6433a414acf8..0104be9d4704 100644
> --- a/net/unix/garbage.c
> +++ b/net/unix/garbage.c
> @@ -299,7 +299,7 @@ static void __unix_gc(struct work_struct *work)
> __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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