Re: [syzbot] [net?] possible deadlock in unix_del_edges

[syzbot <syzbot+7f7f201cc2668a8fd169@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    2b3d5988ae2c Add linux-next specific files for 20240404
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13114d8d180000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9c48fd2523cdee5e
dashboard link: https://syzkaller.appspot.com/bug?extid=7f7f201cc2668a8fd169
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=113c7103180000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1133aaa9180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/136270ed2c7b/disk-2b3d5988.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/466d2f7c1952/vmlinux-2b3d5988.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7dfaf3959891/bzImage-2b3d5988.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7f7f201cc2668a8fd169@syzkaller.appspotmail.com

============================================
WARNING: possible recursive locking detected
6.9.0-rc2-next-20240404-syzkaller #0 Not tainted
--------------------------------------------
kworker/u8:3/51 is trying to acquire lock:
ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: unix_del_edges+0x30/0x590 net/unix/garbage.c:227

but task is already holding lock:
ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: __unix_gc+0xc5/0x1830 net/unix/garbage.c:547

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(unix_gc_lock);
  lock(unix_gc_lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

4 locks held by kworker/u8:3/51:
 #0: ffff888015089148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3193 [inline]
 #0: ffff888015089148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_scheduled_works+0x90a/0x1830 kernel/workqueue.c:3299
 #1: ffffc90000bb7d00 (unix_gc_work){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3194 [inline]
 #1: ffffc90000bb7d00 (unix_gc_work){+.+.}-{0:0}, at: process_scheduled_works+0x945/0x1830 kernel/workqueue.c:3299
 #2: ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
 #2: ffffffff8f6dc178 (unix_gc_lock){+.+.}-{2:2}, at: __unix_gc+0xc5/0x1830 net/unix/garbage.c:547
 #3: ffff88802bd76118 (rlock-AF_UNIX){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
 #3: ffff88802bd76118 (rlock-AF_UNIX){+.+.}-{2:2}, at: unix_collect_skb+0xb8/0x700 net/unix/garbage.c:343

stack backtrace:
CPU: 0 PID: 51 Comm: kworker/u8:3 Not tainted 6.9.0-rc2-next-20240404-syzkaller #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_deadlock kernel/locking/lockdep.c:3062 [inline]
 validate_chain+0x15c1/0x58e0 kernel/locking/lockdep.c:3856
 __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_del_edges+0x30/0x590 net/unix/garbage.c:227
 unix_destroy_fpl+0x59/0x210 net/unix/garbage.c:286
 unix_detach_fds net/unix/af_unix.c:1816 [inline]
 unix_destruct_scm+0x13e/0x210 net/unix/af_unix.c:1873
 skb_release_head_state+0x100/0x250 net/core/skbuff.c:1162
 skb_release_all net/core/skbuff.c:1173 [inline]
 __kfree_skb net/core/skbuff.c:1189 [inline]
 kfree_skb_reason+0x16d/0x3b0 net/core/skbuff.c:1225
 kfree_skb include/linux/skbuff.h:1262 [inline]
 unix_collect_skb+0x5e4/0x700 net/unix/garbage.c:361
 __unix_walk_scc net/unix/garbage.c:481 [inline]
 unix_walk_scc net/unix/garbage.c:506 [inline]
 __unix_gc+0x108c/0x1830 net/unix/garbage.c:559
 process_one_work kernel/workqueue.c:3218 [inline]
 process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3299
 worker_thread+0x86d/0xd70 kernel/workqueue.c:3380
 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:243
 </TASK>


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