[syzbot] possible deadlock in `mptcp_subflow_create_socket`: `nbd` has a lockdep dependency between `reclaim` and `af_socket`

Hello,

syzbot found the following issue on:

* HEAD commit:    262858079afd Add linux-next specific files for 20250926
* git tree:       linux-next
* console output: https://syzkaller.appspot.com/x/log.txt?x=14f88942580000
* kernel config:  https://syzkaller.appspot.com/x/.config?x=997066872b7941b6
* dashboard link: https://syzkaller.appspot.com/bug?extid=fb2c3fa2ba28aec94627
* compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Unfortunately, no reproducer for this issue yet.

Downloadable assets:
* disk image: https://storage.googleapis.com/syzbot-assets/7b65e4026db8/disk-26285807.raw.xz
* vmlinux: https://storage.googleapis.com/syzbot-assets/30f313e0811e/vmlinux-26285807.xz
* kernel image: https://storage.googleapis.com/syzbot-assets/13fbc9bd375c/bzImage-26285807.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:

    Reported-by: syzbot+fb2c3fa2ba28aec94627@syzkaller.appspotmail.com

Report:

```
netlink: 'syz.4.799': attribute type 1 has an invalid length.
netlink: 'syz.4.799': attribute type 1 has an invalid length.
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.4.799/9142 is trying to acquire lock:
ffffffff8e645260 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:318 [inline]
ffffffff8e645260 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4896 [inline]
ffffffff8e645260 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:5220 [inline]
ffffffff8e645260 (fs_reclaim){+.+.}-{0:0}, at: __kmalloc_cache_noprof+0x40/0x6f0 mm/slub.c:5718

but task is already holding lock:
ffff888030248f18 (k-sk_lock-AF_INET/1){+.+.}-{0:0}, at: mptcp_subflow_create_socket+0x137/0x7d0 net/mptcp/subflow.c:1772

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #7 (k-sk_lock-AF_INET/1){+.+.}-{0:0}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       lock_sock_nested+0x48/0x100 net/core/sock.c:3720
       mptcp_subflow_create_socket+0x137/0x7d0 net/mptcp/subflow.c:1772
       __mptcp_socket_create net/mptcp/protocol.c:99 [inline]
       __mptcp_nmpc_sk+0x148/0x760 net/mptcp/protocol.c:131
       mptcp_connect+0x71/0x830 net/mptcp/protocol.c:3729
       __inet_stream_connect+0x2ae/0xe70 net/ipv4/af_inet.c:679
       inet_stream_connect+0x66/0xa0 net/ipv4/af_inet.c:750
       __sys_connect_file net/socket.c:2089 [inline]
       __sys_connect+0x316/0x440 net/socket.c:2108
       __do_sys_connect net/socket.c:2114 [inline]
       __se_sys_connect net/socket.c:2111 [inline]
       __x64_sys_connect+0x7a/0x90 net/socket.c:2111
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #6 (sk_lock-AF_INET){+.+.}-{0:0}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       lock_sock_nested+0x48/0x100 net/core/sock.c:3720
       lock_sock include/net/sock.h:1679 [inline]
       inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:907
       nbd_mark_nsock_dead+0x2e9/0x560 drivers/block/nbd.c:318
       sock_shutdown+0x15e/0x260 drivers/block/nbd.c:411
       nbd_clear_sock drivers/block/nbd.c:1424 [inline]
       nbd_config_put+0x342/0x790 drivers/block/nbd.c:1448
       nbd_release+0xfe/0x140 drivers/block/nbd.c:1753
       bdev_release+0x536/0x650 block/bdev.c:-1
       blkdev_release+0x15/0x20 block/fops.c:702
       __fput+0x44c/0xa70 fs/file_table.c:468
       task_work_run+0x1d4/0x260 kernel/task_work.c:227
       resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
       exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43
       exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
       syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
       syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
       do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #5 (&nsock->tx_lock){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/mutex.c:598 [inline]
       __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:760
       nbd_handle_cmd drivers/block/nbd.c:1140 [inline]
       nbd_queue_rq+0x257/0xf10 drivers/block/nbd.c:1204
       blk_mq_dispatch_rq_list+0x4c0/0x1900 block/blk-mq.c:2129
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xda4/0x1570 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_hw_queue+0x348/0x4f0 block/blk-mq.c:2367
       blk_mq_dispatch_list+0xd0c/0xe00 include/linux/spinlock.h:-1
       blk_mq_flush_plug_list+0x469/0x550 block/blk-mq.c:2976
       __blk_flush_plug+0x3d3/0x4b0 block/blk-core.c:1225
       blk_finish_plug block/blk-core.c:1252 [inline]
       __submit_bio+0x2d3/0x5a0 block/blk-core.c:651
       __submit_bio_noacct_mq block/blk-core.c:724 [inline]
       submit_bio_noacct_nocheck+0x2fb/0xa50 block/blk-core.c:755
       submit_bh fs/buffer.c:2829 [inline]
       block_read_full_folio+0x599/0x830 fs/buffer.c:2447
       filemap_read_folio+0x117/0x380 mm/filemap.c:2444
       do_read_cache_folio+0x350/0x590 mm/filemap.c:4024
       read_mapping_folio include/linux/pagemap.h:999 [inline]
       read_part_sector+0xb6/0x2b0 block/partitions/core.c:722
       adfspart_check_ICS+0xa4/0xa50 block/partitions/acorn.c:360
       check_partition block/partitions/core.c:141 [inline]
       blk_add_partitions block/partitions/core.c:589 [inline]
       bdev_disk_changed+0x75f/0x14b0 block/partitions/core.c:693
       blkdev_get_whole+0x380/0x510 block/bdev.c:748
       bdev_open+0x31e/0xd30 block/bdev.c:957
       blkdev_open+0x457/0x600 block/fops.c:694
       do_dentry_open+0x953/0x13f0 fs/open.c:965
       vfs_open+0x3b/0x340 fs/open.c:1097
       do_open fs/namei.c:3975 [inline]
       path_openat+0x2ee5/0x3830 fs/namei.c:4134
       do_filp_open+0x1fa/0x410 fs/namei.c:4161
       do_sys_openat2+0x121/0x1c0 fs/open.c:1437
       do_sys_open fs/open.c:1452 [inline]
       __do_sys_openat fs/open.c:1468 [inline]
       __se_sys_openat fs/open.c:1463 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1463
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #4 (&cmd->lock){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/mutex.c:598 [inline]
       __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:760
       nbd_queue_rq+0xc8/0xf10 drivers/block/nbd.c:1196
       blk_mq_dispatch_rq_list+0x4c0/0x1900 block/blk-mq.c:2129
       __blk_mq_do_dispatch_sched block/blk-mq-sched.c:168 [inline]
       blk_mq_do_dispatch_sched block/blk-mq-sched.c:182 [inline]
       __blk_mq_sched_dispatch_requests+0xda4/0x1570 block/blk-mq-sched.c:307
       blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
       blk_mq_run_hw_queue+0x348/0x4f0 block/blk-mq.c:2367
       blk_mq_dispatch_list+0xd0c/0xe00 include/linux/spinlock.h:-1
       blk_mq_flush_plug_list+0x469/0x550 block/blk-mq.c:2976
       __blk_flush_plug+0x3d3/0x4b0 block/blk-core.c:1225
       blk_finish_plug block/blk-core.c:1252 [inline]
       __submit_bio+0x2d3/0x5a0 block/blk-core.c:651
       __submit_bio_noacct_mq block/blk-core.c:724 [inline]
       submit_bio_noacct_nocheck+0x2fb/0xa50 block/blk-core.c:755
       submit_bh fs/buffer.c:2829 [inline]
       block_read_full_folio+0x599/0x830 fs/buffer.c:2447
       filemap_read_folio+0x117/0x380 mm/filemap.c:2444
       do_read_cache_folio+0x350/0x590 mm/filemap.c:4024
       read_mapping_folio include/linux/pagemap.h:999 [inline]
       read_part_sector+0xb6/0x2b0 block/partitions/core.c:722
       adfspart_check_ICS+0xa4/0xa50 block/partitions/acorn.c:360
       check_partition block/partitions/core.c:141 [inline]
       blk_add_partitions block/partitions/core.c:589 [inline]
       bdev_disk_changed+0x75f/0x14b0 block/partitions/core.c:693
       blkdev_get_whole+0x380/0x510 block/bdev.c:748
       bdev_open+0x31e/0xd30 block/bdev.c:957
       blkdev_open+0x457/0x600 block/fops.c:694
       do_dentry_open+0x953/0x13f0 fs/open.c:965
       vfs_open+0x3b/0x340 fs/open.c:1097
       do_open fs/namei.c:3975 [inline]
       path_openat+0x2ee5/0x3830 fs/namei.c:4134
       do_filp_open+0x1fa/0x410 fs/namei.c:4161
       do_sys_openat2+0x121/0x1c0 fs/open.c:1437
       do_sys_open fs/open.c:1452 [inline]
       __do_sys_openat fs/open.c:1468 [inline]
       __se_sys_openat fs/open.c:1463 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1463
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #3 (set->srcu){.+.+}-{0:0}:
       lock_sync+0xba/0x160 kernel/locking/lockdep.c:5916
       srcu_lock_sync include/linux/srcu.h:173 [inline]
       __synchronize_srcu+0x96/0x3a0 kernel/rcu/srcutree.c:1429
       elevator_switch+0x12b/0x640 block/elevator.c:588
       elevator_change+0x315/0x4c0 block/elevator.c:691
       elevator_set_default+0x186/0x260 block/elevator.c:767
       blk_register_queue+0x34e/0x3f0 block/blk-sysfs.c:942
       __add_disk+0x677/0xd50 block/genhd.c:528
       add_disk_fwnode+0xfc/0x480 block/genhd.c:597
       add_disk include/linux/blkdev.h:775 [inline]
       nbd_dev_add+0x717/0xae0 drivers/block/nbd.c:1981
       nbd_init+0x168/0x1f0 drivers/block/nbd.c:2688
       do_one_initcall+0x236/0x820 init/main.c:1283
       do_initcall_level+0x104/0x190 init/main.c:1345
       do_initcalls+0x59/0xa0 init/main.c:1361
       kernel_init_freeable+0x334/0x4b0 init/main.c:1593
       kernel_init+0x1d/0x1d0 init/main.c:1483
       ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 (&q->elevator_lock){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/mutex.c:598 [inline]
       __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:760
       elevator_change+0x1e5/0x4c0 block/elevator.c:689
       elevator_set_none+0x42/0xb0 block/elevator.c:782
       blk_mq_elv_switch_none block/blk-mq.c:5032 [inline]
       __blk_mq_update_nr_hw_queues block/blk-mq.c:5075 [inline]
       blk_mq_update_nr_hw_queues+0x598/0x1ab0 block/blk-mq.c:5133
       nbd_start_device+0x17f/0xb10 drivers/block/nbd.c:1486
       nbd_genl_connect+0x135b/0x18f0 drivers/block/nbd.c:2236
       genl_family_rcv_msg_doit+0x215/0x300 net/netlink/genetlink.c:1115
       genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
       genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
       netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2552
       genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
       netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
       netlink_unicast+0x82f/0x9e0 net/netlink/af_netlink.c:1346
       netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1896
       sock_sendmsg_nosec net/socket.c:714 [inline]
       __sock_sendmsg+0x21c/0x270 net/socket.c:729
       ____sys_sendmsg+0x505/0x830 net/socket.c:2617
       ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2671
       __sys_sendmsg net/socket.c:2703 [inline]
       __do_sys_sendmsg net/socket.c:2708 [inline]
       __se_sys_sendmsg net/socket.c:2706 [inline]
       __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2706
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&q->q_usage_counter(io)#49){++++}-{0:0}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       blk_alloc_queue+0x538/0x620 block/blk-core.c:461
       blk_mq_alloc_queue block/blk-mq.c:4399 [inline]
       __blk_mq_alloc_disk+0x15c/0x340 block/blk-mq.c:4446
       nbd_dev_add+0x46c/0xae0 drivers/block/nbd.c:1951
       nbd_init+0x168/0x1f0 drivers/block/nbd.c:2688
       do_one_initcall+0x236/0x820 init/main.c:1283
       do_initcall_level+0x104/0x190 init/main.c:1345
       do_initcalls+0x59/0xa0 init/main.c:1361
       kernel_init_freeable+0x334/0x4b0 init/main.c:1593
       kernel_init+0x1d/0x1d0 init/main.c:1483
       ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (fs_reclaim){+.+.}-{0:0}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __fs_reclaim_acquire mm/page_alloc.c:4269 [inline]
       fs_reclaim_acquire+0x72/0x100 mm/page_alloc.c:4283
       might_alloc include/linux/sched/mm.h:318 [inline]
       slab_pre_alloc_hook mm/slub.c:4896 [inline]
       slab_alloc_node mm/slub.c:5220 [inline]
       __kmalloc_cache_noprof+0x40/0x6f0 mm/slub.c:5718
       kmalloc_noprof include/linux/slab.h:957 [inline]
       kzalloc_noprof include/linux/slab.h:1094 [inline]
       ref_tracker_alloc+0x133/0x460 lib/ref_tracker.c:271
       __netns_tracker_alloc include/net/net_namespace.h:362 [inline]
       netns_tracker_alloc include/net/net_namespace.h:371 [inline]
       get_net_track include/net/net_namespace.h:388 [inline]
       sk_net_refcnt_upgrade+0x10c/0x1b0 net/core/sock.c:2384
       mptcp_subflow_create_socket+0x31a/0x7d0 net/mptcp/subflow.c:1785
       __mptcp_socket_create net/mptcp/protocol.c:99 [inline]
       __mptcp_nmpc_sk+0x148/0x760 net/mptcp/protocol.c:131
       mptcp_pm_nl_create_listen_socket net/mptcp/pm_kernel.c:662 [inline]
       mptcp_pm_nl_add_addr_doit+0xacb/0x1460 net/mptcp/pm_kernel.c:826
       genl_family_rcv_msg_doit+0x215/0x300 net/netlink/genetlink.c:1115
       genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
       genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
       netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2552
       genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
       netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
       netlink_unicast+0x82f/0x9e0 net/netlink/af_netlink.c:1346
       netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1896
       sock_sendmsg_nosec net/socket.c:714 [inline]
       __sock_sendmsg+0x21c/0x270 net/socket.c:729
       ____sys_sendmsg+0x505/0x830 net/socket.c:2617
       ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2671
       __sys_sendmsg net/socket.c:2703 [inline]
       __do_sys_sendmsg net/socket.c:2708 [inline]
       __se_sys_sendmsg net/socket.c:2706 [inline]
       __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2706
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  fs_reclaim --> sk_lock-AF_INET --> k-sk_lock-AF_INET/1

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(k-sk_lock-AF_INET/1);
                               lock(sk_lock-AF_INET);
                               lock(k-sk_lock-AF_INET/1);
  lock(fs_reclaim);

 *** DEADLOCK ***

4 locks held by syz.4.799/9142:
 #0: ffffffff8f9b93d0 (cb_lock){++++}-{4:4}, at: genl_rcv+0x19/0x40 net/netlink/genetlink.c:1218
 #1: ffffffff8f9b91e8 (genl_mutex){+.+.}-{4:4}, at: genl_lock net/netlink/genetlink.c:35 [inline]
 #1: ffffffff8f9b91e8 (genl_mutex){+.+.}-{4:4}, at: genl_op_lock net/netlink/genetlink.c:60 [inline]
 #1: ffffffff8f9b91e8 (genl_mutex){+.+.}-{4:4}, at: genl_rcv_msg+0x10d/0x790 net/netlink/genetlink.c:1209
 #2: ffff88807928ee58 (msk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
 #2: ffff88807928ee58 (msk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_create_listen_socket net/mptcp/pm_kernel.c:661 [inline]
 #2: ffff88807928ee58 (msk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_add_addr_doit+0xa6c/0x1460 net/mptcp/pm_kernel.c:826
 #3: ffff888030248f18 (k-sk_lock-AF_INET/1){+.+.}-{0:0}, at: mptcp_subflow_create_socket+0x137/0x7d0 net/mptcp/subflow.c:1772

stack backtrace:
CPU: 1 UID: 0 PID: 9142 Comm: syz.4.799 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2043
 check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
 __fs_reclaim_acquire mm/page_alloc.c:4269 [inline]
 fs_reclaim_acquire+0x72/0x100 mm/page_alloc.c:4283
 might_alloc include/linux/sched/mm.h:318 [inline]
 slab_pre_alloc_hook mm/slub.c:4896 [inline]
 slab_alloc_node mm/slub.c:5220 [inline]
 __kmalloc_cache_noprof+0x40/0x6f0 mm/slub.c:5718
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 ref_tracker_alloc+0x133/0x460 lib/ref_tracker.c:271
 __netns_tracker_alloc include/net/net_namespace.h:362 [inline]
 netns_tracker_alloc include/net/net_namespace.h:371 [inline]
 get_net_track include/net/net_namespace.h:388 [inline]
 sk_net_refcnt_upgrade+0x10c/0x1b0 net/core/sock.c:2384
 mptcp_subflow_create_socket+0x31a/0x7d0 net/mptcp/subflow.c:1785
 __mptcp_socket_create net/mptcp/protocol.c:99 [inline]
 __mptcp_nmpc_sk+0x148/0x760 net/mptcp/protocol.c:131
 mptcp_pm_nl_create_listen_socket net/mptcp/pm_kernel.c:662 [inline]
 mptcp_pm_nl_add_addr_doit+0xacb/0x1460 net/mptcp/pm_kernel.c:826
 genl_family_rcv_msg_doit+0x215/0x300 net/netlink/genetlink.c:1115
 genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
 genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
 netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2552
 genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
 netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
 netlink_unicast+0x82f/0x9e0 net/netlink/af_netlink.c:1346
 netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1896
 sock_sendmsg_nosec net/socket.c:714 [inline]
 __sock_sendmsg+0x21c/0x270 net/socket.c:729
 ____sys_sendmsg+0x505/0x830 net/socket.c:2617
 ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2671
 __sys_sendmsg net/socket.c:2703 [inline]
 __do_sys_sendmsg net/socket.c:2708 [inline]
 __se_sys_sendmsg net/socket.c:2706 [inline]
 __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2706
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa199d8eec9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fa19abc5038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fa199fe5fa0 RCX: 00007fa199d8eec9
RDX: 0000000000000000 RSI: 0000200000000400 RDI: 0000000000000003
RBP: 00007fa199e11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa199fe6038 R14: 00007fa199fe5fa0 R15: 00007fff33ff5d78
 </TASK>
netlink: 'syz.4.799': attribute type 1 has an invalid length.
```

As noted by Paolo, it looks like this issue is due to nbd introducing a lockdep dependency between `reclaim` and `af_socket`.

Also see commit 9f7c02e031570e8291a63162c6c046dc15ff85b0

Plus this [thread](https://lore.kernel.org/netdev/68bb4160.050a0220.192772.0198.GAE@google.com/T).

Suggestions from Paolo if a fix is needed:

> I think a possible fix would be changing the lockdep annotation for mptcp sockets, but that in turn is hard to do, either:
>
> 1) in the core/sock_lock_init() 
> 2) after socket creation/cloning but outside the relevant socket lock (we can't change the lockdep annotation for a held lock)
>
> * the first is ugly/will break layering
> * the second is problematic because we can't do it before acquiring the lock for cloned socket, and doing after socket release will not remove the dependency vs the old annotation

But maybe the first one is acceptable for a debugging tool? or adding a new callback just for lockdep?

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[syzbot] possible deadlock in `mptcp_subflow_create_socket`: `nbd` has a lockdep dependency between `reclaim` and `af_socket` #599

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Uh oh!

[syzbot] possible deadlock in mptcp_subflow_create_socket: nbd has a lockdep dependency between reclaim and af_socket #599

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[syzbot] possible deadlock in `mptcp_subflow_create_socket`: `nbd` has a lockdep dependency between `reclaim` and `af_socket` #599