Re: [syzbot] [mptcp?] possible deadlock in mptcp_subflow_shutdown (2)

[syzbot <syzbot+7902f127c28c701e913f@syzkaller.appspotmail.com>]

> Hello,
>
> On 10/11/2025 09:41, Paolo Abeni wrote:
>> On 11/10/25 2:29 AM, syzbot wrote:
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    dc77806cf3b4 Merge tag 'rust-fixes-6.18' of git://git.kern..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=17dd9bcd980000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=19d831c6d0386a9c
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=7902f127c28c701e913f
>>> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>>>
>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>
>>> Downloadable assets:
>>> disk image: https://storage.googleapis.com/syzbot-assets/a1c9259ca92c/disk-dc77806c.raw.xz
>>> vmlinux: https://storage.googleapis.com/syzbot-assets/98d084f2ad8b/vmlinux-dc77806c.xz
>>> kernel image: https://storage.googleapis.com/syzbot-assets/c25e628e3491/bzImage-dc77806c.xz
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+7902f127c28c701e913f@syzkaller.appspotmail.com
>>>
>>> ======================================================
>>> WARNING: possible circular locking dependency detected
>>> syzkaller #0 Not tainted
>>> ------------------------------------------------------
>>> syz.7.3695/23717 is trying to acquire lock:
>>> ffff888087316860 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
>>> ffff888087316860 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_subflow_shutdown+0x24/0x380 net/mptcp/protocol.c:2918
>>>
>>> but task is already holding lock:
>>> ffff888026899a60 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
>>> ffff888026899a60 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x1d/0xe0 net/mptcp/protocol.c:3168
>>>
>>> which lock already depends on the new lock.
>>>
>>>
>>> the existing dependency chain (in reverse order) is:
>>>
>>> -> #7 (sk_lock-AF_INET){+.+.}-{0:0}:
>>>        lock_sock_nested+0x41/0xf0 net/core/sock.c:3720
>>>        lock_sock include/net/sock.h:1679 [inline]
>>>        inet_shutdown+0x67/0x440 net/ipv4/af_inet.c:907
>>>        nbd_mark_nsock_dead+0xae/0x5d0 drivers/block/nbd.c:319
>>>        recv_work+0x671/0xa80 drivers/block/nbd.c:1024
>>>        process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
>>>        process_scheduled_works kernel/workqueue.c:3346 [inline]
>>>        worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
>>>        kthread+0x3c5/0x780 kernel/kthread.c:463
>>>        ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
>>>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>>>
>>> -> #6 (&nsock->tx_lock){+.+.}-{4:4}:
>>>        __mutex_lock_common kernel/locking/mutex.c:598 [inline]
>>>        __mutex_lock+0x193/0x1060 kernel/locking/mutex.c:760
>>>        nbd_handle_cmd drivers/block/nbd.c:1146 [inline]
>>>        nbd_queue_rq+0x423/0x12d0 drivers/block/nbd.c:1210
>>>        blk_mq_dispatch_rq_list+0x416/0x1e20 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+0xcb7/0x15f0 block/blk-mq-sched.c:307
>>>        blk_mq_sched_dispatch_requests+0xd8/0x1b0 block/blk-mq-sched.c:329
>>>        blk_mq_run_hw_queue+0x239/0x670 block/blk-mq.c:2367
>>>        blk_mq_dispatch_list+0x514/0x1310 block/blk-mq.c:2928
>>>        blk_mq_flush_plug_list block/blk-mq.c:2976 [inline]
>>>        blk_mq_flush_plug_list+0x130/0x600 block/blk-mq.c:2948
>>>        __blk_flush_plug+0x2c4/0x4b0 block/blk-core.c:1225
>>>        blk_finish_plug block/blk-core.c:1252 [inline]
>>>        blk_finish_plug block/blk-core.c:1249 [inline]
>>>        __submit_bio+0x545/0x690 block/blk-core.c:651
>>>        __submit_bio_noacct_mq block/blk-core.c:724 [inline]
>>>        submit_bio_noacct_nocheck+0x53d/0xc10 block/blk-core.c:755
>>>        submit_bio_noacct+0x5bd/0x1f60 block/blk-core.c:879
>>>        submit_bh fs/buffer.c:2829 [inline]
>>>        block_read_full_folio+0x4db/0x850 fs/buffer.c:2461
>>>        filemap_read_folio+0xc8/0x2a0 mm/filemap.c:2444
>>>        do_read_cache_folio+0x263/0x5c0 mm/filemap.c:4024
>>>        read_mapping_folio include/linux/pagemap.h:999 [inline]
>>>        read_part_sector+0xd4/0x370 block/partitions/core.c:722
>>>        adfspart_check_ICS+0x93/0x940 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+0x723/0x1520 block/partitions/core.c:693
>>>        blkdev_get_whole+0x187/0x290 block/bdev.c:748
>>>        bdev_open+0x2c7/0xe40 block/bdev.c:957
>>>        blkdev_open+0x34e/0x4f0 block/fops.c:701
>>>        do_dentry_open+0x982/0x1530 fs/open.c:965
>>>        vfs_open+0x82/0x3f0 fs/open.c:1097
>>>        do_open fs/namei.c:3975 [inline]
>>>        path_openat+0x1de4/0x2cb0 fs/namei.c:4134
>>>        do_filp_open+0x20b/0x470 fs/namei.c:4161
>>>        do_sys_openat2+0x11b/0x1d0 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+0x174/0x210 fs/open.c:1463
>>>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>>        do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>>>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> -> #5 (&cmd->lock){+.+.}-{4:4}:
>>>        __mutex_lock_common kernel/locking/mutex.c:598 [inline]
>>>        __mutex_lock+0x193/0x1060 kernel/locking/mutex.c:760
>>>        nbd_queue_rq+0xbd/0x12d0 drivers/block/nbd.c:1202
>>>        blk_mq_dispatch_rq_list+0x416/0x1e20 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+0xcb7/0x15f0 block/blk-mq-sched.c:307
>>>        blk_mq_sched_dispatch_requests+0xd8/0x1b0 block/blk-mq-sched.c:329
>>>        blk_mq_run_hw_queue+0x239/0x670 block/blk-mq.c:2367
>>>        blk_mq_dispatch_list+0x514/0x1310 block/blk-mq.c:2928
>>>        blk_mq_flush_plug_list block/blk-mq.c:2976 [inline]
>>>        blk_mq_flush_plug_list+0x130/0x600 block/blk-mq.c:2948
>>>        __blk_flush_plug+0x2c4/0x4b0 block/blk-core.c:1225
>>>        blk_finish_plug block/blk-core.c:1252 [inline]
>>>        blk_finish_plug block/blk-core.c:1249 [inline]
>>>        __submit_bio+0x545/0x690 block/blk-core.c:651
>>>        __submit_bio_noacct_mq block/blk-core.c:724 [inline]
>>>        submit_bio_noacct_nocheck+0x53d/0xc10 block/blk-core.c:755
>>>        submit_bio_noacct+0x5bd/0x1f60 block/blk-core.c:879
>>>        submit_bh fs/buffer.c:2829 [inline]
>>>        block_read_full_folio+0x4db/0x850 fs/buffer.c:2461
>>>        filemap_read_folio+0xc8/0x2a0 mm/filemap.c:2444
>>>        do_read_cache_folio+0x263/0x5c0 mm/filemap.c:4024
>>>        read_mapping_folio include/linux/pagemap.h:999 [inline]
>>>        read_part_sector+0xd4/0x370 block/partitions/core.c:722
>>>        adfspart_check_ICS+0x93/0x940 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+0x723/0x1520 block/partitions/core.c:693
>>>        blkdev_get_whole+0x187/0x290 block/bdev.c:748
>>>        bdev_open+0x2c7/0xe40 block/bdev.c:957
>>>        blkdev_open+0x34e/0x4f0 block/fops.c:701
>>>        do_dentry_open+0x982/0x1530 fs/open.c:965
>>>        vfs_open+0x82/0x3f0 fs/open.c:1097
>>>        do_open fs/namei.c:3975 [inline]
>>>        path_openat+0x1de4/0x2cb0 fs/namei.c:4134
>>>        do_filp_open+0x20b/0x470 fs/namei.c:4161
>>>        do_sys_openat2+0x11b/0x1d0 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+0x174/0x210 fs/open.c:1463
>>>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>>        do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>>>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> -> #4 (set->srcu){.+.+}-{0:0}:
>>>        srcu_lock_sync include/linux/srcu.h:173 [inline]
>>>        __synchronize_srcu+0xa1/0x290 kernel/rcu/srcutree.c:1439
>>>        blk_mq_wait_quiesce_done block/blk-mq.c:283 [inline]
>>>        blk_mq_wait_quiesce_done block/blk-mq.c:280 [inline]
>>>        blk_mq_quiesce_queue block/blk-mq.c:303 [inline]
>>>        blk_mq_quiesce_queue+0x149/0x1b0 block/blk-mq.c:298
>>>        elevator_switch+0x17d/0x810 block/elevator.c:588
>>>        elevator_change+0x391/0x5d0 block/elevator.c:691
>>>        elevator_set_default+0x2e9/0x380 block/elevator.c:767
>>>        blk_register_queue+0x384/0x4e0 block/blk-sysfs.c:942
>>>        __add_disk+0x74a/0xf00 block/genhd.c:528
>>>        add_disk_fwnode+0x13f/0x5d0 block/genhd.c:597
>>>        add_disk include/linux/blkdev.h:775 [inline]
>>>        nbd_dev_add+0x783/0xbb0 drivers/block/nbd.c:1987
>>>        nbd_init+0x1a2/0x3c0 drivers/block/nbd.c:2702
>>>        do_one_initcall+0x123/0x6e0 init/main.c:1283
>>>        do_initcall_level init/main.c:1345 [inline]
>>>        do_initcalls init/main.c:1361 [inline]
>>>        do_basic_setup init/main.c:1380 [inline]
>>>        kernel_init_freeable+0x5c8/0x920 init/main.c:1593
>>>        kernel_init+0x1c/0x2b0 init/main.c:1483
>>>        ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
>>>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>>>
>>> -> #3 (&q->elevator_lock){+.+.}-{4:4}:
>>>        __mutex_lock_common kernel/locking/mutex.c:598 [inline]
>>>        __mutex_lock+0x193/0x1060 kernel/locking/mutex.c:760
>>>        queue_requests_store+0x3a7/0x670 block/blk-sysfs.c:117
>>>        queue_attr_store+0x26b/0x310 block/blk-sysfs.c:869
>>>        sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:142
>>>        kernfs_fop_write_iter+0x3af/0x570 fs/kernfs/file.c:352
>>>        new_sync_write fs/read_write.c:593 [inline]
>>>        vfs_write+0x7d3/0x11d0 fs/read_write.c:686
>>>        ksys_write+0x12a/0x250 fs/read_write.c:738
>>>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>>        do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>>>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> -> #2 (&q->q_usage_counter(io)#52){++++}-{0:0}:
>>>        blk_alloc_queue+0x619/0x760 block/blk-core.c:461
>>>        blk_mq_alloc_queue+0x172/0x280 block/blk-mq.c:4399
>>>        __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4446
>>>        nbd_dev_add+0x492/0xbb0 drivers/block/nbd.c:1957
>>>        nbd_init+0x1a2/0x3c0 drivers/block/nbd.c:2702
>>>        do_one_initcall+0x123/0x6e0 init/main.c:1283
>>>        do_initcall_level init/main.c:1345 [inline]
>>>        do_initcalls init/main.c:1361 [inline]
>>>        do_basic_setup init/main.c:1380 [inline]
>>>        kernel_init_freeable+0x5c8/0x920 init/main.c:1593
>>>        kernel_init+0x1c/0x2b0 init/main.c:1483
>>>        ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
>>>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>>>
>>> -> #1 (fs_reclaim){+.+.}-{0:0}:
>>>        __fs_reclaim_acquire mm/page_alloc.c:4269 [inline]
>>>        fs_reclaim_acquire+0x102/0x150 mm/page_alloc.c:4283
>>>        might_alloc include/linux/sched/mm.h:318 [inline]
>>>        slab_pre_alloc_hook mm/slub.c:4921 [inline]
>>>        slab_alloc_node mm/slub.c:5256 [inline]
>>>        __kmalloc_cache_noprof+0x58/0x780 mm/slub.c:5758
>>>        kmalloc_noprof include/linux/slab.h:957 [inline]
>>>        kzalloc_noprof include/linux/slab.h:1094 [inline]
>>>        ref_tracker_alloc+0x18e/0x5b0 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+0x141/0x1e0 net/core/sock.c:2384
>>>        rds_tcp_tune+0x23d/0x530 net/rds/tcp.c:507
>>>        rds_tcp_conn_path_connect+0x305/0x7f0 net/rds/tcp_connect.c:127
>>>        rds_connect_worker+0x1af/0x2c0 net/rds/threads.c:176
>>>        process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
>>>        process_scheduled_works kernel/workqueue.c:3346 [inline]
>>>        worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
>>>        kthread+0x3c5/0x780 kernel/kthread.c:463
>>>        ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
>>>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>>>
>>> -> #0 (k-sk_lock-AF_INET){+.+.}-{0:0}:
>>>        check_prev_add kernel/locking/lockdep.c:3165 [inline]
>>>        check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>>>        validate_chain kernel/locking/lockdep.c:3908 [inline]
>>>        __lock_acquire+0x126f/0x1c90 kernel/locking/lockdep.c:5237
>>>        lock_acquire kernel/locking/lockdep.c:5868 [inline]
>>>        lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5825
>>>        lock_sock_nested+0x41/0xf0 net/core/sock.c:3720
>>>        lock_sock include/net/sock.h:1679 [inline]
>>>        mptcp_subflow_shutdown+0x24/0x380 net/mptcp/protocol.c:2918
>>>        mptcp_check_send_data_fin+0x248/0x440 net/mptcp/protocol.c:3022
>>>        __mptcp_close+0x90e/0xbe0 net/mptcp/protocol.c:3116
>>>        mptcp_close+0x28/0xe0 net/mptcp/protocol.c:3170
>>>        inet_release+0xed/0x200 net/ipv4/af_inet.c:437
>>>        __sock_release+0xb3/0x270 net/socket.c:662
>>>        sock_close+0x1c/0x30 net/socket.c:1455
>>>        __fput+0x402/0xb70 fs/file_table.c:468
>>>        task_work_run+0x150/0x240 kernel/task_work.c:227
>>>        resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>>>        exit_to_user_mode_loop+0xec/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+0x426/0xfa0 arch/x86/entry/syscall_64.c:100
>>>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>>
>>> other info that might help us debug this:
>>>
>>> Chain exists of:
>>>   k-sk_lock-AF_INET --> &nsock->tx_lock --> sk_lock-AF_INET
>> 
>> It looks like a false positive due to mptcp subflows and nbd connection
>> sockets getting the lockdep annotation. We should possibly/likely use a
>> specific lockdep key for mptcp subflows.
>
> As noted by Paolo offline, it looks like this issue is due to nbd
> introducing a lockdep dependency between reclaim and af_socket, and this
> is similar to a previous report:
>
> #syz dup: [syzbot] [mptcp?] possible deadlock in

can't find the dup bug

> mptcp_subflow_create_socket (2)
>
> Cheers,
> Matt
> -- 
> Sponsored by the NGI0 Core fund.
>