Re: [syzbot] [mptcp?] possible deadlock in mptcp_pm_nl_set_flags

[Matthieu Baerts <matttbe@kernel.org>]

Hello,

On 10/04/2026 16:41, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    591cd656a1bf Linux 7.0-rc7
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1779ce06580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=5a3e5e8c17cc174e
> dashboard link: https://syzkaller.appspot.com/bug?extid=dfa28bb6444d8f169cbb
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-591cd656.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/3e99d0e29df0/vmlinux-591cd656.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/5877fee0a056/bzImage-591cd656.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+dfa28bb6444d8f169cbb@syzkaller.appspotmail.com
> 
> netlink: 8 bytes leftover after parsing attributes in process `syz.8.6915'.
> netlink: 8 bytes leftover after parsing attributes in process `syz.8.6915'.
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Tainted: G             L     
> ------------------------------------------------------
> syz.8.6915/29783 is trying to acquire lock:
> ffffffff8e9ab8a0 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:317 [inline]
> ffffffff8e9ab8a0 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4489 [inline]
> ffffffff8e9ab8a0 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:4843 [inline]
> ffffffff8e9ab8a0 (fs_reclaim){+.+.}-{0:0}, at: kmem_cache_alloc_lru_noprof+0x51/0x6e0 mm/slub.c:4885
> 
> but task is already holding lock:
> ffff888034761ae0 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
> ffff888034761ae0 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_set_flags_all net/mptcp/pm_kernel.c:1482 [inline]
> ffff888034761ae0 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_set_flags+0x605/0xd30 net/mptcp/pm_kernel.c:1551
> 
> which lock already depends on the new lock.
> 
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #6 (sk_lock-AF_INET){+.+.}-{0:0}:
>        lock_sock_nested+0x41/0xf0 net/core/sock.c:3780
>        lock_sock include/net/sock.h:1709 [inline]
>        inet_shutdown+0x67/0x410 net/ipv4/af_inet.c:919
>        nbd_mark_nsock_dead+0xae/0x5c0 drivers/block/nbd.c:318
>        recv_work+0x5fb/0x8c0 drivers/block/nbd.c:1021
>        process_one_work+0xa23/0x19a0 kernel/workqueue.c:3276
>        process_scheduled_works kernel/workqueue.c:3359 [inline]
>        worker_thread+0x5ef/0xe50 kernel/workqueue.c:3440
>        kthread+0x370/0x450 kernel/kthread.c:436
>        ret_from_fork+0x754/0xd80 arch/x86/kernel/process.c:158
>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> 
> -> #5 (&nsock->tx_lock){+.+.}-{4:4}:
>        __mutex_lock_common kernel/locking/mutex.c:614 [inline]
>        __mutex_lock+0x1a2/0x1b90 kernel/locking/mutex.c:776
>        nbd_handle_cmd drivers/block/nbd.c:1143 [inline]
>        nbd_queue_rq+0x428/0x1080 drivers/block/nbd.c:1207
>        blk_mq_dispatch_rq_list+0x422/0x1e70 block/blk-mq.c:2148
>        __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+0xcea/0x1620 block/blk-mq-sched.c:307
>        blk_mq_sched_dispatch_requests+0xd7/0x1c0 block/blk-mq-sched.c:329
>        blk_mq_run_hw_queue+0x23c/0x670 block/blk-mq.c:2386
>        blk_mq_dispatch_list+0x51d/0x1360 block/blk-mq.c:2949
>        blk_mq_flush_plug_list block/blk-mq.c:2997 [inline]
>        blk_mq_flush_plug_list+0x130/0x600 block/blk-mq.c:2969
>        __blk_flush_plug+0x2c4/0x4b0 block/blk-core.c:1230
>        blk_finish_plug block/blk-core.c:1257 [inline]
>        __submit_bio+0x584/0x6c0 block/blk-core.c:649
>        __submit_bio_noacct_mq block/blk-core.c:722 [inline]
>        submit_bio_noacct_nocheck+0x562/0xc10 block/blk-core.c:753
>        submit_bio_noacct+0xd17/0x2010 block/blk-core.c:884
>        blk_crypto_submit_bio include/linux/blk-crypto.h:203 [inline]
>        submit_bh_wbc+0x59c/0x770 fs/buffer.c:2821
>        submit_bh fs/buffer.c:2826 [inline]
>        block_read_full_folio+0x4c8/0x8e0 fs/buffer.c:2458
>        filemap_read_folio+0xfc/0x3b0 mm/filemap.c:2501
>        do_read_cache_folio+0x2d7/0x6b0 mm/filemap.c:4101
>        read_mapping_folio include/linux/pagemap.h:1017 [inline]
>        read_part_sector+0xd1/0x370 block/partitions/core.c:723
>        adfspart_check_ICS+0x93/0x910 block/partitions/acorn.c:360
>        check_partition block/partitions/core.c:142 [inline]
>        blk_add_partitions block/partitions/core.c:590 [inline]
>        bdev_disk_changed+0x7f8/0xc80 block/partitions/core.c:694
>        blkdev_get_whole+0x187/0x290 block/bdev.c:764
>        bdev_open+0x2c7/0xe40 block/bdev.c:973
>        blkdev_open+0x34e/0x4f0 block/fops.c:697
>        do_dentry_open+0x6d8/0x1660 fs/open.c:949
>        vfs_open+0x82/0x3f0 fs/open.c:1081
>        do_open fs/namei.c:4677 [inline]
>        path_openat+0x208c/0x31a0 fs/namei.c:4836
>        do_file_open+0x20e/0x430 fs/namei.c:4865
>        do_sys_openat2+0x10d/0x1e0 fs/open.c:1366
>        do_sys_open fs/open.c:1372 [inline]
>        __do_sys_openat fs/open.c:1388 [inline]
>        __se_sys_openat fs/open.c:1383 [inline]
>        __x64_sys_openat+0x12d/0x210 fs/open.c:1383
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> -> #4 (&cmd->lock){+.+.}-{4:4}:
>        __mutex_lock_common kernel/locking/mutex.c:614 [inline]
>        __mutex_lock+0x1a2/0x1b90 kernel/locking/mutex.c:776
>        nbd_queue_rq+0xba/0x1080 drivers/block/nbd.c:1199
>        blk_mq_dispatch_rq_list+0x422/0x1e70 block/blk-mq.c:2148
>        __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+0xcea/0x1620 block/blk-mq-sched.c:307
>        blk_mq_sched_dispatch_requests+0xd7/0x1c0 block/blk-mq-sched.c:329
>        blk_mq_run_hw_queue+0x23c/0x670 block/blk-mq.c:2386
>        blk_mq_dispatch_list+0x51d/0x1360 block/blk-mq.c:2949
>        blk_mq_flush_plug_list block/blk-mq.c:2997 [inline]
>        blk_mq_flush_plug_list+0x130/0x600 block/blk-mq.c:2969
>        __blk_flush_plug+0x2c4/0x4b0 block/blk-core.c:1230
>        blk_finish_plug block/blk-core.c:1257 [inline]
>        __submit_bio+0x584/0x6c0 block/blk-core.c:649
>        __submit_bio_noacct_mq block/blk-core.c:722 [inline]
>        submit_bio_noacct_nocheck+0x562/0xc10 block/blk-core.c:753
>        submit_bio_noacct+0xd17/0x2010 block/blk-core.c:884
>        blk_crypto_submit_bio include/linux/blk-crypto.h:203 [inline]
>        submit_bh_wbc+0x59c/0x770 fs/buffer.c:2821
>        submit_bh fs/buffer.c:2826 [inline]
>        block_read_full_folio+0x4c8/0x8e0 fs/buffer.c:2458
>        filemap_read_folio+0xfc/0x3b0 mm/filemap.c:2501
>        do_read_cache_folio+0x2d7/0x6b0 mm/filemap.c:4101
>        read_mapping_folio include/linux/pagemap.h:1017 [inline]
>        read_part_sector+0xd1/0x370 block/partitions/core.c:723
>        adfspart_check_ICS+0x93/0x910 block/partitions/acorn.c:360
>        check_partition block/partitions/core.c:142 [inline]
>        blk_add_partitions block/partitions/core.c:590 [inline]
>        bdev_disk_changed+0x7f8/0xc80 block/partitions/core.c:694
>        blkdev_get_whole+0x187/0x290 block/bdev.c:764
>        bdev_open+0x2c7/0xe40 block/bdev.c:973
>        blkdev_open+0x34e/0x4f0 block/fops.c:697
>        do_dentry_open+0x6d8/0x1660 fs/open.c:949
>        vfs_open+0x82/0x3f0 fs/open.c:1081
>        do_open fs/namei.c:4677 [inline]
>        path_openat+0x208c/0x31a0 fs/namei.c:4836
>        do_file_open+0x20e/0x430 fs/namei.c:4865
>        do_sys_openat2+0x10d/0x1e0 fs/open.c:1366
>        do_sys_open fs/open.c:1372 [inline]
>        __do_sys_openat fs/open.c:1388 [inline]
>        __se_sys_openat fs/open.c:1383 [inline]
>        __x64_sys_openat+0x12d/0x210 fs/open.c:1383
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> -> #3 (set->srcu){.+.+}-{0:0}:
>        srcu_lock_sync include/linux/srcu.h:199 [inline]
>        __synchronize_srcu+0xa2/0x300 kernel/rcu/srcutree.c:1481
>        blk_mq_wait_quiesce_done block/blk-mq.c:284 [inline]
>        blk_mq_wait_quiesce_done block/blk-mq.c:281 [inline]
>        blk_mq_quiesce_queue block/blk-mq.c:304 [inline]
>        blk_mq_quiesce_queue+0x149/0x1c0 block/blk-mq.c:299
>        elevator_switch+0x17b/0x7e0 block/elevator.c:576
>        elevator_change+0x352/0x530 block/elevator.c:681
>        elevator_set_default+0x29e/0x360 block/elevator.c:754
>        blk_register_queue+0x412/0x590 block/blk-sysfs.c:946
>        __add_disk+0x73f/0xe40 block/genhd.c:528
>        add_disk_fwnode+0x118/0x5c0 block/genhd.c:597
>        add_disk include/linux/blkdev.h:785 [inline]
>        nbd_dev_add+0x77a/0xb10 drivers/block/nbd.c:1984
>        nbd_init+0x291/0x2b0 drivers/block/nbd.c:2692
>        do_one_initcall+0x11d/0x760 init/main.c:1382
>        do_initcall_level init/main.c:1444 [inline]
>        do_initcalls init/main.c:1460 [inline]
>        do_basic_setup init/main.c:1479 [inline]
>        kernel_init_freeable+0x6e5/0x7a0 init/main.c:1692
>        kernel_init+0x1f/0x1e0 init/main.c:1582
>        ret_from_fork+0x754/0xd80 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}:
>        __mutex_lock_common kernel/locking/mutex.c:614 [inline]
>        __mutex_lock+0x1a2/0x1b90 kernel/locking/mutex.c:776
>        elevator_change+0x1bc/0x530 block/elevator.c:679
>        elevator_set_none+0x92/0xf0 block/elevator.c:769
>        blk_mq_elv_switch_none block/blk-mq.c:5110 [inline]
>        __blk_mq_update_nr_hw_queues block/blk-mq.c:5155 [inline]
>        blk_mq_update_nr_hw_queues+0x4c1/0x15f0 block/blk-mq.c:5220
>        nbd_start_device+0x1a6/0xbd0 drivers/block/nbd.c:1489
>        nbd_genl_connect+0xff2/0x1a40 drivers/block/nbd.c:2239
>        genl_family_rcv_msg_doit+0x214/0x300 net/netlink/genetlink.c:1114
>        genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
>        genl_rcv_msg+0x560/0x800 net/netlink/genetlink.c:1209
>        netlink_rcv_skb+0x159/0x420 net/netlink/af_netlink.c:2550
>        genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
>        netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
>        netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
>        netlink_sendmsg+0x8b0/0xda0 net/netlink/af_netlink.c:1894
>        sock_sendmsg_nosec net/socket.c:727 [inline]
>        __sock_sendmsg net/socket.c:742 [inline]
>        ____sys_sendmsg+0x9e1/0xb70 net/socket.c:2592
>        ___sys_sendmsg+0x190/0x1e0 net/socket.c:2646
>        __sys_sendmsg+0x170/0x220 net/socket.c:2678
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> -> #1 (&q->q_usage_counter(io)#51){++++}-{0:0}:
>        blk_alloc_queue+0x610/0x790 block/blk-core.c:461
>        blk_mq_alloc_queue+0x174/0x290 block/blk-mq.c:4429
>        __blk_mq_alloc_disk+0x29/0x120 block/blk-mq.c:4476
>        nbd_dev_add+0x492/0xb10 drivers/block/nbd.c:1954
>        nbd_init+0x291/0x2b0 drivers/block/nbd.c:2692
>        do_one_initcall+0x11d/0x760 init/main.c:1382
>        do_initcall_level init/main.c:1444 [inline]
>        do_initcalls init/main.c:1460 [inline]
>        do_basic_setup init/main.c:1479 [inline]
>        kernel_init_freeable+0x6e5/0x7a0 init/main.c:1692
>        kernel_init+0x1f/0x1e0 init/main.c:1582
>        ret_from_fork+0x754/0xd80 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 kernel/locking/lockdep.c:3908 [inline]
>        __lock_acquire+0x14b8/0x2630 kernel/locking/lockdep.c:5237
>        lock_acquire kernel/locking/lockdep.c:5868 [inline]
>        lock_acquire+0x1cf/0x380 kernel/locking/lockdep.c:5825
>        __fs_reclaim_acquire mm/page_alloc.c:4348 [inline]
>        fs_reclaim_acquire+0xc4/0x100 mm/page_alloc.c:4362
>        might_alloc include/linux/sched/mm.h:317 [inline]
>        slab_pre_alloc_hook mm/slub.c:4489 [inline]
>        slab_alloc_node mm/slub.c:4843 [inline]
>        kmem_cache_alloc_lru_noprof+0x51/0x6e0 mm/slub.c:4885
>        sock_alloc_inode+0x25/0x1c0 net/socket.c:322
>        alloc_inode+0x68/0x250 fs/inode.c:347
>        new_inode_pseudo include/linux/fs.h:3003 [inline]
>        sock_alloc+0x44/0x280 net/socket.c:637
>        __sock_create+0xc2/0x860 net/socket.c:1569
>        mptcp_subflow_create_socket+0xec/0xa30 net/mptcp/subflow.c:1790
>        __mptcp_subflow_connect+0x3c6/0x1480 net/mptcp/subflow.c:1631
>        mptcp_pm_create_subflow_or_signal_addr+0xc3e/0x18d0 net/mptcp/pm_kernel.c:416
>        mptcp_pm_nl_fullmesh net/mptcp/pm_kernel.c:1460 [inline]
>        mptcp_pm_nl_set_flags_all net/mptcp/pm_kernel.c:1487 [inline]
>        mptcp_pm_nl_set_flags+0x814/0xd30 net/mptcp/pm_kernel.c:1551
>        mptcp_pm_set_flags net/mptcp/pm_netlink.c:277 [inline]
>        mptcp_pm_nl_set_flags_doit+0x1b0/0x290 net/mptcp/pm_netlink.c:282
>        genl_family_rcv_msg_doit+0x214/0x300 net/netlink/genetlink.c:1114
>        genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
>        genl_rcv_msg+0x560/0x800 net/netlink/genetlink.c:1209
>        netlink_rcv_skb+0x159/0x420 net/netlink/af_netlink.c:2550
>        genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
>        netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
>        netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
>        netlink_sendmsg+0x8b0/0xda0 net/netlink/af_netlink.c:1894
>        sock_sendmsg_nosec net/socket.c:727 [inline]
>        __sock_sendmsg net/socket.c:742 [inline]
>        ____sys_sendmsg+0x9e1/0xb70 net/socket.c:2592
>        ___sys_sendmsg+0x190/0x1e0 net/socket.c:2646
>        __sys_sendmsg+0x170/0x220 net/socket.c:2678
>        do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>        do_syscall_64+0x106/0xf80 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 --> &nsock->tx_lock --> sk_lock-AF_INET
> 
>  Possible unsafe locking scenario:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(sk_lock-AF_INET);
>                                lock(&nsock->tx_lock);
>                                lock(sk_lock-AF_INET);
>   lock(fs_reclaim);
> 
>  *** DEADLOCK ***

Same here, if I'm not mistaken, it looks like this issue is also 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 mptcp_subflow_create_socket (2)

If that's not correct, please induplicate it.

I'm not sure how MPTCP is being involved here, I guess it should not and
a simple fix is to forbid it. But maybe this issue is not specific to
MPTCP?

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.