* [syzbot] [net?] possible deadlock in mptcp_close (5)
@ 2026-11-13 15:37 syzbot
0 siblings, 0 replies; only message in thread
From: syzbot @ 2026-11-13 15:37 UTC (permalink / raw)
To: davem, edumazet, horms, kuba, linux-kernel, netdev, pabeni,
syzkaller-bugs
Hello,
syzbot found the following issue on:
HEAD commit: 145480b28ca8 net/mlx5: HWS, fix flow meter release on FTE ..
git tree: net
console output: https://syzkaller.appspot.com/x/log.txt?x=15776b55580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d44b06612b68f56
dashboard link: https://syzkaller.appspot.com/bug?extid=fe510de4a165d4512813
compiler: Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
Unfortunately, I don't have any reproducer for this issue yet.
Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6f77cec177cc/disk-145480b2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/846aaa5e713a/vmlinux-145480b2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/db5f4ddbee05/bzImage-145480b2.xz
AI-generated root cause summary:
When attaching a socket to an NBD device in nbd_add_socket(),
nbd_reclassify_socket() attempts to assign the socket a distinct lockdep class
(sk_lock-AF_INET-NBD) to decouple it from general networking socket locks. If
the socket lock happens to be held or busy during addition,
sock_allow_reclassification() returns false, causing nbd_reclassify_socket() to
silently skip reclassification and leave the socket with the default
sk_lock-AF_INET class. Consequently, when NBD acquires lock_sock under
nsock->tx_lock (e.g. during recv_work or request dispatch under fs_reclaim),
lockdep records the chain fs_reclaim -> &nsock->tx_lock -> sk_lock-AF_INET. When
an unrelated userspace AF_INET socket (here, an MPTCP socket during
mptcp_close()) subsequently performs a GFP_KERNEL allocation while holding
lock_sock, lockdep triggers a false-positive circular locking dependency warning
between fs_reclaim and sk_lock-AF_INET.
full analysis: https://syzkaller.appspot.com/x/ai_root_cause?x=fe510de4a165d4512813
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+fe510de4a165d4512813@syzkaller.appspotmail.com
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.3453/19253 is trying to acquire lock:
ffffffff90294f80 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:316 [inline]
ffffffff90294f80 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slub.c:4673 [inline]
ffffffff90294f80 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:5011 [inline]
ffffffff90294f80 (fs_reclaim){+.+.}-{0:0}, at: kmem_cache_alloc_node_noprof+0x5f/0x600 mm/slub.c:5105
but task is already holding lock:
ffff88803354a720 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1717 [inline]
ffff88803354a720 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x1d/0x1a0 net/mptcp/protocol.c:3537
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+0x41/0x100 net/core/sock.c:3906
lock_sock include/net/sock.h:1717 [inline]
inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:913
nbd_mark_nsock_dead+0x2cb/0x550 drivers/block/nbd.c:319
recv_work+0x1d14/0x1e30 drivers/block/nbd.c:1030
process_one_work kernel/workqueue.c:3495 [inline]
process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3578
worker_thread+0xa47/0xfb0 kernel/workqueue.c:3659
kthread+0x38b/0x480 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 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:646 [inline]
__mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
nbd_handle_cmd drivers/block/nbd.c:1143 [inline]
nbd_queue_rq+0x373/0xff0 drivers/block/nbd.c:1207
blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2141
__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+0xd36/0x1580 block/blk-mq-sched.c:307
blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
blk_mq_run_work_fn+0x167/0x2f0 block/blk-mq.c:2556
process_one_work kernel/workqueue.c:3495 [inline]
process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3578
worker_thread+0xa47/0xfb0 kernel/workqueue.c:3659
kthread+0x38b/0x480 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
-> #4 (&cmd->lock){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
nbd_queue_rq+0xc1/0xff0 drivers/block/nbd.c:1199
blk_mq_dispatch_rq_list+0x499/0x1990 block/blk-mq.c:2141
__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+0xd36/0x1580 block/blk-mq-sched.c:307
blk_mq_sched_dispatch_requests+0xd7/0x190 block/blk-mq-sched.c:329
blk_mq_run_work_fn+0x167/0x2f0 block/blk-mq.c:2556
process_one_work kernel/workqueue.c:3495 [inline]
process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3578
worker_thread+0xa47/0xfb0 kernel/workqueue.c:3659
kthread+0x38b/0x480 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
-> #3 (set->srcu){.+.+}-{0:0}:
srcu_lock_sync include/linux/srcu.h:225 [inline]
__synchronize_srcu+0xca/0x2f0 kernel/rcu/srcutree.c:1662
elevator_switch+0x1e8/0x7b0 block/elevator.c:576
elevator_change+0x2fa/0x480 block/elevator.c:681
elevator_set_default+0x375/0x440 block/elevator.c:754
blk_register_queue+0x404/0x4f0 block/blk-sysfs.c:992
__add_disk+0x6b8/0xdb0 block/genhd.c:531
add_disk_fwnode+0x100/0x3a0 block/genhd.c:600
add_disk include/linux/blkdev.h:801 [inline]
nbd_dev_add+0x733/0xb60 drivers/block/nbd.c:2026
nbd_init+0x168/0x1f0 drivers/block/nbd.c:2743
do_one_initcall+0x250/0x870 init/main.c:1419
do_initcall_level+0x10a/0x1a0 init/main.c:1481
do_initcalls+0x59/0xa0 init/main.c:1497
kernel_init_freeable+0x29d/0x3e0 init/main.c:1732
kernel_init+0x22/0x1d0 init/main.c:1622
ret_from_fork+0x514/0xb70 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:646 [inline]
__mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
elevator_change+0x1af/0x480 block/elevator.c:679
elevator_set_none+0xb5/0x140 block/elevator.c:769
blk_mq_elv_switch_none block/blk-mq.c:5160 [inline]
__blk_mq_update_nr_hw_queues block/blk-mq.c:5205 [inline]
blk_mq_update_nr_hw_queues+0x60b/0x1b70 block/blk-mq.c:5273
nbd_start_device+0x189/0xb30 drivers/block/nbd.c:1530
nbd_genl_connect+0x167d/0x1cd0 drivers/block/nbd.c:2310
genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2585
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
netlink_unicast_kernel net/netlink/af_netlink.c:1336 [inline]
netlink_unicast+0x8ef/0xa80 net/netlink/af_netlink.c:1362
netlink_sendmsg+0x7f0/0xb20 net/netlink/af_netlink.c:1921
sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
__sock_sendmsg net/socket.c:818 [inline]
____sys_sendmsg+0x49f/0x790 net/socket.c:2713
___sys_sendmsg+0x2f7/0x3d0 net/socket.c:2770
__sys_sendmsg net/socket.c:2802 [inline]
__do_sys_sendmsg net/socket.c:2807 [inline]
__se_sys_sendmsg net/socket.c:2805 [inline]
__x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2805
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #1 (&q->q_usage_counter(io)#49){++++}-{0:0}:
blk_alloc_queue+0x544/0x690 block/blk-core.c:505
blk_mq_alloc_queue block/blk-mq.c:4460 [inline]
__blk_mq_alloc_disk+0x194/0x390 block/blk-mq.c:4507
nbd_dev_add+0x494/0xb60 drivers/block/nbd.c:1996
nbd_init+0x168/0x1f0 drivers/block/nbd.c:2743
do_one_initcall+0x250/0x870 init/main.c:1419
do_initcall_level+0x10a/0x1a0 init/main.c:1481
do_initcalls+0x59/0xa0 init/main.c:1497
kernel_init_freeable+0x29d/0x3e0 init/main.c:1732
kernel_init+0x22/0x1d0 init/main.c:1622
ret_from_fork+0x514/0xb70 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:3209 [inline]
check_prevs_add kernel/locking/lockdep.c:3328 [inline]
validate_chain kernel/locking/lockdep.c:3952 [inline]
__lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
__fs_reclaim_acquire mm/page_alloc.c:4365 [inline]
fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4379
might_alloc include/linux/sched/mm.h:316 [inline]
slab_pre_alloc_hook mm/slub.c:4673 [inline]
slab_alloc_node mm/slub.c:5011 [inline]
kmem_cache_alloc_node_noprof+0x5f/0x600 mm/slub.c:5105
__alloc_skb+0x1db/0x7a0 net/core/skbuff.c:705
alloc_skb include/linux/skbuff.h:1390 [inline]
nlmsg_new include/net/netlink.h:1055 [inline]
mptcp_event+0x13b/0x990 net/mptcp/pm_netlink.c:582
__mptcp_close+0x871/0xd10 net/mptcp/protocol.c:3521
mptcp_close+0x28/0x1a0 net/mptcp/protocol.c:3539
inet_release+0x143/0x190 net/ipv4/af_inet.c:443
__sock_release+0xa0/0x1f0 net/socket.c:735
sock_close+0x1c/0x30 net/socket.c:1529
__fput+0x408/0xa40 fs/file_table.c:512
fput_close_sync+0x11f/0x240 fs/file_table.c:618
filp_close_sync+0x6e/0xa0 fs/open.c:1573
__range_close fs/file.c:919 [inline]
__do_sys_close_range fs/file.c:1003 [inline]
__se_sys_close_range+0xa77/0xef0 fs/file.c:954
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
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 ***
locks held by syz.0.3453/19253: 2, last CPU#1:
#0: ffff8880529b4740 (&sb->s_type->i_mutex_key#13){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:1026 [inline]
#0: ffff8880529b4740 (&sb->s_type->i_mutex_key#13){+.+.}-{4:4}, at: __sock_release+0x7a/0x1f0 net/socket.c:734
#1: ffff88803354a720 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1717 [inline]
#1: ffff88803354a720 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x1d/0x1a0 net/mptcp/protocol.c:3537
stack backtrace:
CPU: 1 UID: 0 PID: 19253 Comm: syz.0.3453 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2087
check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2219
check_prev_add kernel/locking/lockdep.c:3209 [inline]
check_prevs_add kernel/locking/lockdep.c:3328 [inline]
validate_chain kernel/locking/lockdep.c:3952 [inline]
__lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
__fs_reclaim_acquire mm/page_alloc.c:4365 [inline]
fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4379
might_alloc include/linux/sched/mm.h:316 [inline]
slab_pre_alloc_hook mm/slub.c:4673 [inline]
slab_alloc_node mm/slub.c:5011 [inline]
kmem_cache_alloc_node_noprof+0x5f/0x600 mm/slub.c:5105
__alloc_skb+0x1db/0x7a0 net/core/skbuff.c:705
alloc_skb include/linux/skbuff.h:1390 [inline]
nlmsg_new include/net/netlink.h:1055 [inline]
mptcp_event+0x13b/0x990 net/mptcp/pm_netlink.c:582
__mptcp_close+0x871/0xd10 net/mptcp/protocol.c:3521
mptcp_close+0x28/0x1a0 net/mptcp/protocol.c:3539
inet_release+0x143/0x190 net/ipv4/af_inet.c:443
__sock_release+0xa0/0x1f0 net/socket.c:735
sock_close+0x1c/0x30 net/socket.c:1529
__fput+0x408/0xa40 fs/file_table.c:512
fput_close_sync+0x11f/0x240 fs/file_table.c:618
filp_close_sync+0x6e/0xa0 fs/open.c:1573
__range_close fs/file.c:919 [inline]
__do_sys_close_range fs/file.c:1003 [inline]
__se_sys_close_range+0xa77/0xef0 fs/file.c:954
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3143da94d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff94a73f18 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: ffffffffffffffda RBX: 00007fff94a74000 RCX: 00007f3143da94d9
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 000000000005c95b R08: 0000000000000001 R09: 0000000000000000
R10: 0000001b31620000 R11: 0000000000000246 R12: 00007fff94a74040
R13: 00007f314402609c R14: 000000000005c9a5 R15: 00007f3144026090
</TASK>
---
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2026-11-13 15:37 [syzbot] [net?] possible deadlock in mptcp_close (5) syzbot
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