From: syzbot <syzbot+a761911cb2560f3ca2f7@syzkaller.appspotmail.com>
To: davem@davemloft.net, edumazet@kernel.org, horms@kernel.org,
kuba@kernel.org, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org, pabeni@redhat.com,
syzkaller-bugs@googlegroups.com
Subject: [syzbot] [net?] possible deadlock in sk_stream_wait_memory
Date: Thu, 05 Nov 2026 03:11:43 -0800 [thread overview]
Message-ID: <6aec646f.aca85a03.3b899.0000.GAE@google.com> (raw)
Hello,
syzbot found the following issue on:
HEAD commit: 03897066069f tipc: raise the minimum bearer MTU to fit tun..
git tree: net
console output: https://syzkaller.appspot.com/x/log.txt?x=12215969580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d30e1495ee3ac817
dashboard link: https://syzkaller.appspot.com/bug?extid=a761911cb2560f3ca2f7
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/4416adab3ebe/disk-03897066.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2165a756da2b/vmlinux-03897066.xz
kernel image: https://storage.googleapis.com/syzbot-assets/d949ba699d8b/bzImage-03897066.xz
AI-generated root cause summary:
In drivers/block/nbd.c, nbd_add_socket() reclassifies user-supplied socket locks
to sk_lock-AF_INET-NBD using nbd_reclassify_socket(). Because the socket was
obtained via sockfd_lookup(), userspace retains access to the socket file
descriptor and can issue sendto()/sendmsg() calls on it. During userspace
transmission, tcp_sendmsg() holds lock_sock(sk) (sk_lock-AF_INET-NBD) while
copying user data, creating a dependency on &mm->mmap_lock via page faults.
Concurrently, memory allocations while holding &mm->mmap_lock can trigger direct
reclaim (fs_reclaim) to write back dirty pages through the NBD block device
(nbd_queue_rq), which acquires &nsock->tx_lock and subsequently lock_sock(sk)
(sk_lock-AF_INET-NBD), creating an ABBA circular locking dependency between
&mm->mmap_lock and sk_lock-AF_INET-NBD.
full analysis: https://syzkaller.appspot.com/x/ai_root_cause?x=a761911cb2560f3ca2f7
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a761911cb2560f3ca2f7@syzkaller.appspotmail.com
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.4.7408/2091 is trying to acquire lock:
ffff888036f0a800 (&mm->mmap_lock){++++}-{4:4}, at: __might_fault+0xaf/0x130 mm/memory.c:7530
but task is already holding lock:
ffff8880760ed1e0 (sk_lock-AF_INET-NBD){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1717 [inline]
ffff8880760ed1e0 (sk_lock-AF_INET-NBD){+.+.}-{0:0}, at: sk_stream_wait_memory+0x8de/0xff0 net/core/stream.c:148
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #7 (sk_lock-AF_INET-NBD){+.+.}-{0:0}:
lock_sock_nested+0x41/0x100 net/core/sock.c:3908
lock_sock include/net/sock.h:1717 [inline]
inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:911
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
-> #6 (&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:2131
__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:2546
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 (&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:2131
__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:2546
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 (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
-> #3 (&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:5114 [inline]
__blk_mq_update_nr_hw_queues block/blk-mq.c:5159 [inline]
blk_mq_update_nr_hw_queues+0x5ef/0x19e0 block/blk-mq.c:5224
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:2584
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
netlink_unicast+0x8ef/0xa80 net/netlink/af_netlink.c:1361
netlink_sendmsg+0x7f0/0xb20 net/netlink/af_netlink.c:1920
sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
__sock_sendmsg net/socket.c:818 [inline]
____sys_sendmsg+0x54e/0x850 net/socket.c:2715
___sys_sendmsg+0x2a5/0x360 net/socket.c:2769
__sys_sendmsg net/socket.c:2801 [inline]
__do_sys_sendmsg net/socket.c:2806 [inline]
__se_sys_sendmsg net/socket.c:2804 [inline]
__x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2804
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
-> #2 (&q->q_usage_counter(io)#50){++++}-{0:0}:
blk_alloc_queue+0x544/0x690 block/blk-core.c:505
blk_mq_alloc_queue block/blk-mq.c:4439 [inline]
__blk_mq_alloc_disk+0x194/0x390 block/blk-mq.c:4486
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
-> #1 (fs_reclaim){+.+.}-{0:0}:
__fs_reclaim_acquire mm/page_alloc.c:4360 [inline]
fs_reclaim_acquire+0x71/0x100 mm/page_alloc.c:4374
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_noprof+0x5f/0x600 mm/slub.c:5047
mt_alloc_one lib/maple_tree.c:139 [inline]
mas_alloc_nodes+0x291/0x350 lib/maple_tree.c:1049
mas_preallocate+0x42a/0x840 lib/maple_tree.c:5016
vma_iter_prealloc mm/vma.h:698 [inline]
vma_link+0x123/0x460 mm/vma.c:1922
insert_vm_struct+0x3db/0x5b0 mm/vma.c:3608
create_init_stack_vma+0x607/0xa40 mm/vma_exec.c:155
bprm_mm_init fs/exec.c:279 [inline]
alloc_bprm+0x507/0x640 fs/exec.c:1570
class_bprm_constructor fs/exec.c:1580 [inline]
kernel_execve+0x85/0x9a0 fs/exec.c:1976
try_to_run_init_process+0x13/0x60 init/main.c:1548
kernel_init+0xb2/0x1d0 init/main.c:1678
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 (&mm->mmap_lock){++++}-{4:4}:
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
__might_fault+0xcb/0x130 mm/memory.c:7530
_copy_from_iter+0x100/0x1680 lib/iov_iter.c:271
copy_from_iter include/linux/uio.h:236 [inline]
copy_from_iter_full include/linux/uio.h:253 [inline]
skb_do_copy_data_nocache include/net/sock.h:2333 [inline]
skb_copy_to_page_nocache include/net/sock.h:2359 [inline]
tcp_sendmsg_locked+0x214a/0x53c0 net/ipv4/tcp.c:1321
tcp_sendmsg+0x2f/0x50 net/ipv4/tcp.c:1462
sock_sendmsg_nosec+0x10f/0x190 net/socket.c:801
__sock_sendmsg net/socket.c:818 [inline]
__sys_sendto+0x408/0x5a0 net/socket.c:2283
__do_sys_sendto net/socket.c:2290 [inline]
__se_sys_sendto net/socket.c:2286 [inline]
__x64_sys_sendto+0xde/0x100 net/socket.c:2286
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:
&mm->mmap_lock --> &nsock->tx_lock --> sk_lock-AF_INET-NBD
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(sk_lock-AF_INET-NBD);
lock(&nsock->tx_lock);
lock(sk_lock-AF_INET-NBD);
rlock(&mm->mmap_lock);
*** DEADLOCK ***
locks held by syz.4.7408/2091: 1, last CPU#0:
#0: ffff8880760ed1e0 (sk_lock-AF_INET-NBD){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1717 [inline]
#0: ffff8880760ed1e0 (sk_lock-AF_INET-NBD){+.+.}-{0:0}, at: sk_stream_wait_memory+0x8de/0xff0 net/core/stream.c:148
stack backtrace:
CPU: 0 UID: 0 PID: 2091 Comm: syz.4.7408 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026
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
__might_fault+0xcb/0x130 mm/memory.c:7530
_copy_from_iter+0x100/0x1680 lib/iov_iter.c:271
copy_from_iter include/linux/uio.h:236 [inline]
copy_from_iter_full include/linux/uio.h:253 [inline]
skb_do_copy_data_nocache include/net/sock.h:2333 [inline]
skb_copy_to_page_nocache include/net/sock.h:2359 [inline]
tcp_sendmsg_locked+0x214a/0x53c0 net/ipv4/tcp.c:1321
tcp_sendmsg+0x2f/0x50 net/ipv4/tcp.c:1462
sock_sendmsg_nosec+0x10f/0x190 net/socket.c:801
__sock_sendmsg net/socket.c:818 [inline]
__sys_sendto+0x408/0x5a0 net/socket.c:2283
__do_sys_sendto net/socket.c:2290 [inline]
__se_sys_sendto net/socket.c:2286 [inline]
__x64_sys_sendto+0xde/0x100 net/socket.c:2286
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: 0033:0x7f2f21b9fb19
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:00007f2f22997028 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f2f21e25fa0 RCX: 00007f2f21b9fb19
RDX: ffffffffffffff94 RSI: 0000200000000100 RDI: 0000000000000004
RBP: 00007f2f21c37b70 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000000000b R11: 0000000000000246 R12: 0000000000000000
R13: 00007f2f21e26038 R14: 00007f2f21e25fa0 R15: 00007fffcbec2ed8
</TASK>
---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.
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https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
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If the report is a duplicate of another one, reply with:
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If you want to undo deduplication, reply with:
#syz undup
reply other threads:[~2026-11-05 11:11 UTC|newest]
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