Message-ID: <6853f49e.a00a0220.137b3.0006.GAE@google.com>
Date: Thu, 19 Jun 2025 04:29:34 -0700
From: syzbot <syzbot+ea702c2366971b7fc6e4@...kaller.appspotmail.com>
To: axboe@...nel.dk, josef@...icpanda.com, linux-block@...r.kernel.org, 
	linux-kernel@...r.kernel.org, nbd@...er.debian.org, 
	syzkaller-bugs@...glegroups.com
Subject: [syzbot] [nbd?] possible deadlock in nbd_open

Hello,

syzbot found the following issue on:

HEAD commit:    306e57988197 Merge patch "riscv: defconfig: run savedefcon..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git for-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10721e82580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=618b9468db3872f5
dashboard link: https://syzkaller.appspot.com/bug?extid=ea702c2366971b7fc6e4
compiler:       riscv64-linux-gnu-gcc (Debian 12.2.0-13) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
userspace arch: riscv64

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/a741b348759c/non_bootable_disk-306e5798.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5fee9bbe87f3/vmlinux-306e5798.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e959580bb405/Image-306e5798.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ea702c2366971b7fc6e4@...kaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.16.0-rc1-syzkaller-g306e57988197 #0 Not tainted
------------------------------------------------------
syz.4.1189/8748 is trying to acquire lock:
ffffaf801a39a230 (&nbd->config_lock){+.+.}-{4:4}, at: nbd_open+0x3c0/0x6fe drivers/block/nbd.c:1703

but task is already holding lock:
ffffffff88e269e8 (nbd_index_mutex){+.+.}-{4:4}, at: nbd_open+0x98/0x6fe drivers/block/nbd.c:1690

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (nbd_index_mutex){+.+.}-{4:4}:
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x1ac/0x448 kernel/locking/lockdep.c:5828
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x166/0x1292 kernel/locking/mutex.c:747
       mutex_lock_nested+0x14/0x1c kernel/locking/mutex.c:799
       nbd_open+0x98/0x6fe drivers/block/nbd.c:1690
       blkdev_get_whole+0x8a/0x266 block/bdev.c:730
       bdev_open+0x292/0xce6 block/bdev.c:957
       blkdev_open+0x264/0x39a block/fops.c:676
       do_dentry_open+0x97e/0x171e fs/open.c:964
       vfs_open+0xbe/0x380 fs/open.c:1094
       do_open fs/namei.c:3887 [inline]
       path_openat+0x1bd8/0x2a82 fs/namei.c:4046
       do_filp_open+0x19c/0x380 fs/namei.c:4073
       do_sys_openat2+0x11a/0x1c2 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]
       __riscv_sys_openat+0x178/0x1fe fs/open.c:1463
       syscall_handler+0x94/0x118 arch/riscv/include/asm/syscall.h:112
       do_trap_ecall_u+0x396/0x530 arch/riscv/kernel/traps.c:341
       handle_exception+0x146/0x152 arch/riscv/kernel/entry.S:197

-> #2 (&disk->open_mutex){+.+.}-{4:4}:
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x1ac/0x448 kernel/locking/lockdep.c:5828
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x166/0x1292 kernel/locking/mutex.c:747
       mutex_lock_nested+0x14/0x1c kernel/locking/mutex.c:799
       __del_gendisk+0x132/0xac6 block/genhd.c:706
       del_gendisk+0xf6/0x19a block/genhd.c:819
       nbd_dev_remove+0x3c/0xf2 drivers/block/nbd.c:268
       nbd_dev_remove_work+0x1c/0x26 drivers/block/nbd.c:284
       process_one_work+0x96a/0x1f32 kernel/workqueue.c:3238
       process_scheduled_works kernel/workqueue.c:3321 [inline]
       worker_thread+0x5ce/0xde8 kernel/workqueue.c:3402
       kthread+0x39c/0x7d4 kernel/kthread.c:464
       ret_from_fork_kernel+0x2a/0xbb2 arch/riscv/kernel/process.c:214
       ret_from_fork_kernel_asm+0x16/0x18 arch/riscv/kernel/entry.S:327

-> #1 (&set->update_nr_hwq_lock){++++}-{4:4}:
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x1ac/0x448 kernel/locking/lockdep.c:5828
       down_write+0x9c/0x19a kernel/locking/rwsem.c:1577
       blk_mq_update_nr_hw_queues+0x3e/0xb86 block/blk-mq.c:5041
       nbd_start_device+0x140/0xb2c drivers/block/nbd.c:1476
       nbd_genl_connect+0xae0/0x1b24 drivers/block/nbd.c:2201
       genl_family_rcv_msg_doit+0x206/0x2e6 net/netlink/genetlink.c:1115
       genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
       genl_rcv_msg+0x514/0x78e net/netlink/genetlink.c:1210
       netlink_rcv_skb+0x206/0x3be net/netlink/af_netlink.c:2534
       genl_rcv+0x36/0x4c net/netlink/genetlink.c:1219
       netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
       netlink_unicast+0x4f0/0x82c net/netlink/af_netlink.c:1339
       netlink_sendmsg+0x85e/0xdd6 net/netlink/af_netlink.c:1883
       sock_sendmsg_nosec net/socket.c:712 [inline]
       __sock_sendmsg+0xcc/0x160 net/socket.c:727
       ____sys_sendmsg+0x63e/0x79c net/socket.c:2566
       ___sys_sendmsg+0x144/0x1e6 net/socket.c:2620
       __sys_sendmsg+0x188/0x246 net/socket.c:2652
       __do_sys_sendmsg net/socket.c:2657 [inline]
       __se_sys_sendmsg net/socket.c:2655 [inline]
       __riscv_sys_sendmsg+0x70/0xa2 net/socket.c:2655
       syscall_handler+0x94/0x118 arch/riscv/include/asm/syscall.h:112
       do_trap_ecall_u+0x396/0x530 arch/riscv/kernel/traps.c:341
       handle_exception+0x146/0x152 arch/riscv/kernel/entry.S:197

-> #0 (&nbd->config_lock){+.+.}-{4:4}:
       check_noncircular+0x132/0x146 kernel/locking/lockdep.c:2178
       check_prev_add kernel/locking/lockdep.c:3168 [inline]
       check_prevs_add kernel/locking/lockdep.c:3287 [inline]
       validate_chain kernel/locking/lockdep.c:3911 [inline]
       __lock_acquire+0x12b2/0x24ea kernel/locking/lockdep.c:5240
       lock_acquire kernel/locking/lockdep.c:5871 [inline]
       lock_acquire+0x1ac/0x448 kernel/locking/lockdep.c:5828
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x166/0x1292 kernel/locking/mutex.c:747
       mutex_lock_nested+0x14/0x1c kernel/locking/mutex.c:799
       nbd_open+0x3c0/0x6fe drivers/block/nbd.c:1703
       blkdev_get_whole+0x8a/0x266 block/bdev.c:730
       bdev_open+0x292/0xce6 block/bdev.c:957
       blkdev_open+0x264/0x39a block/fops.c:676
       do_dentry_open+0x97e/0x171e fs/open.c:964
       vfs_open+0xbe/0x380 fs/open.c:1094
       do_open fs/namei.c:3887 [inline]
       path_openat+0x1bd8/0x2a82 fs/namei.c:4046
       do_filp_open+0x19c/0x380 fs/namei.c:4073
       do_sys_openat2+0x11a/0x1c2 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]
       __riscv_sys_openat+0x178/0x1fe fs/open.c:1463
       syscall_handler+0x94/0x118 arch/riscv/include/asm/syscall.h:112
       do_trap_ecall_u+0x396/0x530 arch/riscv/kernel/traps.c:341
       handle_exception+0x146/0x152 arch/riscv/kernel/entry.S:197

other info that might help us debug this:

Chain exists of:
  &nbd->config_lock --> &disk->open_mutex --> nbd_index_mutex

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(nbd_index_mutex);
                               lock(&disk->open_mutex);
                               lock(nbd_index_mutex);
  lock(&nbd->config_lock);

 *** DEADLOCK ***

2 locks held by syz.4.1189/8748:
 #0: ffffaf801a3a2358 (&disk->open_mutex){+.+.}-{4:4}, at: bdev_open+0x3d0/0xce6 block/bdev.c:945
 #1: ffffffff88e269e8 (nbd_index_mutex){+.+.}-{4:4}, at: nbd_open+0x98/0x6fe drivers/block/nbd.c:1690

stack backtrace:
CPU: 0 UID: 0 PID: 8748 Comm: syz.4.1189 Not tainted 6.16.0-rc1-syzkaller-g306e57988197 #0 PREEMPT 
Hardware name: riscv-virtio,qemu (DT)
Call Trace:
[<ffffffff80078bbe>] dump_backtrace+0x2e/0x3c arch/riscv/kernel/stacktrace.c:132
[<ffffffff8000327a>] show_stack+0x30/0x3c arch/riscv/kernel/stacktrace.c:138
[<ffffffff8006103e>] __dump_stack lib/dump_stack.c:94 [inline]
[<ffffffff8006103e>] dump_stack_lvl+0x12e/0x1a6 lib/dump_stack.c:120
[<ffffffff800610d2>] dump_stack+0x1c/0x24 lib/dump_stack.c:129
[<ffffffff802ce116>] print_circular_bug+0x254/0x29a kernel/locking/lockdep.c:2046
[<ffffffff802ce28e>] check_noncircular+0x132/0x146 kernel/locking/lockdep.c:2178
[<ffffffff802d1468>] check_prev_add kernel/locking/lockdep.c:3168 [inline]
[<ffffffff802d1468>] check_prevs_add kernel/locking/lockdep.c:3287 [inline]
[<ffffffff802d1468>] validate_chain kernel/locking/lockdep.c:3911 [inline]
[<ffffffff802d1468>] __lock_acquire+0x12b2/0x24ea kernel/locking/lockdep.c:5240
[<ffffffff802d32de>] lock_acquire kernel/locking/lockdep.c:5871 [inline]
[<ffffffff802d32de>] lock_acquire+0x1ac/0x448 kernel/locking/lockdep.c:5828
[<ffffffff8630e424>] __mutex_lock_common kernel/locking/mutex.c:602 [inline]
[<ffffffff8630e424>] __mutex_lock+0x166/0x1292 kernel/locking/mutex.c:747
[<ffffffff8630f564>] mutex_lock_nested+0x14/0x1c kernel/locking/mutex.c:799
[<ffffffff82c3fb3a>] nbd_open+0x3c0/0x6fe drivers/block/nbd.c:1703
[<ffffffff81531ca6>] blkdev_get_whole+0x8a/0x266 block/bdev.c:730
[<ffffffff81532f2e>] bdev_open+0x292/0xce6 block/bdev.c:957
[<ffffffff81539df6>] blkdev_open+0x264/0x39a block/fops.c:676
[<ffffffff80be5b4e>] do_dentry_open+0x97e/0x171e fs/open.c:964
[<ffffffff80bea7aa>] vfs_open+0xbe/0x380 fs/open.c:1094
[<ffffffff80c3d1ba>] do_open fs/namei.c:3887 [inline]
[<ffffffff80c3d1ba>] path_openat+0x1bd8/0x2a82 fs/namei.c:4046
[<ffffffff80c3e200>] do_filp_open+0x19c/0x380 fs/namei.c:4073
[<ffffffff80beb5f4>] do_sys_openat2+0x11a/0x1c2 fs/open.c:1437
[<ffffffff80bebff6>] do_sys_open fs/open.c:1452 [inline]
[<ffffffff80bebff6>] __do_sys_openat fs/open.c:1468 [inline]
[<ffffffff80bebff6>] __se_sys_openat fs/open.c:1463 [inline]
[<ffffffff80bebff6>] __riscv_sys_openat+0x178/0x1fe fs/open.c:1463
[<ffffffff800769ae>] syscall_handler+0x94/0x118 arch/riscv/include/asm/syscall.h:112
[<ffffffff862fcc32>] do_trap_ecall_u+0x396/0x530 arch/riscv/kernel/traps.c:341
[<ffffffff863250ca>] handle_exception+0x146/0x152 arch/riscv/kernel/entry.S:197


---
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@...glegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

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