------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 4511 at lib/refcount.c:28 refcount_warn_saturate+0x11b/0x1a0 lib/refcount.c:28
Modules linked in:

CPU: 1 PID: 4511 Comm: syz.0.48 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
RIP: 0010:refcount_warn_saturate+0x11b/0x1a0 lib/refcount.c:28
Code: 09 01 48 c7 c7 a0 6b 59 8a e8 81 b2 c3 05 0f 0b eb c4 e8 18 f5 9c fd c6 05 2d 20 7b 09 01 48 c7 c7 00 6c 59 8a e8 65 b2 c3 05 <0f> 0b eb a8 e8 fc f4 9c fd c6 05 0e 20 7b 09 01 48 c7 c7 40 6b 59
RSP: 0000:ffffc90003b9fa20 EFLAGS: 00010246
RAX: 809670ca5773f700 RBX: 0000000000000003 RCX: 0000000000080000
RDX: ffffc9000e621000 RSI: 0000000000026244 RDI: 0000000000026245
RBP: 0000000000000000 R08: dffffc0000000000 R09: ffffed1017204f2c
R10: ffffed1017204f2c R11: 1ffff11017204f2b R12: 1ffff92000773f83
R13: ffff8880202e8600 R14: 0000000000000003 R15: ffffffff84d38350
FS:  00007f06a51106c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f74a88bee9c CR3: 000000001f3d8000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 bt_tags_iter+0x23a/0x2e0 block/blk-mq-tag.c:307
 __sbitmap_for_each_set include/linux/sbitmap.h:263 [inline]
 sbitmap_for_each_set+0x26b/0x480 include/linux/sbitmap.h:284
 bt_tags_for_each block/blk-mq-tag.c:334 [inline]
 __blk_mq_all_tag_iter block/blk-mq-tag.c:345 [inline]
 blk_mq_tagset_busy_iter+0x27d/0x360 block/blk-mq-tag.c:386
 nbd_clear_que drivers/block/nbd.c:849 [inline]
 nbd_clear_sock drivers/block/nbd.c:1240 [inline]
 nbd_clear_sock_ioctl drivers/block/nbd.c:1379 [inline]
 __nbd_ioctl drivers/block/nbd.c:1407 [inline]
 nbd_ioctl+0x2db/0xe40 drivers/block/nbd.c:1467
 blkdev_ioctl+0x33c/0x690 block/ioctl.c:604
 block_ioctl+0xac/0xf0 block/fops.c:496
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:874 [inline]
 __se_sys_ioctl+0xfa/0x170 fs/ioctl.c:860
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f06a6ec9fc9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f06a5110038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f06a7121090 RCX: 00007f06a6ec9fc9
RDX: 0000000000000000 RSI: 000000000000ab04 RDI: 0000000000000004
RBP: 00007f06a6f4cf91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f06a7121128 R14: 00007f06a7121090 R15: 00007ffe980330b8
 </TASK>