[syzbot] [block?] possible deadlock in lo_ioctl

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 6b8c8af514d7 Add linux-next specific files for 20260807
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1615c079580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6d343b54cc50df0f
dashboard link: https://syzkaller.appspot.com/bug?extid=cc0de396bac84da51919
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), 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/71ff4edcb608/disk-6b8c8af5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1f5fa331d8f2/vmlinux-6b8c8af5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f993558ec63b/bzImage-6b8c8af5.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cc0de3...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G L
------------------------------------------------------
syz.4.8555/6352 is trying to acquire lock:
ffff88802688d118 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:1101 [inline]
ffff88802688d118 (&q->limits_lock){+.+.}-{4:4}, at: loop_set_dio drivers/block/loop.c:1460 [inline]
ffff88802688d118 (&q->limits_lock){+.+.}-{4:4}, at: lo_simple_ioctl drivers/block/loop.c:1528 [inline]
ffff88802688d118 (&q->limits_lock){+.+.}-{4:4}, at: lo_ioctl+0x16ca/0x2150 drivers/block/loop.c:1594

but task is already holding lock:
ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline]
ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: loop_set_dio drivers/block/loop.c:1455 [inline]
ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: lo_simple_ioctl drivers/block/loop.c:1528 [inline]
ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: lo_ioctl+0x1688/0x2150 drivers/block/loop.c:1594

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->q_usage_counter(io)#26){++++}-{0:0}:
bio_queue_enter block/blk.h:95 [inline]
blk_mq_submit_bio+0x7fa/0x2a40 block/blk-mq.c:3127
__submit_bio_noacct_mq block/blk-core.c:756 [inline]
submit_bio_noacct_nocheck+0x2f4/0xa40 block/blk-core.c:790
bh_submit fs/buffer.c:1146 [inline]
__bread_slow fs/buffer.c:1157 [inline]
__bread_gfp+0x1e9/0x380 fs/buffer.c:1403
sb_bread include/linux/buffer_head.h:344 [inline]
fat12_ent_bread+0x156/0x570 fs/fat/fatent.c:77
fat_ent_read_block+0x391/0x4b0 fs/fat/fatent.c:450
fat_alloc_clusters+0x434/0xe30 fs/fat/fatent.c:498
fat_alloc_new_dir+0x160/0xb90 fs/fat/dir.c:1184
vfat_mkdir+0x13a/0x300 fs/fat/namei_vfat.c:861
vfs_mkdir+0x40c/0x620 fs/namei.c:5410
filename_mkdirat+0x285/0x510 fs/namei.c:5443
__do_sys_mkdirat fs/namei.c:5464 [inline]
__se_sys_mkdirat+0x35/0x150 fs/namei.c:5461
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&sbi->fat_lock){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
lock_fat fs/fat/fatent.c:285 [inline]
fat_count_free_clusters+0x157/0xb40 fs/fat/fatent.c:729
fat_statfs+0x135/0x400 fs/fat/inode.c:874
statfs_by_dentry fs/statfs.c:66 [inline]
vfs_statfs+0x147/0x2d0 fs/statfs.c:90
loop_get_discard_config drivers/block/loop.c:774 [inline]
loop_update_limits+0x375/0x920 drivers/block/loop.c:984
loop_configure+0x7d2/0xdc0 drivers/block/loop.c:1084
lo_ioctl+0xad5/0x2150 drivers/block/loop.c:1564
blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&q->limits_lock){+.+.}-{4:4}:
check_prev_add kernel/locking/lockdep.c:3181 [inline]
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
queue_limits_start_update include/linux/blkdev.h:1101 [inline]
loop_set_dio drivers/block/loop.c:1460 [inline]
lo_simple_ioctl drivers/block/loop.c:1528 [inline]
lo_ioctl+0x16ca/0x2150 drivers/block/loop.c:1594
blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 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:
&q->limits_lock --> &sbi->fat_lock --> &q->q_usage_counter(io)#26

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&q->q_usage_counter(io)#26);
lock(&sbi->fat_lock);
lock(&q->q_usage_counter(io)#26);
lock(&q->limits_lock);

*** DEADLOCK ***

locks held by syz.4.8555/6352: 3, last CPU#1:
#0: ffff8880267e7430 (&lo->lo_mutex){+.+.}-{4:4}, at: lo_simple_ioctl drivers/block/loop.c:1520 [inline]
#0: ffff8880267e7430 (&lo->lo_mutex){+.+.}-{4:4}, at: lo_ioctl+0x7c4/0x2150 drivers/block/loop.c:1594
#1: ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline]
#1: ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: loop_set_dio drivers/block/loop.c:1455 [inline]
#1: ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: lo_simple_ioctl drivers/block/loop.c:1528 [inline]
#1: ffff88802688cae0 (&q->q_usage_counter(io)#26){++++}-{0:0}, at: lo_ioctl+0x1688/0x2150 drivers/block/loop.c:1594
#2: ffff88802688cb18 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline]
#2: ffff88802688cb18 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: loop_set_dio drivers/block/loop.c:1455 [inline]
#2: ffff88802688cb18 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: lo_simple_ioctl drivers/block/loop.c:1528 [inline]
#2: ffff88802688cb18 (&q->q_usage_counter(queue)#10){+.+.}-{0:0}, at: lo_ioctl+0x1688/0x2150 drivers/block/loop.c:1594

stack backtrace:
CPU: 1 UID: 0 PID: 6352 Comm: syz.4.8555 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2059
check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2191
check_prev_add kernel/locking/lockdep.c:3181 [inline]
check_prevs_add kernel/locking/lockdep.c:3300 [inline]
validate_chain kernel/locking/lockdep.c:3924 [inline]
__lock_acquire+0x15ff/0x2e50 kernel/locking/lockdep.c:5254
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5908
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x19d/0x1550 kernel/locking/mutex.c:821
queue_limits_start_update include/linux/blkdev.h:1101 [inline]
loop_set_dio drivers/block/loop.c:1460 [inline]
lo_simple_ioctl drivers/block/loop.c:1528 [inline]
lo_ioctl+0x16ca/0x2150 drivers/block/loop.c:1594
blkdev_ioctl+0x5e3/0x740 block/ioctl.c:798
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f450119e0d9
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:00007f45020e0028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f4501426090 RCX: 00007f450119e0d9
RDX: 0000000000000000 RSI: 0000000000004c08 RDI: 0000000000000008
RBP: 00007f4501235024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4501426128 R14: 00007f4501426090 R15: 00007ffee924f4a8
</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 syzk...@googlegroups.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