[syzbot] [exfat?] possible deadlock in fat_count_free_clusters

[syzbot <syzbot+a5d8c609c02f508672cc@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    929beafbe7ac Add linux-next specific files for 20241108
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1621bd87980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=75175323f2078363
dashboard link: https://syzkaller.appspot.com/bug?extid=a5d8c609c02f508672cc
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9705ecb6a595/disk-929beafb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/dbdd1f64b9b8/vmlinux-929beafb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3f70d07a929b/bzImage-929beafb.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a5d8c609c02f508672cc@syzkaller.appspotmail.com

FAT-fs (loop3): error, invalid access to FAT (entry 0x0000616b)
======================================================
WARNING: possible circular locking dependency detected
6.12.0-rc6-next-20241108-syzkaller #0 Not tainted
------------------------------------------------------
syz.3.2125/17744 is trying to acquire lock:
ffff8880691980b0 (&sbi->fat_lock){+.+.}-{4:4}, at: lock_fat fs/fat/fatent.c:281 [inline]
ffff8880691980b0 (&sbi->fat_lock){+.+.}-{4:4}, at: fat_count_free_clusters+0x156/0xe70 fs/fat/fatent.c:724

but task is already holding lock:
ffff88802533deb0 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:944 [inline]
ffff88802533deb0 (&q->limits_lock){+.+.}-{4:4}, at: loop_reconfigure_limits+0x287/0x9f0 drivers/block/loop.c:1003

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&q->limits_lock){+.+.}-{4:4}:
       lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
       __mutex_lock_common kernel/locking/mutex.c:585 [inline]
       __mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
       queue_limits_start_update include/linux/blkdev.h:944 [inline]
       loop_reconfigure_limits+0x287/0x9f0 drivers/block/loop.c:1003
       loop_set_block_size drivers/block/loop.c:1473 [inline]
       lo_simple_ioctl drivers/block/loop.c:1496 [inline]
       lo_ioctl+0x1351/0x1f50 drivers/block/loop.c:1559
       blkdev_ioctl+0x57d/0x6a0 block/ioctl.c:693
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&q->q_usage_counter(io)#17){++++}-{0:0}:
       lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
       bio_queue_enter block/blk.h:75 [inline]
       blk_mq_submit_bio+0x1510/0x2490 block/blk-mq.c:3095
       __submit_bio+0x2c2/0x560 block/blk-core.c:629
       __submit_bio_noacct_mq block/blk-core.c:710 [inline]
       submit_bio_noacct_nocheck+0x4d3/0xe30 block/blk-core.c:739
       submit_bh fs/buffer.c:2819 [inline]
       __bread_slow fs/buffer.c:1264 [inline]
       __bread_gfp+0x23c/0x400 fs/buffer.c:1488
       sb_bread include/linux/buffer_head.h:346 [inline]
       fat12_ent_bread+0x155/0x540 fs/fat/fatent.c:77
       fat_ent_read_block+0x3e4/0x530 fs/fat/fatent.c:445
       fat_alloc_clusters+0x4ee/0x11c0 fs/fat/fatent.c:493
       fat_add_cluster fs/fat/inode.c:107 [inline]
       __fat_get_block fs/fat/inode.c:154 [inline]
       fat_get_block+0x4c4/0xd00 fs/fat/inode.c:189
       __block_write_begin_int+0x50c/0x1a70 fs/buffer.c:2116
       block_write_begin fs/buffer.c:2226 [inline]
       cont_write_begin+0x6e2/0x9d0 fs/buffer.c:2577
       fat_write_begin+0x76/0x140 fs/fat/inode.c:228
       generic_perform_write+0x344/0x6d0 mm/filemap.c:4055
       generic_file_write_iter+0xae/0x310 mm/filemap.c:4182
       new_sync_write fs/read_write.c:586 [inline]
       vfs_write+0xaeb/0xd30 fs/read_write.c:679
       ksys_write+0x18f/0x2b0 fs/read_write.c:731
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&sbi->fat_lock){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3161 [inline]
       check_prevs_add kernel/locking/lockdep.c:3280 [inline]
       validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
       __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
       lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
       __mutex_lock_common kernel/locking/mutex.c:585 [inline]
       __mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
       lock_fat fs/fat/fatent.c:281 [inline]
       fat_count_free_clusters+0x156/0xe70 fs/fat/fatent.c:724
       fat_statfs+0x139/0x450 fs/fat/inode.c:834
       statfs_by_dentry fs/statfs.c:66 [inline]
       vfs_statfs+0x13b/0x2c0 fs/statfs.c:90
       loop_config_discard drivers/block/loop.c:798 [inline]
       loop_reconfigure_limits+0x5fe/0x9f0 drivers/block/loop.c:1012
       loop_configure+0x77e/0xeb0 drivers/block/loop.c:1093
       lo_ioctl+0x846/0x1f50
       blkdev_ioctl+0x57d/0x6a0 block/ioctl.c:693
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:906 [inline]
       __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
       do_syscall_x64 arch/x86/entry/common.c:52 [inline]
       do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &sbi->fat_lock --> &q->q_usage_counter(io)#17 --> &q->limits_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&q->limits_lock);
                               lock(&q->q_usage_counter(io)#17);
                               lock(&q->limits_lock);
  lock(&sbi->fat_lock);

 *** DEADLOCK ***

2 locks held by syz.3.2125/17744:
 #0: ffff88802541fb60 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_global_lock_killable drivers/block/loop.c:120 [inline]
 #0: ffff88802541fb60 (&lo->lo_mutex){+.+.}-{4:4}, at: loop_configure+0x1f7/0xeb0 drivers/block/loop.c:1044
 #1: ffff88802533deb0 (&q->limits_lock){+.+.}-{4:4}, at: queue_limits_start_update include/linux/blkdev.h:944 [inline]
 #1: ffff88802533deb0 (&q->limits_lock){+.+.}-{4:4}, at: loop_reconfigure_limits+0x287/0x9f0 drivers/block/loop.c:1003

stack backtrace:
CPU: 0 UID: 0 PID: 17744 Comm: syz.3.2125 Not tainted 6.12.0-rc6-next-20241108-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
 print_circular_bug+0x13a/0x1b0 kernel/locking/lockdep.c:2074
 check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2206
 check_prev_add kernel/locking/lockdep.c:3161 [inline]
 check_prevs_add kernel/locking/lockdep.c:3280 [inline]
 validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
 __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
 lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
 __mutex_lock_common kernel/locking/mutex.c:585 [inline]
 __mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
 lock_fat fs/fat/fatent.c:281 [inline]
 fat_count_free_clusters+0x156/0xe70 fs/fat/fatent.c:724
 fat_statfs+0x139/0x450 fs/fat/inode.c:834
 statfs_by_dentry fs/statfs.c:66 [inline]
 vfs_statfs+0x13b/0x2c0 fs/statfs.c:90
 loop_config_discard drivers/block/loop.c:798 [inline]
 loop_reconfigure_limits+0x5fe/0x9f0 drivers/block/loop.c:1012
 loop_configure+0x77e/0xeb0 drivers/block/loop.c:1093
 lo_ioctl+0x846/0x1f50
 blkdev_ioctl+0x57d/0x6a0 block/ioctl.c:693
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9752d7e719
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:00007f9753abd038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f9752f36058 RCX: 00007f9752d7e719
RDX: 00000000200002c0 RSI: 0000000000004c0a RDI: 0000000000000008
RBP: 00007f9752df139e R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f9752f36058 R15: 00007ffe36e679e8
 </TASK>


---
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