[syzbot] [block?] possible deadlock in md_alloc

[syzbot] [block?] possible deadlock in md_alloc

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a39c95080621 Add linux-next specific files for 20261029
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=1406117f980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=82467d52d846ff27
dashboard link: https://syzkaller.appspot.com/bug?extid=cd254889291ef8666036
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16477d31580000

AI-generated root cause summary:
A circular locking dependency exists between major_names_lock, md's internal
disks_mutex, and mddev->reconfig_mutex. In blk_probe_dev(), major_names_lock is
held while invoking the driver's autoload probe callback md_probe(), which calls
md_alloc() and acquires disks_mutex followed by mddev->reconfig_mutex.
Concurrently, md ioctls (such as ADD_NEW_DISK) hold mddev->reconfig_mutex while
calling bdev_file_open_by_dev(), which triggers block device autoloading and
attempts to acquire major_names_lock via blk_request_module() / blk_probe_dev(),
resulting in a potential ABBA deadlock.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=cd254889291ef8666036

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cd254889291ef8666036@syzkaller.appspotmail.com

md: could not open device unknown-block(8,200).
md: error, md_import_device() returned -6
md: could not open device unknown-block(8,250).
md: error, md_import_device() returned -6
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz-executor140/6002 is trying to acquire lock:
ffffffff91329200 (disks_mutex){+.+.}-{4:4}, at: md_alloc+0x40/0xf40 drivers/md/md.c:6355

but task is already holding lock:
ffffffff9096c060 (major_names_lock){+.+.}-{4:4}, at: blk_probe_dev block/genhd.c:885 [inline]
ffffffff9096c060 (major_names_lock){+.+.}-{4:4}, at: blk_request_module+0x35/0x2a0 block/genhd.c:901

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (major_names_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
       blk_probe_dev block/genhd.c:885 [inline]
       blk_request_module+0x35/0x2a0 block/genhd.c:901
       blkdev_get_no_open+0x3f/0xe0 block/bdev.c:870
       bdev_file_open_by_dev+0xa0/0x240 block/bdev.c:1091
       md_import_device+0x2e9/0x650 drivers/md/md.c:3861
       md_add_new_disk+0x522/0x1850 drivers/md/md.c:7681
       md_ioctl+0x2027/0x2630 drivers/md/md.c:8525
       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+0x166/0x520 arch/x86/entry/syscall_64.c:84
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&mddev->reconfig_mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
       mddev_lock_nointr drivers/md/md.h:734 [inline]
       md_alloc+0x81e/0xf40 drivers/md/md.c:6413
       md_alloc_and_put+0x1a/0x1f0 drivers/md/md.c:6452
       add_named_array+0x240/0x350 drivers/md/md.c:-1
       param_attr_store+0x214/0x310 kernel/params.c:591
       module_attr_store+0x65/0x90 kernel/params.c:906
       kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
       new_sync_write fs/read_write.c:595 [inline]
       vfs_write+0x612/0xba0 fs/read_write.c:687
       ksys_write+0x150/0x270 fs/read_write.c:1036
       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

-> #0 (disks_mutex){+.+.}-{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
       __mutex_lock_common kernel/locking/mutex.c:646 [inline]
       __mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
       md_alloc+0x40/0xf40 drivers/md/md.c:6355
       md_alloc_and_put+0x1a/0x1f0 drivers/md/md.c:6452
       blk_probe_dev block/genhd.c:888 [inline]
       blk_request_module+0x27d/0x2a0 block/genhd.c:-1
       blkdev_get_no_open+0x3f/0xe0 block/bdev.c:870
       blkdev_open+0x1d7/0x600 block/fops.c:678
       do_dentry_open+0x80e/0x1360 fs/open.c:1000
       vfs_open_consume+0x4a/0x350 fs/open.c:1131
       do_open fs/namei.c:4845 [inline]
       path_openat+0x16fc/0x20f0 fs/namei.c:5010
       do_file_open+0x23e/0x4a0 fs/namei.c:5039
       do_sys_openat2+0x115/0x200 fs/open.c:1442
       do_sys_open fs/open.c:1448 [inline]
       __do_sys_openat fs/open.c:1788 [inline]
       __se_sys_openat fs/open.c:1783 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1783
       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:
  disks_mutex --> &mddev->reconfig_mutex --> major_names_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(major_names_lock);
                               lock(&mddev->reconfig_mutex);
                               lock(major_names_lock);
  lock(disks_mutex);

 *** DEADLOCK ***

locks held by syz-executor140/6002: 1, last CPU#1:
 #0: ffffffff9096c060 (major_names_lock){+.+.}-{4:4}, at: blk_probe_dev block/genhd.c:885 [inline]
 #0: ffffffff9096c060 (major_names_lock){+.+.}-{4:4}, at: blk_request_module+0x35/0x2a0 block/genhd.c:901

stack backtrace:
CPU: 1 UID: 0 PID: 6002 Comm: syz-executor140 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
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
 __mutex_lock_common kernel/locking/mutex.c:646 [inline]
 __mutex_lock+0x197/0x15a0 kernel/locking/mutex.c:821
 md_alloc+0x40/0xf40 drivers/md/md.c:6355
 md_alloc_and_put+0x1a/0x1f0 drivers/md/md.c:6452
 blk_probe_dev block/genhd.c:888 [inline]
 blk_request_module+0x27d/0x2a0 block/genhd.c:-1
 blkdev_get_no_open+0x3f/0xe0 block/bdev.c:870
 blkdev_open+0x1d7/0x600 block/fops.c:678
 do_dentry_open+0x80e/0x1360 fs/open.c:1000
 vfs_open_consume+0x4a/0x350 fs/open.c:1131
 do_open fs/namei.c:4845 [inline]
 path_openat+0x16fc/0x20f0 fs/namei.c:5010
 do_file_open+0x23e/0x4a0 fs/namei.c:5039
 do_sys_openat2+0x115/0x200 fs/open.c:1442
 do_sys_open fs/open.c:1448 [inline]
 __do_sys_openat fs/open.c:1788 [inline]
 __se_sys_openat fs/open.c:1783 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1783
 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: 0033:0x7f9cb93efb37
Code: 48 89 fa 4c 89 df e8 98 1d 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
RSP: 002b:00007ffd0a7a14d0 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 000055558698d400 RCX: 00007f9cb93efb37
RDX: 0000000000000002 RSI: 00007ffd0a7a15f0 RDI: ffffffffffffff9c
RBP: 00007ffd0a7a15b0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00007ffd0a7a15f0
R13: 00000000ffffffff R14: 00007f9cb942909a R15: 0000000000000002
 </TASK>
md: async del_gendisk mode will be removed in future, please upgrade to mdadm-4.5+
block device autoloading is deprecated and will be removed.
md: async del_gendisk mode will be removed in future, please upgrade to mdadm-4.5+
block device autoloading is deprecated and will be removed.


---
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