Re: [syzbot] [overlayfs?] possible deadlock in mnt_want_write (8)

On Tue, Jun 9, 2026 at 1:15 AM syzbot
<[email protected]> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    ec7ef10b77a6 Merge branch 'for-next/core' into for-kernelci
> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> console output: https://syzkaller.appspot.com/x/log.txt?x=17a17dd2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=522222e78af9b9d2
> dashboard link: https://syzkaller.appspot.com/bug?extid=1e5626d6fd59853cbfaa
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> userspace arch: arm64
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/22c81873d970/disk-ec7ef10b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ba2c817c4ea9/vmlinux-ec7ef10b.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/04c4e1c1bb56/Image-ec7ef10b.gz.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email protected]
>
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Tainted: G             L
> ------------------------------------------------------
> syz.5.673/7842 is trying to acquire lock:
> ffff0000d814a410 (sb_writers#3){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:493
>
> but task is already holding lock:
> ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
> ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_nlink_start+0xc8/0x350 fs/overlayfs/util.c:1173
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (&ovl_i_lock_key[depth]){+.+.}-{4:4}:
>        __mutex_lock_common kernel/locking/mutex.c:646 [inline]
>        __mutex_lock+0x160/0xef8 kernel/locking/mutex.c:820
>        mutex_lock_interruptible_nested+0x24/0x30 kernel/locking/mutex.c:898
>        ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
>        ovl_copy_up_start+0x58/0x264 fs/overlayfs/util.c:735
>        ovl_copy_up_one fs/overlayfs/copy_up.c:1182 [inline]
>        ovl_copy_up_flags+0x768/0x28a4 fs/overlayfs/copy_up.c:1243
>        ovl_copy_up+0x24/0x34 fs/overlayfs/copy_up.c:1282
>        ovl_link+0x8c/0x26c fs/overlayfs/dir.c:774
>        vfs_link+0x40c/0x580 fs/namei.c:5774
>        filename_linkat+0x224/0x478 fs/namei.c:5842
>        __do_sys_linkat fs/namei.c:5868 [inline]
>        __se_sys_linkat fs/namei.c:5863 [inline]
>        __arm64_sys_linkat+0xe8/0x118 fs/namei.c:5863
>        __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>        invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
>        el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
>        do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
>        el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
>        el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
>        el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
>
> -> #1 (&ovl_i_mutex_key[depth]){+.+.}-{4:4}:
>        down_write+0x50/0xc0 kernel/locking/rwsem.c:1625
>        inode_lock include/linux/fs.h:1029 [inline]
>        lock_two_nondirectories+0xc4/0x148 fs/inode.c:1254
>        ext4_move_extents+0x194/0x3580 fs/ext4/move_extent.c:589
>        __ext4_ioctl fs/ext4/ioctl.c:1657 [inline]
>        ext4_ioctl+0x2a14/0x4234 fs/ext4/ioctl.c:1922
>        vfs_ioctl fs/ioctl.c:51 [inline]
>        __do_sys_ioctl fs/ioctl.c:597 [inline]
>        __se_sys_ioctl fs/ioctl.c:583 [inline]
>        __arm64_sys_ioctl+0x14c/0x1c4 fs/ioctl.c:583
>        __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>        invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
>        el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
>        do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
>        el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
>        el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
>        el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
>
> -> #0 (sb_writers#3){.+.+}-{0:0}:
>        check_prev_add kernel/locking/lockdep.c:3165 [inline]
>        check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>        validate_chain kernel/locking/lockdep.c:3908 [inline]
>        __lock_acquire+0x1780/0x2f44 kernel/locking/lockdep.c:5237
>        lock_acquire+0x140/0x368 kernel/locking/lockdep.c:5868
>        percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
>        percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
>        __sb_start_write include/linux/fs/super.h:19 [inline]
>        sb_start_write+0x5c/0x294 include/linux/fs/super.h:125
>        mnt_want_write+0x44/0x9c fs/namespace.c:493
>        ovl_want_write fs/overlayfs/util.c:38 [inline]
>        ovl_nlink_start+0x154/0x350 fs/overlayfs/util.c:1177
>        ovl_do_remove+0x1c0/0xa04 fs/overlayfs/dir.c:954
>        ovl_rmdir+0x24/0x34 fs/overlayfs/dir.c:993
>        vfs_rmdir+0x34c/0x5a0 fs/namei.c:5366
>        filename_rmdir+0x208/0x3dc fs/namei.c:5421
>        __do_sys_unlinkat fs/namei.c:5596 [inline]
>        __se_sys_unlinkat fs/namei.c:5589 [inline]
>        __arm64_sys_unlinkat+0xc8/0x104 fs/namei.c:5589
>        __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>        invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
>        el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
>        do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
>        el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
>        el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
>        el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
>
> other info that might help us debug this:
>
> Chain exists of:
>   sb_writers#3 --> &ovl_i_mutex_key[depth] --> &ovl_i_lock_key[depth]
>
>  Possible unsafe locking scenario:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&ovl_i_lock_key[depth]);
>                                lock(&ovl_i_mutex_key[depth]);
>                                lock(&ovl_i_lock_key[depth]);
>   rlock(sb_writers#3);
>
>  *** DEADLOCK ***
>
> 4 locks held by syz.5.673/7842:
>  #0: ffff0000d924c410 (sb_writers#17){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:493
>  #1: ffff0000dc944300 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:1074 [inline]
>  #1: ffff0000dc944300 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: __start_dirop+0xa0/0xf4 fs/namei.c:2914
>  #2: ffff0000dc946ec0 (&ovl_i_mutex_dir_key[depth]){++++}-{4:4}, at: inode_lock include/linux/fs.h:1029 [inline]
>  #2: ffff0000dc946ec0 (&ovl_i_mutex_dir_key[depth]){++++}-{4:4}, at: vfs_rmdir+0x110/0x5a0 fs/namei.c:5351
>  #3: ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
>  #3: ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_nlink_start+0xc8/0x350 fs/overlayfs/util.c:1173
>
> stack backtrace:
> CPU: 1 UID: 0 PID: 7842 Comm: syz.5.673 Tainted: G             L      syzkaller #0 PREEMPT
> Tainted: [L]=SOFTLOCKUP
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
> Call trace:
>  show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
>  __dump_stack+0x30/0x40 lib/dump_stack.c:94
>  dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
>  dump_stack+0x1c/0x28 lib/dump_stack.c:129
>  print_circular_bug+0x328/0x330 kernel/locking/lockdep.c:2043
>  check_noncircular+0x158/0x174 kernel/locking/lockdep.c:2175
>  check_prev_add kernel/locking/lockdep.c:3165 [inline]
>  check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>  validate_chain kernel/locking/lockdep.c:3908 [inline]
>  __lock_acquire+0x1780/0x2f44 kernel/locking/lockdep.c:5237
>  lock_acquire+0x140/0x368 kernel/locking/lockdep.c:5868
>  percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
>  percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
>  __sb_start_write include/linux/fs/super.h:19 [inline]
>  sb_start_write+0x5c/0x294 include/linux/fs/super.h:125
>  mnt_want_write+0x44/0x9c fs/namespace.c:493
>  ovl_want_write fs/overlayfs/util.c:38 [inline]
>  ovl_nlink_start+0x154/0x350 fs/overlayfs/util.c:1177
>  ovl_do_remove+0x1c0/0xa04 fs/overlayfs/dir.c:954
>  ovl_rmdir+0x24/0x34 fs/overlayfs/dir.c:993
>  vfs_rmdir+0x34c/0x5a0 fs/namei.c:5366
>  filename_rmdir+0x208/0x3dc fs/namei.c:5421
>  __do_sys_unlinkat fs/namei.c:5596 [inline]
>  __se_sys_unlinkat fs/namei.c:5589 [inline]
>  __arm64_sys_unlinkat+0xc8/0x104 fs/namei.c:5589
>  __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>  invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
>  el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
>  do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
>  el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
>  el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
>  el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
>
>
> ---
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> If the report is a duplicate of another one, reply with:
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#syz dup: possible deadlock in lock_two_nondirectories (2)

Thanks,
Amir.