In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix assertion of exclop condition when starting balance

Balance as exclusive state is compatible with paused balance and device
add, which makes some things more complicated. The assertion of valid
states when starting from paused balance needs to take into account two
more states, the combinations can be hit when there are several threads
racing to start balance and device add. This won't typically happen when
the commands are started from command line.

Scenario 1: With exclusive_operation state == BTRFS_EXCLOP_NONE.

Concurrently adding multiple devices to the same mount point and
btrfs_exclop_finish executed finishes before assertion in
btrfs_exclop_balance, exclusive_operation will changed to
BTRFS_EXCLOP_NONE state which lead to assertion failed:

  fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE ||
  fs_info->exclusive_operation == BTRFS_EXCLOP_DEV_ADD,
  in fs/btrfs/ioctl.c:456
  Call Trace:
   <TASK>
   btrfs_exclop_balance+0x13c/0x310
   ? memdup_user+0xab/0xc0
   ? PTR_ERR+0x17/0x20
   btrfs_ioctl_add_dev+0x2ee/0x320
   btrfs_ioctl+0x9d5/0x10d0
   ? btrfs_ioctl_encoded_write+0xb80/0xb80
   __x64_sys_ioctl+0x197/0x210
   do_syscall_64+0x3c/0xb0
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

Scenario 2: With exclusive_operation state == BTRFS_EXCLOP_BALANCE_PAUSED.

Concurrently adding multiple devices to the same mount point and
btrfs_exclop_balance executed finish before the latter thread execute
assertion in btrfs_exclop_balance, exclusive_operation will changed to
BTRFS_EXCLOP_BALANCE_PAUSED state which lead to assertion failed:

  fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE ||
  fs_info->exclusive_operation == BTRFS_EXCLOP_DEV_ADD ||
  fs_info->exclusive_operation == BTRFS_EXCLOP_NONE,
  fs/btrfs/ioctl.c:458
  Call Trace:
   <TASK>
   btrfs_exclop_balance+0x240/0x410
   ? memdup_user+0xab/0xc0
   ? PTR_ERR+0x17/0x20
   btrfs_ioctl_add_dev+0x2ee/0x320
   btrfs_ioctl+0x9d5/0x10d0
   ? btrfs_ioctl_encoded_write+0xb80/0xb80
   __x64_sys_ioctl+0x197/0x210
   do_syscall_64+0x3c/0xb0
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

An example of the failed assertion is below, which shows that the
paused balance is also needed to be checked.

  root@syzkaller:/home/xsk# ./repro
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  Failed to add device /dev/vda, errno 14
  [  416.611428][ T7970] BTRFS info (device loop0): fs_info exclusive_operation: 0
  Failed to add device /dev/vda, errno 14
  [  416.613973][ T7971] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.615456][ T7972] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.617528][ T7973] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.618359][ T7974] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.622589][ T7975] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.624034][ T7976] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.626420][ T7977] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.627643][ T7978] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Failed to add device /dev/vda, errno 14
  [  416.629006][ T7979] BTRFS info (device loop0): fs_info exclusive_operation: 3
  [  416.630298][ T7980] BTRFS info (device loop0): fs_info exclusive_operation: 3
  Fai
---truncated---

https://git.kernel.org/stable/c/17eaeee4c5f24946aad0298d51f32981c3161d13

https://git.kernel.org/stable/c/6062e9e335a3bf409b5118bfe4cc10aff4b6adb1

https://git.kernel.org/stable/c/7877dc1136ada770622d22041be306539902951b

https://git.kernel.org/stable/c/ac868bc9d136cde6e3eb5de77019a63d57a540ff