btrfs: check superblock to ensure the fs was not modified at thaw time

btrfs: check superblock to ensure the fs was not modified at thaw time

[BACKGROUND]
There is an incident report that, one user hibernated the system, with
one btrfs on removable device still mounted.

Then by some incident, the btrfs got mounted and modified by another
system/OS, then back to the hibernated system.

After resuming from the hibernation, new write happened into the victim btrfs.

Now the fs is completely broken, since the underlying btrfs is no longer
the same one before the hibernation, and the user lost their data due to
various transid mismatch.

[REPRODUCER]
We can emulate the situation using the following small script:

  truncate -s 1G $dev
  mkfs.btrfs -f $dev
  mount $dev $mnt
  fsstress -w -d $mnt -n 500
  sync
  xfs_freeze -f $mnt
  cp $dev $dev.backup

  # There is no way to mount the same cloned fs on the same system,
  # as the conflicting fsid will be rejected by btrfs.
  # Thus here we have to wipe the fs using a different btrfs.
  mkfs.btrfs -f $dev.backup

  dd if=$dev.backup of=$dev bs=1M
  xfs_freeze -u $mnt
  fsstress -w -d $mnt -n 20
  umount $mnt
  btrfs check $dev

The final fsck will fail due to some tree blocks has incorrect fsid.

This is enough to emulate the problem hit by the unfortunate user.

[ENHANCEMENT]
Although such case should not be that common, it can still happen from
time to time.

From the view of btrfs, we can detect any unexpected super block change,
and if there is any unexpected change, we just mark the fs read-only,
and thaw the fs.

By this we can limit the damage to minimal, and I hope no one would lose
their data by this anymore.

Suggested-by: Goffredo Baroncelli <kreijack@libero.it>
Link: https://lore.kernel.org/linux-btrfs/83bf3b4b-7f4c-387a-b286-9251e3991e34@bluemole.com/
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>