Re: [PATCH RFC RESEND] btrfs: harden agaist duplicate fsid

[David Sterba <dsterba@suse.cz>]

On Mon, Oct 15, 2018 at 10:45:17AM +0800, Anand Jain wrote:
> (Thanks for the comments on requiring to warn_on if we fail the device change.)
> (This fixes an ugly bug, I appreciate if you have any further comments).
> 
> Its not that impossible to imagine that a device OR a btrfs image is
> been copied just by using the dd or the cp command. Which in case both
> the copies of the btrfs will have the same fsid. If on the system with
> automount enabled, the copied FS gets scanned.
> 
> We have a known bug in btrfs, that we let the device path be changed
> after the device has been mounted. So using this loop hole the new
> copied device would appears as if its mounted immediately after its
> been copied.
> 
> For example:
> 
> Initially.. /dev/mmcblk0p4 is mounted as /
> 
> lsblk
> NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
> mmcblk0     179:0    0 29.2G  0 disk
> |-mmcblk0p4 179:4    0    4G  0 part /
> |-mmcblk0p2 179:2    0  500M  0 part /boot
> |-mmcblk0p3 179:3    0  256M  0 part [SWAP]
> `-mmcblk0p1 179:1    0  256M  0 part /boot/efi
> 
> btrfs fi show
>    Label: none  uuid: 07892354-ddaa-4443-90ea-f76a06accaba
>    Total devices 1 FS bytes used 1.40GiB
>    devid    1 size 4.00GiB used 3.00GiB path /dev/mmcblk0p4
> 
> Copy mmcblk0 to sda
>    dd if=/dev/mmcblk0 of=/dev/sda
> 
> And immediately after the copy completes the change in the device
> superblock is notified which the automount scans using
> btrfs device scan and the new device sda becomes the mounted root
> device.
> 
> lsblk
> NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
> sda           8:0    1 14.9G  0 disk
> |-sda4        8:4    1    4G  0 part /
> |-sda2        8:2    1  500M  0 part
> |-sda3        8:3    1  256M  0 part
> `-sda1        8:1    1  256M  0 part
> mmcblk0     179:0    0 29.2G  0 disk
> |-mmcblk0p4 179:4    0    4G  0 part
> |-mmcblk0p2 179:2    0  500M  0 part /boot
> |-mmcblk0p3 179:3    0  256M  0 part [SWAP]
> `-mmcblk0p1 179:1    0  256M  0 part /boot/efi
> 
> btrfs fi show /
>  Label: none  uuid: 07892354-ddaa-4443-90ea-f76a06accaba
>  Total devices 1 FS bytes used 1.40GiB
>  devid    1 size 4.00GiB used 3.00GiB path /dev/sda4
> 
> The bug is quite nasty that you can't either unmount /dev/sda4 or
> /dev/mmcblk0p4. And the problem does not get solved until you take
> sda out of the system on to another system to change its fsid
> using the 'btrfstune -u' command.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>

The fix for that case looks ok to me, block devices with duplicate UUIDs
are problematic. I'll add the patch to for-next, thanks.