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

[Anand Jain <anand.jain@oracle.com>]

On 11/13/2018 11:21 PM, David Sterba wrote:
> 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>
> 
> I'm adding the patch to misc-next now, with an update message that
> matches the format when a device is scanned.
> 
> "BTRFS: device fsid %pU devid %llu moved old:%s new:%s\n",
> 
> That way it should be possible to grep for all messages that are related
> to the scanning ioctl.

  Right. Looks fine to me. Thanks, Anand