create volume from snapshot will lose encrypted head when source volume is encrypted in RBD.

Bug #1922408 reported by haixin on 2021-04-03
This bug affects 1 person
Affects Status Importance Assigned to Milestone

Bug Description

1:create an encrypted rbd volume A, which size is 2G.
2:create A's snapshot snap_A.
3:create new encrypted volume B form snap_A, the size of B is 2G too.

   we know that, currently create rbd encrypted volume, we use qume-img create to create an encrypted file, and then use rbd import into rbd cluster, and the encrypted head takes up the first 1 MB capacity, so in the rbd cluster, the real size of volume A is 2G + 1M.
   in rbd driver, at function def create_volume_from_snapshot(), after clone new volume B, will resize volume B to size of A. the code below:
   if int(volume.size)
  we can see that if volume.size is also 2G. then we resize B to 2G. this will lead to volume B lose it't encrypted head.

  if volume B lose encrypted head, bug volume B has encrypt_key_id in database, this will lead to volume B can not attach to VM.

haixin (haixin77) on 2021-04-03
Changed in cinder:
assignee: nobody → haixin (haixin77)
Eric Harney (eharney) on 2021-04-05
tags: added: ceph rbd
Changed in cinder:
importance: Undecided → Medium
Changed in cinder:
assignee: haixin (haixin77) → Sofia Enriquez (lsofia-enriquez)
Changed in cinder:
assignee: Sofia Enriquez (lsofia-enriquez) → haixin (haixin77)
tags: added: snapshot
Changed in cinder:
status: New → In Progress
milestone: none → 19.0.0
Brian Rosmaita (brian-rosmaita) wrote :

Could you explain why you think the header is missing from the new volume? I'd think that what would happen is the new volume would contain the header plus the data, and the resize operation would truncate the data. But the header would still be there.

haixin (haixin77) wrote :

1:create an encrypted rbd volume A, which size is 2G.
2:create A's snapshot snap_A.
3:create new encrypted volume B form snap_A, the size of B is 2G too.

def create_volume_from_snap():
    if int(volume.size)

before exec _resize the size of B is 2G + 1M, after exec _resize. B will be truncate to 2G
,this operation would cut the header, after resize operation, use cryptsetup Dumpluks this volume
,will not found encryption header.

the possilbel resolve:

Usually the source volume would be the same size or smaller than the destination volume and they *must* share the same volume-type.

In particular RBD workflow would be something like this:
A source luks volume would be 1026M, we write some data and create a snap from it. We like to create a new luks volume from a snapshot so the create_volume_from_snapshot() method performs a RBD clone first and then a resize if needed.

In addition the _clone() method creates a clone (copy-on-write child) of the parent snapshot. Object size will be identical to that of the parent image unless specified (we don't in cinder) so size will be the same as the parent snapshot.

If the desired size of the destination luks volume is 1G the create_volume_from_snapshot() won't perform any resize and will be 1026M as the parent. This solves bug #1922408 because we don't force it to resize and because of that we don't truncate the data anymore.

The second case scenario is when we would like to increase the size of the destination volume. As far as I test it this won't face the encryption header problem but we still need to calculate the difference size to provide the size that the user is expecting.

That's why the fix proposed calculate the new_size based on :
size difference = desired size - size of source volume
new size = current size + size difference

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