Patch:

https://listman.redhat.com/archives/libvir-list/2021-November/msg00453.html

Fixed upstream:

commit 1e085019cadcafe87dd96975900c5798dfe46060
Author: Peter Krempa <pkrempa>
Date:   Tue Nov 16 14:45:53 2021 +0100

    qemuDomainPrepareStorageSourceBlockdev: Set default encryption engine also when preparing virStorageSource
    
    Originally the default encryption engine is populated in the disk
    post-parse callback code. This works for disks but for any additional
    images introduced either via the block copy API or via the backup API we
    don't populate the default.
    
    In case when the backup or block copy is requested on an encrypted image
    this would then lead to an error:
    
     error: internal error: Unexpected enum value 0 for virStorageEncryptionEngine
    
    This patch adds another point where we populate the default which is
    when setting up a virStorageSource for actual usage.
    
    We keep the original setting in the post-parse callback as that's the
    only point that is recorded in the XML file after definition.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2023674
    Fixes: ab1d46d6128
    Signed-off-by: Peter Krempa <pkrempa>
    Reviewed-by: Ján Tomko <jtomko>

v7.9.0-187-g1e085019ca

pre-verified with upstream build and result is PASS


[root@yisun-upstream-test ~]# rpm -qa | grep libvirt-7
libvirt-7.10.0-1.fc34.x86_64

[root@yisun-upstream-test ~]# cat sec.xml
<secret ephemeral='no' private='yes'>
<description>LUKS Secret</description>
<usage type='volume'>
<volume>/tmp/scratch</volume>
</usage>
</secret>

[root@yisun-upstream-test ~]# virsh secret-define sec.xml
Secret d533acbb-37ac-4983-a82d-1be85148b0c1 created

[root@yisun-upstream-test ~]# MYSECRET=`printf %s "redhat" | base64`
[root@yisun-upstream-test ~]# virsh secret-set-value d533acbb-37ac-4983-a82d-1be85148b0c1 $MYSECRET
error: Passing secret value as command-line argument is insecure!
Secret value set

[root@yisun-upstream-test ~]# virsh domstate lmn
running

[root@yisun-upstream-test ~]# virsh domblklist lmn
 Target   Source
---------------------------------------------
 sda      /var/lib/libvirt/images/lmn.qcow2

[root@yisun-upstream-test ~]# cat backup.xml
<domainbackup mode="pull">
  <server name="localhost" port="10809" />
  <disks>
    <disk backup="yes" name="sda" type="file">
      <scratch file="/tmp/scratch_file">
        <encryption format="luks">
          <secret type="passphrase" uuid="d533acbb-37ac-4983-a82d-1be85148b0c1" />
        </encryption>
      </scratch>
    </disk>
  </disks>
</domainbackup>


[root@yisun-upstream-test ~]# virsh backup-begin lmn  backup.xml
Backup started


[root@yisun-upstream-test ~]# qemu-img info /tmp/scratch_file -U
image: /tmp/scratch_file
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 2.19 MiB
encrypted: yes
cluster_size: 65536
backing file: /var/lib/libvirt/images/lmn.qcow2
backing file format: qcow2
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    encrypt:
        ivgen alg: plain64
        hash alg: sha256
        cipher alg: aes-256
        uuid: ea6631ad-bb66-456e-adeb-2c88c1bbaced
        format: luks
        cipher mode: xts
        slots:
            [0]:
                active: true
                iters: 1253078
                key offset: 4096
                stripes: 4000
            [1]:
                active: false
                key offset: 262144
            [2]:
                active: false
                key offset: 520192
            [3]:
                active: false
                key offset: 778240
            [4]:
                active: false
                key offset: 1036288
            [5]:
                active: false
                key offset: 1294336
            [6]:
                active: false
                key offset: 1552384
            [7]:
                active: false
                key offset: 1810432
        payload offset: 2068480
        master key iters: 333062
    corrupt: false
    extended l2: false

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (new packages: libvirt), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2022:2390