qcow2, vmdk: Restrict creation with secondary file using protocol

qcow2, vmdk: Restrict creation with secondary file using protocol

Ever since CVE-2024-4467 (see commit 7ead9469 in qemu v9.1.0), we have
intentionally treated the opening of secondary files whose name is
specified in the contents of the primary file, such as a qcow2
data_file, as something that must be a local file and not a protocol
prefix (it is still possible to open a qcow2 file that wraps an NBD
data image by using QMP commands, but that is from the explicit action
of the QMP overriding any string encoded in the qcow2 file).  At the
time, we did not prevent the use of protocol prefixes on the secondary
image while creating a qcow2 file, but it results in a qcow2 file that
records an empty string for the data_file, rather than the protocol
passed in during creation:

$ qemu-img create -f raw datastore.raw 2G
$ qemu-nbd -e 0 -t -f raw datastore.raw &
$ qemu-img create -f qcow2 -o data_file=nbd://localhost:10809/ \
  datastore_nbd.qcow2 2G
Formatting 'datastore_nbd.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=2147483648 data_file=nbd://localhost:10809/ lazy_refcounts=off refcount_bits=16
$ qemu-img info datastore_nbd.qcow2 | grep data
$ qemu-img info datastore_nbd.qcow2 | grep data
image: datastore_nbd.qcow2
    data file:
    data file raw: false
    filename: datastore_nbd.qcow2

And since an empty string was recorded in the file, attempting to open
the image without using QMP to supply the NBD data store fails, with a
somewhat confusing error message:

$ qemu-io -f qcow2 datastore_nbd.qcow2
qemu-io: can't open device datastore_nbd.qcow2: The 'file' block driver requires a file name

Although the ability to create an image with a convenience reference
to a protocol data file is not a security hole (unlike the case with
open, the image is not untrusted if we are the ones creating it), the
above demo shows that it is still inconsistent.  Thus, it makes more
sense if we also insist that image creation rejects a protocol prefix
when using the same syntax.  Now, the above attempt produces:

$ qemu-img create -f qcow2 -o data_file=nbd://localhost:10809/ \
  datastore_nbd.qcow2 2G
Formatting 'datastore_nbd.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=2147483648 data_file=nbd://localhost:10809/ lazy_refcounts=off refcount_bits=16
qemu-img: datastore_nbd.qcow2: Could not create 'nbd://localhost:10809/': No such file or directory

with datastore_nbd.qcow2 no longer created.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250915213919.3121401-6-eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>