There are many existing qcow2 images that specify a backing file but
no format. This has been the source of CVEs in the past, but has
become more prominent of a problem now that libvirt has switched to
-blockdev. With older -drive, at least the probing was always done by
qemu (so the only risk of a changed format between successive boots of
a guest was if qemu was upgraded and probed differently). But with
newer -blockdev, libvirt must specify a format; if libvirt guesses raw
where the image was formatted, this results in data corruption visible
to the guest; conversely, if libvirt guesses qcow2 where qemu was
using raw, this can result in potential security holes, so modern
libvirt instead refuses to use images without explicit backing format.
The change in libvirt to reject images without explicit backing format
has pointed out that a number of tools have been far too reliant on
probing in the past. It's time to set a better example in our own
iotests of properly setting this parameter.
iotest calls to create, rebase, and convert are all impacted to some
degree. It's a bit annoying that we are inconsistent on command line
- while all of those accept -o backing_file=...,backing_fmt=..., the
shortcuts are different: create and rebase have -b and -F, while
convert has -B but no -F. (amend has no shortcuts, but the previous
patch just deprecated the use of amend to change backing chains).
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200706203954.341758-9-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Without HEAD^, the following happens when you attempt a large write
request to a qcow2 file such that the number of bytes covered by all
clusters involved in a single allocation will exceed INT_MAX:
(A) handle_alloc_space() decides to fill the whole area with zeroes and
fails because bdrv_co_pwrite_zeroes() fails (the request is too
large).
(B) If handle_alloc_space() does not do anything, but merge_cow()
decides that the requests can be merged, it will create a too long
IOV that later cannot be written.
(C) Otherwise, all parts will be written separately, so those requests
will work.
In either B or C, though, qcow2_alloc_cluster_link_l2() will have an
overflow: We use an int (i) to iterate over nb_clusters, and then
calculate the L2 entry based on "i << s->cluster_bits" -- which will
overflow if the range covers more than INT_MAX bytes. This then leads
to image corruption because the L2 entry will be wrong (it will be
recognized as a compressed cluster).
Even if that were not the case, the .cow_end area would be empty
(because handle_alloc() will cap avail_bytes and nb_bytes at INT_MAX, so
their difference (which is the .cow_end size) will be 0).
So this test checks that on such large requests, the image will not be
corrupted. Unfortunately, we cannot check whether COW will be handled
correctly, because that data is discarded when it is written to null-co
(but we have to use null-co, because writing 2 GB of data in a test is
not quite reasonable).
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Eric Blake
Max Reitz