On 2/24/20 5:38 AM, Peter Krempa wrote:
On Sat, Feb 22, 2020 at 05:23:41 -0600, Eric Blake wrote:
Creating an image that requires format probing of the backing image is
inherently unsafe (we've had several CVEs over the years based on
probes leaking information to the guest on a subsequent boot). If our
probing algorithm ever changes, or if other tools like libvirt
determine a different probe result than we do, then subsequent use of
that backing file under a different format will present corrupted data
to the guest. Start a deprecation clock so that future qemu-img can
refuse to create unsafe backing chains that would rely on probing.
However, there is one time where probing is safe: when we first create
an image, no guest has yet used the new image, so as long as we record
what we probed, all future uses of the image will see the same data -
I disagree. If you are creating an overlay on top of an existing image
it's not safe to probe the format any more generally. (obviously you'd
have to trust the image and express the trust somehow)
The image may have been used in a VM as raw and that means that the VM
might have recorded a valid qcow2 header into it. Creating the overlay
with probing would legitimize this.
Let's assume we have a malicious image written by the guest but we
simulate it by:
b) Now with this patchset:
$ ./qemu-img create -f qcow2 -b /tmp/malicious /tmp/post-patch.qcow2
qemu-img: warning: Deprecated use of non-raw backing file without explicit backing format, using detected format of qcow2
Formatting '/tmp/post-patch.qcow2', fmt=qcow2 size=2560 backing_file=/tmp/malicious backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
You now get a warning, but "backing file format" is now recorded in the
overlay. Now this is WAY worse than it was before. The overlay now
legitimizes the format recorded by the malicious guest which circumvents
libvirt's protections. The warning is very easy to miss, and if you run
it in scripts you might never get to see it. We can't allow that.
Good point. I'll respin this series where v2 never writes the implicit
format except for a raw image (because probing raw is not only safe to
record, but also prevents the guest from ever changing that probe, and
the real risk we are interested in preventing is when a formerly raw
image later probes as non-raw).
so the code now records the probe results as if the user had passed
-F. When this happens, it is unconditionally safe to record a probe
of 'raw', but any other probe is still worth warning the user in case
While it's safe I don't think it should be encouraged. IMO -F should be
made mandatory with -b.
Making it mandatory will require the completion of the deprecation
period. For 5.0 and 5.1, the best we can do is the warning, but for 5.2
(assuming v2 of this series is acceptable), it WILL become a hard error.
our probe differed from their expectations. Similarly, if the backing
file name uses the json: psuedo-protocol, the backing name includes
the format.
Not necessarily. The backing store string can be e.g.:
$ ./qemu-img create -f qcow1 -b 'json:{"driver":"file","filename":"/tmp/malicious"}' /tmp/json.qcow2
Formatting '/tmp/json.qcow1', fmt=qcow2 size=197120 backing_file=json:{"driver":"file",,"filename":"/tmp/malicious"} cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ qemu-img info /tmp/json.qcow1
image: /tmp/json.qcow1
file format: qcow1
virtual size: 191 KiB (197120 bytes)
disk size: 195 KiB
cluster_size: 65535
backing file: json:{"driver":"file","filename":"/tmp/malicious"}
Format specific information:
compat: 0.1
lazy refcounts: false
refcount bits: 15
corrupt: false
Now this has the old semantics but we didn't even get the warning. But
at least the backing file format is not written into the overlay.
Hmm. json:{"driver":"qcow2",...} encodes the format, but your argument
is that json:{"driver":"file",...} encodes only the protocol but not the
format. We want the warning when there is no format, but with json:, it
becomes harder to tell if a format is present or not. I'll have to
think about what to do in that case.
+++ b/block.c
@@ -6013,6 +6013,15 @@ void bdrv_img_create(const char *filename, const char *fmt,
"Could not open backing image to determine size.\n");
goto out;
} else {
+ if (!backing_fmt && !strstart(backing_file, "json:", NULL)) {
+ backing_fmt = bs->drv->format_name;
+ qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt, NULL);
We must never write the detected format into the overlay. Not even when
we print a warning. This can legitimize a malicious file if the user
mises the warning.
Point taken. v2 will address this differently.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
