[PATCH 3/3] qemu-img: Deprecate use of -b without -F

Peter Krempa pkrempa at redhat.com
Mon Feb 24 11:38:27 UTC 2020


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:

$ qemu-img  create -f qcow2 -F raw -b /etc/passwd /tmp/malicious
Formatting '/tmp/malicious', fmt=qcow2 size=2560 backing_file=/etc/passwd backing_fmt=raw cluster_size=65536 lazy_refcounts=off refcount_bits=16


Now we want to create an overlay.

a) without this patchset:

$ qemu-img create -f qcow2 -b /tmp/malicious /tmp/pre-patch.qcow2
Formatting '/tmp/pre-patch.qcow2', fmt=qcow2 size=2560 backing_file=/tmp/malicious cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ qemu-img info /tmp/pre-patch.qcow2
image: /tmp/pre-patch.qcow2
file format: qcow2
virtual size: 2.5 KiB (2560 bytes)
disk size: 196 KiB
cluster_size: 65536
backing file: /tmp/malicious
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

There's no 'backing file format'. When used by libvirt we'd not allow
the VM to touch the backing file of /tmp/malicious in pre-blockdev era
and in libvirt-6.0 we'd report an error right away.

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
$ qemu-img info /tmp/post-patch.qcow2
image: /tmp/post-patch.qcow2
file format: qcow2
virtual size: 2.5 KiB (2560 bytes)
disk size: 196 KiB
cluster_size: 65536
backing file: /tmp/malicious
backing file format: qcow2
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

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.


> 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.

> 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.


> iotest 114 specifically wants to create an unsafe image for later
> amendment rather than defaulting to our new default of recording a
> probed format, so it needs an update.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
>  block.c                    | 17 ++++++++++++++++-
>  qemu-deprecated.texi       | 12 ++++++++++++
>  qemu-img.c                 |  8 +++++++-
>  tests/qemu-iotests/114     |  4 ++--
>  tests/qemu-iotests/114.out |  1 +
>  5 files changed, 38 insertions(+), 4 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 695decbfd7b7..6595683ac52a 100644
> --- a/block.c
> +++ 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.




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