Jeff Cody <jc...@redhat.com> writes:
> On Tue, Oct 28, 2014 at 05:03:40PM +0100, Markus Armbruster wrote:
>> If the user neglects to specify the image format, QEMU probes the
>> image to guess it automatically, for convenience.
>>
>> Relying on format probing is insecure for raw images (CVE-2008-2004).
>> If the guest writes a suitable header to the device, the next probe
>> will recognize a format chosen by the guest. A malicious guest can
>> abuse this to gain access to host files, e.g. by crafting a QCOW2
>> header with backing file /etc/shadow.
>>
>> Commit 1e72d3b (April 2008) provided -drive parameter format to let
>> users disable probing. Commit f965509 (March 2009) extended QCOW2 to
>> optionally store the backing file format, to let users disable backing
>> file probing. QED has had a flag to suppress probing since the
>> beginning (2010), set whenever a raw backing file is assigned.
>>
>> Despite all this work (and time!), we're still insecure by default. I
>> think we're doing our users a disservice by sticking to the fatally
>> flawed probing. "Broken by default" is just wrong, and "convenience"
>> is no excuse.
>>
>> I believe we can retain 90% of the convenience without compromising
>> security by keying on image file name instead of image contents: if
>> the file name ends in .img or .iso, assume raw, if it ends in .qcow2,
>> assume qcow2, and so forth.
>>
>> Naturally, this would break command lines where the filename doesn't
>> provide the correct clue. So don't do it just yet, only warn if the
>> the change would lead to a different result. Looks like this:
>>
>> qemu: -drive file=my.img: warning: insecure format probing of image
>> 'my.img'
>> To get rid of this warning, specify format=qcow2 explicitly, or change
>> the image name to end with '.qcow2'
>>
>> This should steer users away from insecure format probing. After a
>> suitable grace period, we can hopefully drop format probing
>> alltogether.
>>
>> Example 0: file=WHATEVER,format=F
>>
>> Never warns, because the explicit format suppresses probing.
>>
>> Example 1: file=FOO.img
>>
>> Warns when probing of FOO.img results in anything but raw. In
>> particular, it warns when the guest just p0wned you.
>>
>> Example 2: file=FOO.qcow2 with backing file name FOO.img and no
>> backing image format.
>>
>> Warns when probing of FOO.qcow2 results in anything but qcow2, or
>> probing of FOO.img results in anything but raw.
>>
>> This patch is RFC because of open questions:
>>
>> * Should tools warn, too? Probing isn't insecure there, but a "this
>> may pick a different format in the future" warning may be
>> appropriate.
>>
>> * I didn't specify recognized extensions for formats "bochs", "cloop",
>> "parallels", "vpc", because I have no idea which ones to recognize.
>>
>
> Format 'vpc' should probably recognize both extensions "vpc" and
> "vhd". The actual format is VHD, so most MS tools will probably
> create files with .vhd extensions.
>
> (This creates an overlap with vhdx; see my response to Eric's email).
Going to discuss it there.
>> Additionally, some tests still need to be updated.
>>
>> Signed-off-by: Markus Armbruster <arm...@redhat.com>
>
>
> [ ...]
>
>> diff --git a/block/vhdx.c b/block/vhdx.c
>> index 12bfe75..d2c3a20 100644
>> --- a/block/vhdx.c
>> +++ b/block/vhdx.c
>> @@ -1945,6 +1945,7 @@ static BlockDriver bdrv_vhdx = {
>> .format_name = "vhdx",
>> .instance_size = sizeof(BDRVVHDXState),
>> .bdrv_probe = vhdx_probe,
>> + .fname_ext = { "vhd" },
>
> This should also have "vhdx", I think.
Okay. I looked for confirmation in Wikipedia, and found:
Hyper-V, like Microsoft Virtual Server and Windows Virtual PC, saves
each guest OS to a single virtual hard disk file with the extension
.VHD, except in Windows 8 and Windows Server 2012 where it can be
the newer .vhdx.
https://en.wikipedia.org/wiki/Hyper-V#VHD_compatibility_with_Virtual_Server_2005_and_Virtual_PC_2004.2F2007
Makes me wonder whether .vhd is really used for both vhdx and vpc format
images. What have you seen in the wild?
>> .bdrv_open = vhdx_open,
>> .bdrv_close = vhdx_close,
>> .bdrv_reopen_prepare = vhdx_reopen_prepare,
[...]
