Re: [Qemu-devel] [PATCH RFC 2/2] block: Warn on insecure format probing

[Markus Armbruster]

Jeff Cody <address@hidden> 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 <address@hidden>
>
>
> [ ...]
>
>> 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,
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