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

[Jeff Cody]

On Wed, Oct 29, 2014 at 07:37:02AM +0100, Markus Armbruster wrote:
> 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?
>

I need to resurrect my Windows Server Hyper-V test machine, and see
what it generates by default.  Most likely '.vhdx'

However, even so, it seems entirely plausible that a 4-letter
extension may end up represented as a 3-digit extension, and be .vhd,
even if that is not the 'official' name.

> >>      .bdrv_open              = vhdx_open,
> >>      .bdrv_close             = vhdx_close,
> >>      .bdrv_reopen_prepare    = vhdx_reopen_prepare,
> [...]