Re: [Qemu-devel] [PATCH v3 7/9] raw: Prohibit dangerous writes for probed images

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Nov 20, 2014 at 04:27:12PM +0100, Kevin Wolf 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.
> 
> All of these additions that allow to avoid format probing have to be
> specified explicitly. The default still allows the attack.
> 
> In order to fix this, commit 79368c8 (July 2010) put probed raw images
> in a restricted mode, in which they wouldn't be able to overwrite the
> first few bytes of the image so that they would identify as a different
> image. If a write to the first sector would write one of the signatures
> of another driver, qemu would instead zero out the first four bytes.
> This patch was later reverted in commit 8b33d9e (September 2010) because
> it didn't get the handling of unaligned qiov members right.
> 
> Today's block layer that is based on coroutines and has qiov utility
> functions makes it much easier to get this functionality right, so this
> patch implements it.
> 
> The other differences of this patch to the old one are that it doesn't
> silently write something different than the guest requested by zeroing
> out some bytes (it fails the request instead) and that it doesn't
> maintain a list of signatures in the raw driver (it calls the usual
> probe function instead).
> 
> Note that this change doesn't introduce new breakage for false positive
> cases where the guest legitimately writes data into the first sector
> that matches the signatures of an image format (e.g. for nested virt):
> These cases were broken before, only the failure mode changes from
> corruption after the next restart (when the wrong format is probed) to
> failing the problematic write request.
> 
> Also note that like in the original patch, the restrictions only apply
> if the image format has been guessed by probing. Explicitly specifying a
> format allows guests to write anything they like.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  block.c                   |  5 ++--
>  block/raw_bsd.c           | 64 ++++++++++++++++++++++++++++++++++++++++++++++-
>  include/block/block_int.h |  3 +++
>  3 files changed, 69 insertions(+), 3 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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