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

[Jeff Cody]

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).

> 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.  

>      .bdrv_open              = vhdx_open,
>      .bdrv_close             = vhdx_close,
>      .bdrv_reopen_prepare    = vhdx_reopen_prepare,
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 673d3f5..91a42d2 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -2225,6 +2225,7 @@ static BlockDriver bdrv_vmdk = {
>      .format_name                  = "vmdk",
>      .instance_size                = sizeof(BDRVVmdkState),
>      .bdrv_probe                   = vmdk_probe,
> +    .fname_ext                    = { "vdmk" },
>      .bdrv_open                    = vmdk_open,
>      .bdrv_check                   = vmdk_check,
>      .bdrv_reopen_prepare          = vmdk_reopen_prepare,
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 8898c6c..55609ae 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -91,6 +91,7 @@ struct BlockDriver {
>  
>      int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char 
> *filename);
>      int (*bdrv_probe_device)(const char *filename);
> +    const char *fname_ext[2];
>  
>      /* Any driver implementing this callback is expected to be able to handle
>       * NULL file names in its .bdrv_open() implementation */
> -- 
> 1.9.3
>