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Re: [Qemu-devel] [PATCH RFC 2/2] block: Warn on insecure format probing
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH RFC 2/2] block: Warn on insecure format probing |
Date: |
Mon, 03 Nov 2014 09:11:54 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Jeff Cody <address@hidden> writes:
> 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.
If .vhd turns out to be ambiguous in practice, we need to deal with it.
I got an idea how, but I'd prefer to discuss it after we figured out how
to address our "inscure by default" issue.
>> >> .bdrv_open = vhdx_open,
>> >> .bdrv_close = vhdx_close,
>> >> .bdrv_reopen_prepare = vhdx_reopen_prepare,
>> [...]