linux-kernel - Re: [RFC/GIT PULL] Linux KVM tool for v3.2

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Message-ID: <m3pqh0tnoi.fsf@blackfin.pond.sub.org>
Date:	Thu, 10 Nov 2011 08:57:17 +0100
From:	Markus Armbruster <armbru@...hat.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [RFC/GIT PULL] Linux KVM tool for v3.2

Pekka Enberg <penberg@...nel.org> writes:

> Hi Anthony,
>
>> On 11/04/2011 03:38 AM, Pekka Enberg wrote:
>>> Hi Linus,
>>>
>>> Please consider pulling the latest KVM tool tree from:
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/penberg/linux.git
>>> kvmtool/for-linus
>>>
>>
>> [snip]
>>
>>> tools/kvm/virtio/net.c | 423 ++++++++
>>> tools/kvm/virtio/pci.c | 319 ++++++
>>> tools/kvm/virtio/rng.c | 185 ++++
>>> 186 files changed, 19071 insertions(+), 179 deletions(-)
>
> On Wed, 9 Nov 2011, Anthony Liguori wrote:
>> So let's assume for a moment that a tool like this should live in
>> the kernel. What's disturbing about a PULL request like this is the
>> lack of reviewability of it and the lack of any real review from
>> people that understand what's going on in this code base.
>>
>> There are no Acked-by's by people that really understand what the
>> code is doing or that have domain expertise in filesystems and
>> networking.
>>
>> There are major functionality short comings in this code base, data
>> corruptors, and CVEs.  I'm not saying that the kvm-tool developers
>> are bad developers, but the code is not at the appropriate quality
>> level for the kernel.  It just looks pretty on the surface to people
>> that are used to the kernel coding style.
>>
>> To highlight a few of the issues:
[...]
>> 2) The qcow2 code is a filesystem implemented in userspace.  Image
>> formats are file systems.  It really should be reviewed by the
>> filesystem maintainers. There is absolutely no attempt made to
>> synchronize the metadata during write operations which means that
>> you do not have crash consistency of the meta data.
>>
>> If you experience a power failure or kvm-tool crashs, your image
>> will get corrupted.  I highly doubt a file system would ever be
>> merged into Linux that was this naive about data integrity.
>
> The QCOW2 is lagging behind because we lost the main developer. It's
> forced as read-only for the issues you mention. If you think it's a
> merge blocker, we can drop it completely from the tree until the
> issues are sorted out.
>
> I personally don't see the issue of having it as a read-only filesystem.
>
>> 3) The block probing code replicates a well known CVE from three
>> years ago[1]. Using kvm-tool, a malicious guest could write the
>> qcow2 signature to the zero sector and use that to attack the host.
>
> We don't support QCOW2 snapshots so I don't see how the "arbitrary
> file" thing can happen.

You don't need snapshots for the hole.

Start with a clean read/write raw image.  Probing declares it raw.
Guest writes QCOW signature to it, with a backing file of its choice.

Restart with the same image.  Probing declares it QCOW2.  Guest can read
the backing file.  Oops.

Probing images works when all image types can be probed reliably, and
the guest can't mess with the probing.  Requires distinctive signatures
the guest can't change.  Raw images spoil it.

> It's pretty sad though that we're replicating a known security issue :-/

Maybe I'm wrong, but I got the impression you've been replicating quite
a few of QEMU's early mistakes.

I hope you can create something better than QEMU, I really, really do.
But to successfully build a second system, you need to learn the right
lessons from the first system.  Are you sure you do?

>> [1] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-2004
[...]
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