Re: [Qemu-devel] [RFC PATCH v1 10/22] sev: add SEV debug decrypt command

[Eduardo Habkost]

On Wed, Sep 14, 2016 at 06:46:20PM +0300, Michael S. Tsirkin wrote:
> On Wed, Sep 14, 2016 at 04:06:33PM +0100, Daniel P. Berrange wrote:
> > On Wed, Sep 14, 2016 at 05:48:17PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, Sep 14, 2016 at 03:15:07PM +0100, Daniel P. Berrange wrote:
> > > > On Wed, Sep 14, 2016 at 04:50:51PM +0300, Michael S. Tsirkin wrote:
> > > > > On Wed, Sep 14, 2016 at 02:37:49PM +0100, Daniel P. Berrange wrote:
> > > > > > On Wed, Sep 14, 2016 at 04:32:44PM +0300, Michael S. Tsirkin wrote:
> > > > > > > On Wed, Sep 14, 2016 at 02:23:14PM +0100, Daniel P. Berrange 
> > > > > > > wrote:
> > > > > > > > On Wed, Sep 14, 2016 at 03:07:58PM +0200, Paolo Bonzini wrote:
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > On 14/09/2016 15:05, Michael S. Tsirkin wrote:
> > > > > > > > > > I assumed that with debug on, memory is still encrypted but 
> > > > > > > > > > the
> > > > > > > > > > hypervisor can break encryption, and as the cover letter 
> > > > > > > > > > states, the
> > > > > > > > > > hypervisor is assumed benign. If true I don't see a need to
> > > > > > > > > > give users more rope.
> > > > > > > > > 
> > > > > > > > > The hypervisor is assumed benign but vulnerable.
> > > > > > > > > 
> > > > > > > > > So, if somebody breaks the hypervisor, you would like to make 
> > > > > > > > > it as hard
> > > > > > > > > as possible for the attacker to do evil stuff to the guests.  
> > > > > > > > > If the
> > > > > > > > > attacker can just ask the secure processor "decrypt some 
> > > > > > > > > memory for me",
> > > > > > > > > then the encryption is effectively broken.
> > > > > > > > 
> > > > > > > > So there's going to be a tradeoff here between use of SEV and 
> > > > > > > > use of
> > > > > > > > certain other features. eg, it seems that if you're using SEV, 
> > > > > > > > then
> > > > > > > > any concept of creating & analysing guest core dumps from the 
> > > > > > > > host
> > > > > > > > is out.
> > > > > > > 
> > > > > > > I don't see why - as long as we don't trigger dumps, there's no 
> > > > > > > leak :)
> > > > > > 
> > > > > > If the facility to trigger dumps is available, then the memory
> > > > > > encryption feature of SEV is as useful as a chocolate teapot,
> > > > > > as the would be attacker can simply trigger a dump
> > > > > 
> > > > > If attacker can trigger things, IOW execute code in hypervisor,
> > > > > then encrypting memory is not useful anyway.
> > > > 
> > > > The presentation at KVM forum claimed it *would* protect against
> > > > this, and that things like core dump of unencrypted memory would
> > > > not be permitted, so there's a disconnect between that preso and
> > > > what you're saying.
> > > > 
> > > > Regards,
> > > > Daniel
> > > 
> > > You mean presentation claimed protection against leaks to a malicious
> > > active attacker within a hypervisor?  I guess the presentation covers
> > > more than this patchset does then.  And the disconnect would be with
> > > what the patchset cover letter says, not just with what I say.  Clearly
> > > encrypting memory is not enough to protect against a malicious
> > > hypervisor. E.g. just running info cpus is enough to leak information
> > > from guest.
> > 
> > It was explicit about the fact that the host admin would not have any
> > way to get access to the full contents of guest memory, without the
> > guest admin granting it. Only those non-encrypted pages used for I/O
> > transfer between host & guest would be accessible.
> > 
> > Regards,
> > Daniel
> 
> If you like, that's the vision. I'd rather discuss the patchset in
> question though. It encrypts all memory but this does not protect against
> all attackers, only passive ones. If you disable debugging,
> it seems to additionally reduce the amount of information that can be
> leaked to an active attacker in the hypervisor at one go.
> 
> Paolo seems to think it's useful, but it's a far cry from a deal
> breaker, and your email just makes me worry that it has been oversold to
> the point where everyone will start disabling debugging everywhere in
> production and claim that otherwise it's a security problem.  IMO a much
> better in-tree documentation is needed so people know what they are
> getting in return.
> 
> Attestation seems mostly unrelated. The whitepaper says
>       With this attestation, a guest owner can ensure that the hypervisor did
>       not interfere with the initialization of SEV before transmitting
>       confidential information to the guest.
> which seems to imply an active attacker that is able to interfere
> with the hypervisor during guest initialization but not afterwards.

I believe this assumes a compromised hypervisor both before and
after guest launch, but this assumes the hypervisor:
1) Won't be able to change guest memory before attestation
   without being detected.
2) Won't be able to attack the guest after memory is encrypted.

> So I have no idea why that's useful at the moment - I suspect
> it's part of the future vision when there are protections
> against all active attackers in place, but for now it seems to extend the
> firmware/software interface unnecessarily.

"Protection against all active attackers" is a very broad
requirement. Effective protection against a given subset of
attacks would be reasonable enough to me.

-- 
Eduardo