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

[Eduardo Habkost]

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.

I believe the whole point of SEV attestation and key management
is to make "if attacker can executed code in hypervisor,
encrypting memory is not useful" _not_ true, isn't it?

Or are there known vulnerabilities that would allow a compromised
hypervisor to decrypt memory even after successful
encryption+attestation?

-- 
Eduardo