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

[Michael S. Tsirkin]

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.


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