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
