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

[Michael S. Tsirkin]

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 :)

> It seems that SEV on its own is insufficient - there is at least some
> interaction with storage. eg merely running a guest with SEV is not
> going to guarantee security if the guest OS is able to swap out to a
> non-encrypted disk. You could run LUKS inside the guest but that has
> a number of downsides. How to provide the decryption key for LUKS
> at startup without guest admin interaction. Then there is the issue
> that if you take snapshots of the guest disk in the host, this is
> weakening the security of LUKS, since you're keeping around copies
> of the same logical guest sector with different contents which
> allows for improve crytoanalysis. These are reasons for using LUKS
> on the host instead of in the guest, but then the decryption kjeys
> for LUKS are in the QEMU process in memory which is (IIUC) not going
> to be protected by SEV ?  Unles there's a way for QEMU to do allocations
> which are SEV protected for its own purposes ?
> 
> Regards,
> Daniel
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