Re: [RFC PATCH 00/13] Add support for Mirror VM.

[James Bottomley]

On Wed, 2021-08-18 at 18:30 +0100, Dr. David Alan Gilbert wrote:
> * James Bottomley (jejb@linux.ibm.com) wrote:
> > On Wed, 2021-08-18 at 16:43 +0100, Dr. David Alan Gilbert wrote:
> > > * James Bottomley (jejb@linux.ibm.com) wrote:
> > [...]
> > > > Given the lack of SMI, we can't guarantee that with plain SEV
> > > > and -ES. Once we move to -SNP, we can use VMPLs to achieve
> > > > this.
> > > 
> > > Doesn't the MH have access to different slots and running on
> > > separate vCPUs; so it's still got some separation?
> > 
> > Remember that the OVMF code is provided by the host, but its
> > attested to and run by the guest.  Once the guest takes control
> > (i.e. after OVMF boots the next thing), we can't guarantee that it
> > wont overwrite the MH code, so the host must treat the MH as
> > untrusted.
> 
> Yeh; if it's in a romimage I guess we could write protect it?
> (Not that I'd trust it still)

Yes, but unfortunately OVMF (and edk2 in general) has another pitfall
for you: the initial pflash may be a read only ROM image, but it
uncompresses itself to low RAM and executes itself out of there. 
Anything in either PEI or DXE (which is where the migration handler
lies) is RAM based after decompression.

> > > > But realistically, given the above API, even if the guest is
> > > > malicious, what can it do?  I think it's simply return bogus
> > > > pages that cause a crash on start after migration, which
> > > > doesn't look like a huge risk to the cloud to me (it's more a
> > > > self destructive act on behalf of the guest).
> > > 
> > > I'm a bit worried about the data structures that are shared
> > > between the migration code in qemu and the MH; the code in qemu
> > > is going to have to be paranoid about not trusting anything
> > > coming from the MH.
> > 
> > Given that we have to treat the host MH structure as untrusted,
> > this is definitely something we have to do.  Although the primary
> > API is simply "here's a buffer, please fill it", so there's not
> > much checking to do, we just have to be careful that we don't
> > expose any more of the buffer than the guest needs to write to ...
> > and, obviously, clean it before exposing it to the guest.
> 
> I was assuming life got a bit more complicated than that; and we had
> to have lists of pages we were requesting, and a list of pages that
> were cooked and the qemu thread and the helper thread all had to work
> in parallel.  So I'm guessing some list or bookkeeeping that we need
> to be very careful of.

I was more or less imagining a GPA address and length, so range based,
but it could be we need something more sophisticated ... Tobin will
look after that part.  However, either way, we just need to be careful.

Regards,

James