Re: SEV guest regression in 4.18

[Brijesh Singh]

On 08/24/2018 11:24 AM, Sean Christopherson wrote:
> On Fri, Aug 24, 2018 at 10:41:27AM -0500, Brijesh Singh wrote:
> > On 08/23/2018 11:16 AM, Paolo Bonzini wrote:
> > > On 23/08/2018 17:29, Sean Christopherson wrote:
> > > > On Thu, Aug 23, 2018 at 01:26:55PM +0200, Paolo Bonzini wrote:
> > > > > On 22/08/2018 22:11, Brijesh Singh wrote:
> > > > > > Yes, this is one of approach I have in mind. It will avoid splitting
> > > > > > the larger pages; I am thinking that early in boot code we can lookup
> > > > > > for this special section and decrypt it in-place and probably maps with
> > > > > > C=0. Only downside, it will increase data section footprint a bit
> > > > > > because we need to align this section to PM_SIZE.
> > > > >
> > > > > If you can ensure it doesn't span a PMD, maybe it does not need to be
> > > > > aligned; you could establish a C=0 mapping of the whole 2M around it.
> > > >
> > > > Wouldn't that result in exposing/leaking whatever code/data happened
> > > > to reside on the same 2M page (or corrupting it if the entire page
> > > > isn't decrypted)?  Or are you suggesting that we'd also leave the
> > > > encrypted mapping intact?
> > >
> > > Yes, exactly the latter, because...
> >
> >
> > Hardware does not enforce coherency between the encrypted and
> > unencrypted mapping for the same physical page. So, creating a
> > two mapping of same physical address will lead a possible data
> > corruption.
>
> But couldn't we avoid corruption by ensuring data accessed via the
> unencrypted mapping is cache line aligned and sized?  The CPU could
> speculatively bring the encrypted version into the cache but it
> should never get into a modified state (barring a software bug, but
> that would be a problem regardless of encryption).

Yes, if we can ensure that accessed are cache line aligned and sized
then we should be fine.