Re: [PATCH v2 0/8] x86/coco: Mark CoCo VM pages not present when changing encrypted state

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Tue, Nov 28, 2023 at 07:12:33PM +0000, Michael Kelley wrote:
> From: kirill.shutemov@xxxxxxxxxxxxxxx <kirill.shutemov@xxxxxxxxxxxxxxx> Sent: Friday, November 24, 2023 2:06 AM
> > 
> > On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@xxxxxxxxx wrote:
> > > From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > >
> > > In a CoCo VM when a page transitions from encrypted to decrypted, or vice
> > > versa, attributes in the PTE must be updated *and* the hypervisor must
> > > be notified of the change.
> > 
> > Strictly speaking it is not true for TDX. Conversion to shared can be
> > implicit: set shared bit and touch the page will do the conversion. MapGPA
> > is optional.
> 
> Interesting.  Given that, is there a reason to use the explicit
> hypervisor callbacks in for private->shared transitions in 
> __set_mem_enc_pgtable()?   It probably doesn't have direct relevance
> to this patch series, but I'm just trying to understand the tradeoffs of
> the implicit vs. explicit approach.  And am I correct that
> shared->private transitions must use the explicit approach?

It must be explicit in sense, that the memory has to be accepted before
use. MapGPA() is still optional.

I don't like this implicit tricks. I spent a lot of time debugging an
issue that was obscured by this semantics.

But I think it is going to say :/

> > > Because there are two separate steps, there's
> > > a window where the settings are inconsistent.  Normally the code that
> > > initiates the transition (via set_memory_decrypted() or
> > > set_memory_encrypted()) ensures that the memory is not being accessed
> > > during a transition, so the window of inconsistency is not a problem.
> > > However, the load_unaligned_zeropad() function can read arbitrary memory
> > > pages at arbitrary times, which could read a transitioning page during
> > > the window.  In such a case, CoCo VM specific exceptions are taken
> > > (depending on the CoCo architecture in use).  Current code in those
> > > exception handlers recovers and does "fixup" on the result returned by
> > > load_unaligned_zeropad().  Unfortunately, this exception handling can't
> > > work in paravisor scenarios (TDX Paritioning and SEV-SNP in vTOM mode)
> > > if the exceptions are routed to the paravisor.  The paravisor can't
> > > do load_unaligned_zeropad() fixup, so the exceptions would need to
> > > be forwarded from the paravisor to the Linux guest, but there are
> > > no architectural specs for how to do that.
> > 
> > Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees
> > cross-page access to shared memory while no fixup entry for the page in
> > L1. It would give L2 chance to handle the situation in a transparent way.
> > 
> > Maybe I miss something, I donno.
> 
> I'm recounting what the Hyper-V paravisor folks say without knowing all the
> details. :-(   But it seems like any kind of forwarding scheme needs to be a
> well-defined contract that would work for both TDX and SEV-SNP.   The
> paravisor in L1 might or might not be Linux-based, so the contract must be OS
> independent.  And the L2 guest might or might not be Linux, so there's
> potential for some other kind of error to be confused with a Linux
> load_unaligned_zeropad() reference.

Okay, fair enough. I have hard time reasoning if it is okay for L2 which
is not Linux.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov