Re: [RFC PATCH 00/62] Linux as SEV-ES Guest Support

[Joerg Roedel]

On Tue, Feb 11, 2020 at 07:48:12PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Feb 11, 2020, at 5:53 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> 
> > 
> > 
> >    * Putting some NMI-load on the guest will make it crash usually
> >      within a minute
> 
> Suppose you do CPUID or some MMIO and get #VC. You fill in the GHCB to
> ask for help. Some time between when you start filling it out and when
> you do VMGEXIT, you get NMI. If the NMI does its own GHCB access [0],
> it will clobber the outer #VCâa state, resulting in a failure when
> VMGEXIT happens. Thereâs a related failure mode if the NMI is after
> the VMGEXIT but before the result is read.
> 
> I suspect you can fix this by saving the GHCB at the beginning of
> do_nmi and restoring it at the end. This has the major caveat that it
> will not work if do_nmi comes from user mode and schedules, but I
> donât believe this can happen.
> 
> [0] Due to the NMI_COMPLETE catastrophe, there is a 100% chance that
> this happens.

Very true, thank you! You probably saved me a few hours of debugging
this further :)
I will implement better handling for nested #VC exceptions, which
hopefully solves the NMI crashes.

Thanks again,

       Joerg