Re: [PATCH Part1 RFC v4 22/36] x86/sev: move MSR-based VMGEXITs for CPUID to helper

Michael Roth
2021-08-19T15:37:56Z

On Thu, Aug 19, 2021 at 11:45:35AM +0200, Borislav Petkov wrote:
> Finally drop this bouncing npmccallum at RH email address from the Cc
> list.
>
> On Wed, Jul 07, 2021 at 01:14:52PM -0500, Brijesh Singh wrote:
> > From: Michael Roth <michael.roth@amd.com>
> >
> > This code will also be used later for SEV-SNP-validated CPUID code in
> > some cases, so move it to a common helper.
> >
> > Signed-off-by: Michael Roth <michael.roth@amd.com>
> > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> > ---
> > arch/x86/kernel/sev-shared.c | 84 +++++++++++++++++++++++++-----------
> > 1 file changed, 58 insertions(+), 26 deletions(-)
> >
> > diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> > index be4025f14b4f..4884de256a49 100644
> > --- a/arch/x86/kernel/sev-shared.c
> > +++ b/arch/x86/kernel/sev-shared.c
> > @@ -184,6 +184,58 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
> > return ret;
> > }
> >
> > +static int sev_es_cpuid_msr_proto(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
>
> Since it is not only SEV-ES, then it should be prefixed with "sev_" like
> we do for the other such functions. I guess simply
>
> sev_cpuid()
>
> ?

That makes sense, but I think it helps in making sense of the security
aspects of the code to know that sev_cpuid() would be fetching cpuid
information from the hypervisor. "msr_proto" is meant to be an indicator
that it will be using the GHCB MSR protocol to do it, but maybe just
"_hyp" is enough to get the idea across? I use the convention elsewhere
in the series as well.

So sev_cpuid_hyp() maybe?

>
> > + u32 *ecx, u32 *edx)
> > +{
> > + u64 val;
> > +
> > + if (eax) {
>
> What's the protection for? Is it ever going to be called with NULL ptrs
> for the regs? That's not the case in this patchset at least...

In "enable SEV-SNP-validated CPUID in #VC handler", it does:

sev_snp_cpuid() -> sev_snp_cpuid_hyp(),

which will call this with NULL e{a,b,c,d}x arguments in some cases. There
are enough call-sites in sev_snp_cpuid() that it seemed worthwhile to
add the guards so we wouldn't need to declare dummy variables for arguments.

>
> --
> Regards/Gruss,
> Boris.
>
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