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

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Message-ID: <20210819153741.h6yloeihz5vl6hvu@amd.com>
Date:   Thu, 19 Aug 2021 10:37:41 -0500
From:   Michael Roth <michael.roth@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Brijesh Singh <brijesh.singh@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        brijesh.ksingh@...il.com
Subject: Re: [PATCH Part1 RFC v4 22/36] x86/sev: move MSR-based VMGEXITs for
 CPUID to helper

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@....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@....com>
> > Signed-off-by: Brijesh Singh <brijesh.singh@....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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