linux-kernel - Re: [PATCH] KVM: SVM: Treat SVM as unsupported when running as an SEV guest

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Date:   Wed, 3 Feb 2021 09:02:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Treat SVM as unsupported when running as an SEV
 guest

On 02/02/21 22:20, Sean Christopherson wrote:
> Don't let KVM load when running as an SEV guest, regardless of what
> CPUID says.  Memory is encrypted with a key that is not accessible to
> the host (L0), thus it's impossible for L0 to emulate SVM, e.g. it'll
> see garbage when reading the VMCB.
> 
> Technically, KVM could decrypt all memory that needs to be accessible to
> the L0 and use shadow paging so that L0 does not need to shadow NPT, but
> exposing such information to L0 largely defeats the purpose of running as
> an SEV guest.  This can always be revisited if someone comes up with a
> use case for running VMs inside SEV guests.
> 
> Note, VMLOAD, VMRUN, etc... will also #GP on GPAs with C-bit set, i.e. KVM
> is doomed even if the SEV guest is debuggable and the hypervisor is willing
> to decrypt the VMCB.  This may or may not be fixed on CPUs that have the
> SVME_ADDR_CHK fix.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> 
> FWIW, I did get nested SVM working on SEV by decrypting all structures
> that are shadowed by L0, albeit with many restrictions.  So even though
> there's unlikely to be a legitimate use case, I don't think KVM (as L0)
> needs to be changed to disallow nSVM for SEV guests, userspace is
> ultimately the one that should hide SVM from L1.
> 
>   arch/x86/kvm/svm/svm.c    | 5 +++++
>   arch/x86/mm/mem_encrypt.c | 1 +
>   2 files changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 687876211ebe..9fb367cb4f15 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -448,6 +448,11 @@ static int has_svm(void)
>   		return 0;
>   	}
>   
> +	if (sev_active()) {
> +		pr_info("KVM is unsupported when running as an SEV guest\n");
> +		return 0;
> +	}
> +
>   	return 1;
>   }
>   
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index c79e5736ab2b..c3d5f0236f35 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -382,6 +382,7 @@ bool sev_active(void)
>   {
>   	return sev_status & MSR_AMD64_SEV_ENABLED;
>   }
> +EXPORT_SYMBOL_GPL(sev_active);
>   
>   /* Needs to be called from non-instrumentable code */
>   bool noinstr sev_es_active(void)
> 

Queued, thanks.

Paolo