[PATCH] KVM: SVM: Treat SVM as unsupported when running as an SEV guest

[PATCH] KVM: SVM: Treat SVM as unsupported when running as an SEV guest

[Sean Christopherson]

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@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.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)
-- 
2.30.0.365.g02bc693789-goog

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

[Paolo Bonzini]

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@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.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