KVM: SVM: Treat SVM as unsupported when running as an SEV guest

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>
Message-Id: <20210202212017.2486595-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index f923e14e87df2e12a97af016b2e3c72aa92eac16..3442d44ca53b854ca978185a351ef078d416ce80 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -454,6 +454,11 @@ 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 c79e5736ab2b7ebf43182e773c9c35457edcf366..c3d5f0236f353cd42159ab487163d5bc511df95b 100644 (file)
--- 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)