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

commit ccd85d90ce092bdb047a7f6580f3955393833b22 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 94b0cb833045127334431d1d0022d77c602082d8..f4ae3871e412a957ef97faf0e767846ad232fcc4 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -438,6 +438,11 @@ 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 bc0833713be955444359d83ad013125f295f1dfd..f80d10d39cf6d03d5d00643536089490db05b2c0 100644 (file)
--- a/arch/x86/mm/mem_encrypt.c
+++ b/arch/x86/mm/mem_encrypt.c
@@ -351,6 +351,7 @@ 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)