KVM: SVM: Never reject emulation due to SMAP errata for !SEV guests

Always signal that emulation is possible for !SEV guests regardless of
whether or not the CPU provided a valid instruction byte stream.  KVM can
read all guest state (memory and registers) for !SEV guests, i.e. can
fetch the code stream from memory even if the CPU failed to do so because
of the SMAP errata.

Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
Cc: stable@vger.kernel.org
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-2-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 ec05f8bfc21baec6e839559e6b6c810900e59893..a46e1c6abf0e63f7d868c2a7b8c11845184c3b56 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4258,8 +4258,13 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
        bool smep, smap, is_user;
        unsigned long cr4;
 
+       /* Emulation is always possible when KVM has access to all guest state. */
+       if (!sev_guest(vcpu->kvm))
+               return true;
+
        /*
-        * When the guest is an SEV-ES guest, emulation is not possible.
+        * Emulation is impossible for SEV-ES guests as KVM doesn't have access
+        * to guest register state.
         */
        if (sev_es_guest(vcpu->kvm))
                return false;
@@ -4319,9 +4324,6 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
        smap = cr4 & X86_CR4_SMAP;
        is_user = svm_get_cpl(vcpu) == 3;
        if (smap && (!smep || is_user)) {
-               if (!sev_guest(vcpu->kvm))
-                       return true;
-
                pr_err_ratelimited("KVM: SEV Guest triggered AMD Erratum 1096\n");
                kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
        }