KVM: SEV: Return appropriate error codes if SEV-ES scratch setup fails

Return appropriate error codes if setting up the GHCB scratch area for an
SEV-ES guest fails.  In particular, returning -EINVAL instead of -ENOMEM
when allocating the kernel buffer could be confusing as userspace would
likely suspect a guest issue.

Fixes: 8f423a8 ("KVM: SVM: Support MMIO for an SEV-ES guest")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211109222350.2266045-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -2530,7 +2530,7 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
 }
 
 #define GHCB_SCRATCH_AREA_LIMIT		(16ULL * PAGE_SIZE)
-static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
+static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
 	struct ghcb *ghcb = svm->sev_es.ghcb;
@@ -2541,14 +2541,14 @@ static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 	scratch_gpa_beg = ghcb_get_sw_scratch(ghcb);
 	if (!scratch_gpa_beg) {
 		pr_err("vmgexit: scratch gpa not provided\n");
-		return false;
+		return -EINVAL;
 	}
 
 	scratch_gpa_end = scratch_gpa_beg + len;
 	if (scratch_gpa_end < scratch_gpa_beg) {
 		pr_err("vmgexit: scratch length (%#llx) not valid for scratch address (%#llx)\n",
 		       len, scratch_gpa_beg);
-		return false;
+		return -EINVAL;
 	}
 
 	if ((scratch_gpa_beg & PAGE_MASK) == control->ghcb_gpa) {
@@ -2566,7 +2566,7 @@ static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 		    scratch_gpa_end > ghcb_scratch_end) {
 			pr_err("vmgexit: scratch area is outside of GHCB shared buffer area (%#llx - %#llx)\n",
 			       scratch_gpa_beg, scratch_gpa_end);
-			return false;
+			return -EINVAL;
 		}
 
 		scratch_va = (void *)svm->sev_es.ghcb;
@@ -2579,18 +2579,18 @@ static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 		if (len > GHCB_SCRATCH_AREA_LIMIT) {
 			pr_err("vmgexit: scratch area exceeds KVM limits (%#llx requested, %#llx limit)\n",
 			       len, GHCB_SCRATCH_AREA_LIMIT);
-			return false;
+			return -EINVAL;
 		}
 		scratch_va = kzalloc(len, GFP_KERNEL_ACCOUNT);
 		if (!scratch_va)
-			return false;
+			return -ENOMEM;
 
 		if (kvm_read_guest(svm->vcpu.kvm, scratch_gpa_beg, scratch_va, len)) {
 			/* Unable to copy scratch area from guest */
 			pr_err("vmgexit: kvm_read_guest for scratch area failed\n");
 
 			kfree(scratch_va);
-			return false;
+			return -EFAULT;
 		}
 
 		/*
@@ -2606,7 +2606,7 @@ static bool setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 	svm->sev_es.ghcb_sa = scratch_va;
 	svm->sev_es.ghcb_sa_len = len;
 
-	return true;
+	return 0;
 }
 
 static void set_ghcb_msr_bits(struct vcpu_svm *svm, u64 value, u64 mask,
@@ -2745,10 +2745,10 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 	ghcb_set_sw_exit_info_1(ghcb, 0);
 	ghcb_set_sw_exit_info_2(ghcb, 0);
 
-	ret = -EINVAL;
 	switch (exit_code) {
 	case SVM_VMGEXIT_MMIO_READ:
-		if (!setup_vmgexit_scratch(svm, true, control->exit_info_2))
+		ret = setup_vmgexit_scratch(svm, true, control->exit_info_2);
+		if (ret)
 			break;
 
 		ret = kvm_sev_es_mmio_read(vcpu,
@@ -2757,7 +2757,8 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 					   svm->sev_es.ghcb_sa);
 		break;
 	case SVM_VMGEXIT_MMIO_WRITE:
-		if (!setup_vmgexit_scratch(svm, false, control->exit_info_2))
+		ret = setup_vmgexit_scratch(svm, false, control->exit_info_2);
+		if (ret)
 			break;
 
 		ret = kvm_sev_es_mmio_write(vcpu,
@@ -2800,6 +2801,7 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 		vcpu_unimpl(vcpu,
 			    "vmgexit: unsupported event - exit_info_1=%#llx, exit_info_2=%#llx\n",
 			    control->exit_info_1, control->exit_info_2);
+		ret = -EINVAL;
 		break;
 	default:
 		ret = svm_invoke_exit_handler(vcpu, exit_code);
@@ -2812,6 +2814,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 {
 	int count;
 	int bytes;
+	int r;
 
 	if (svm->vmcb->control.exit_info_2 > INT_MAX)
 		return -EINVAL;
@@ -2820,8 +2823,9 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 	if (unlikely(check_mul_overflow(count, size, &bytes)))
 		return -EINVAL;
 
-	if (!setup_vmgexit_scratch(svm, in, bytes))
-		return -EINVAL;
+	r = setup_vmgexit_scratch(svm, in, bytes);
+	if (r)
+		return r;
 
 	return kvm_sev_es_string_io(&svm->vcpu, size, port, svm->sev_es.ghcb_sa,
 				    count, in);