linux-kernel - [PATCH 3/3] KVM: SEV: remove ghcb variable declarations

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Date:   Fri,  4 Aug 2023 13:33:55 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pgonda@...gle.com, seanjc@...gle.com, theflow@...gle.com,
        vkuznets@...hat.com, thomas.lendacky@....com
Subject: [PATCH 3/3] KVM: SEV: remove ghcb variable declarations

To avoid possible time-of-check/time-of-use issues, the GHCB should
almost never be accessed outside dump_ghcb, sev_es_sync_to_ghcb
and sev_es_sync_from_ghcb.  The only legitimate uses are to set the
exitinfo fields and to find the address of the scratch area embedded
in the ghcb.  Accessing ghcb_usage also goes through svm->sev_es.ghcb
in sev_es_validate_vmgexit(), but that is because anyway the value is
not used.

Removing a shortcut variable that contains the value of svm->sev_es.ghcb
makes these cases a bit more verbose, but it limits the chance of someone
reading the ghcb by mistake.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/sev.c | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index ca4ba5fe9a01..d3aec1f2cad2 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2454,12 +2454,9 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
 	struct kvm_vcpu *vcpu = &svm->vcpu;
-	struct ghcb *ghcb;
 	u64 exit_code;
 	u64 reason;
 
-	ghcb = svm->sev_es.ghcb;
-
 	/*
 	 * Retrieve the exit code now even though it may not be marked valid
 	 * as it could help with debugging.
@@ -2467,7 +2464,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 	exit_code = kvm_ghcb_get_sw_exit_code(control);
 
 	/* Only GHCB Usage code 0 is supported */
-	if (ghcb->ghcb_usage) {
+	if (svm->sev_es.ghcb->ghcb_usage) {
 		reason = GHCB_ERR_INVALID_USAGE;
 		goto vmgexit_err;
 	}
@@ -2561,7 +2558,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 vmgexit_err:
 	if (reason == GHCB_ERR_INVALID_USAGE) {
 		vcpu_unimpl(vcpu, "vmgexit: ghcb usage %#x is not valid\n",
-			    ghcb->ghcb_usage);
+			    svm->sev_es.ghcb->ghcb_usage);
 	} else if (reason == GHCB_ERR_INVALID_EVENT) {
 		vcpu_unimpl(vcpu, "vmgexit: exit code %#llx is not valid\n",
 			    exit_code);
@@ -2571,8 +2568,8 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 		dump_ghcb(svm);
 	}
 
-	ghcb_set_sw_exit_info_1(ghcb, 2);
-	ghcb_set_sw_exit_info_2(ghcb, reason);
+	ghcb_set_sw_exit_info_1(svm->sev_es.ghcb, 2);
+	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, reason);
 
 	/* Resume the guest to "return" the error code. */
 	return 1;
@@ -2637,7 +2634,6 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
 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;
 	u64 ghcb_scratch_beg, ghcb_scratch_end;
 	u64 scratch_gpa_beg, scratch_gpa_end;
 	void *scratch_va;
@@ -2713,8 +2709,8 @@ static int setup_vmgexit_scratch(struct vcpu_svm *svm, bool sync, u64 len)
 	return 0;
 
 e_scratch:
-	ghcb_set_sw_exit_info_1(ghcb, 2);
-	ghcb_set_sw_exit_info_2(ghcb, GHCB_ERR_INVALID_SCRATCH_AREA);
+	ghcb_set_sw_exit_info_1(svm->sev_es.ghcb, 2);
+	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, GHCB_ERR_INVALID_SCRATCH_AREA);
 
 	return 1;
 }
@@ -2827,7 +2823,6 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct vmcb_control_area *control = &svm->vmcb->control;
 	u64 ghcb_gpa, exit_code;
-	struct ghcb *ghcb;
 	int ret;
 
 	/* Validate the GHCB */
@@ -2852,17 +2847,16 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 	}
 
 	svm->sev_es.ghcb = svm->sev_es.ghcb_map.hva;
-	ghcb = svm->sev_es.ghcb_map.hva;
 
-	trace_kvm_vmgexit_enter(vcpu->vcpu_id, ghcb);
+	trace_kvm_vmgexit_enter(vcpu->vcpu_id, svm->sev_es.ghcb);
 
 	sev_es_sync_from_ghcb(svm);
 	ret = sev_es_validate_vmgexit(svm);
 	if (ret)
 		return ret;
 
-	ghcb_set_sw_exit_info_1(ghcb, 0);
-	ghcb_set_sw_exit_info_2(ghcb, 0);
+	ghcb_set_sw_exit_info_1(svm->sev_es.ghcb, 0);
+	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, 0);
 
 	exit_code = kvm_ghcb_get_sw_exit_code(control);
 	switch (exit_code) {
@@ -2902,13 +2896,13 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 			break;
 		case 1:
 			/* Get AP jump table address */
-			ghcb_set_sw_exit_info_2(ghcb, sev->ap_jump_table);
+			ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, sev->ap_jump_table);
 			break;
 		default:
 			pr_err("svm: vmgexit: unsupported AP jump table request - exit_info_1=%#llx\n",
 			       control->exit_info_1);
-			ghcb_set_sw_exit_info_1(ghcb, 2);
-			ghcb_set_sw_exit_info_2(ghcb, GHCB_ERR_INVALID_INPUT);
+			ghcb_set_sw_exit_info_1(svm->sev_es.ghcb, 2);
+			ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, GHCB_ERR_INVALID_INPUT);
 		}
 
 		ret = 1;
-- 
2.39.0