linux-kernel - [RFC PATCH] KVM: SVM: Disallow SEV if NPT is disabled

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Date:   Fri, 31 Jul 2020 13:51:16 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: [RFC PATCH] KVM: SVM: Disallow SEV if NPT is disabled

Forcefully turn off SEV if NPT is disabled, e.g. via module param.  SEV
requires NPT as the C-bit only exists if NPT is active.

Fixes: e9df09428996f ("KVM: SVM: Add sev module_param")
Cc: stable@...r.kernel.org
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Brijesh Singh <brijesh.singh@....com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---

RFC as it's entirely possible that I am completely misunderstanding how
SEV works.  Compile tested only.

 arch/x86/kvm/svm/svm.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 783330d0e7b88..e30629593458b 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -860,8 +860,14 @@ static __init int svm_hardware_setup(void)
 		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
 	}
 
+	if (!boot_cpu_has(X86_FEATURE_NPT))
+		npt_enabled = false;
+
+	if (npt_enabled && !npt)
+		npt_enabled = false;
+
 	if (sev) {
-		if (boot_cpu_has(X86_FEATURE_SEV) &&
+		if (boot_cpu_has(X86_FEATURE_SEV) && npt_enabled &&
 		    IS_ENABLED(CONFIG_KVM_AMD_SEV)) {
 			r = sev_hardware_setup();
 			if (r)
@@ -879,12 +885,6 @@ static __init int svm_hardware_setup(void)
 			goto err;
 	}
 
-	if (!boot_cpu_has(X86_FEATURE_NPT))
-		npt_enabled = false;
-
-	if (npt_enabled && !npt)
-		npt_enabled = false;
-
 	kvm_configure_mmu(npt_enabled, PG_LEVEL_1G);
 	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
 
-- 
2.28.0