linux-kernel - [PATCH 1/9] KVM: SVM: Never reject emulation due to SMAP errata for !SEV guests

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Message-Id: <20220120010719.711476-2-seanjc@google.com>
Date:   Thu, 20 Jan 2022 01:07:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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>,
        Liam Merwick <liam.merwick@...cle.com>
Subject: [PATCH 1/9] 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@...r.kernel.org
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Brijesh Singh <brijesh.singh@....com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 6d31d357a83b..aa1649b8cd8f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4257,8 +4257,13 @@ 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;
@@ -4318,9 +4323,6 @@ 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);
 	}
-- 
2.34.1.703.g22d0c6ccf7-goog