linux-kernel - [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with protected state

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Message-Id: <d0904f0d049300267665bd4abf96c3d7e7aa4825.1621701837.git.thomas.lendacky@amd.com>
Date:   Sat, 22 May 2021 11:43:58 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <Ashish.Kalra@....com>
Subject: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with protected state

When processing a hypercall for a guest with protected state, currently
SEV-ES guests, the guest CS segment register can't be checked to
determine if the guest is in 64-bit mode. For an SEV-ES guest, it is
expected that communication between the guest and the hypervisor is
performed to shared memory using the GHCB. In order to use the GHCB, the
guest must have been in long mode, otherwise writes by the guest to the
GHCB would be encrypted and not be able to be comprehended by the
hypervisor. Given that, assume that the guest is in 64-bit mode when
processing a hypercall from a guest with protected state.

Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
Reported-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/kvm/x86.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9b6bca616929..e715c69bb882 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8403,7 +8403,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 
 	trace_kvm_hypercall(nr, a0, a1, a2, a3);
 
-	op_64_bit = is_64_bit_mode(vcpu);
+	/*
+	 * If running with protected guest state, the CS register is not
+	 * accessible. The hypercall register values will have had to been
+	 * provided in 64-bit mode, so assume the guest is in 64-bit.
+	 */
+	op_64_bit = is_64_bit_mode(vcpu) || vcpu->arch.guest_state_protected;
 	if (!op_64_bit) {
 		nr &= 0xFFFFFFFF;
 		a0 &= 0xFFFFFFFF;
-- 
2.31.0