linux-kernel - [PATCH 2/5] KVM: svm: Disallow EFER.LMSLE on hardware that doesn't support it

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Message-ID: <20220916045832.461395-3-jmattson@google.com>
Date:   Thu, 15 Sep 2022 21:58:29 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Avi Kivity <avi@...hat.com>, Babu Moger <babu.moger@....com>,
        Borislav Petkov <bp@...en8.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joerg Roedel <joerg.roedel@....com>,
        Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wyes Karny <wyes.karny@....com>, x86@...nel.org
Cc:     Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 2/5] KVM: svm: Disallow EFER.LMSLE on hardware that doesn't
 support it

KVM has never properly virtualized EFER.LMSLE. When the "nested"
module parameter is true, it allows an SVM guest to set EFER.LMSLE,
and it passes the bit through in the VMCB, but the KVM emulator
doesn't perform the required data segment limit checks in 64-bit mode.

With Zen3, AMD has dropped support for EFER.LMSLE. Hence, if a Zen3
guest sets EFER.LMSLE, the next VMRUN will fail with "invalid VMCB."

When the host reports X86_FEATURE_NO_LMSLE, treat EFER.LMSLE as a
reserved bit in the guest. Now, if a guest tries to set EFER.LMSLE on
a host without support for EFER.LMSLE, the WRMSR will raise a #GP.

At the moment, the #GP may come as a surprise, but it's an improvement
over the failed VMRUN. The #GP will be vindicated anon.

Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm")
Signed-off-by: Jim Mattson <jmattson@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index f3813dbacb9f..7c4fd594166c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5012,7 +5012,9 @@ static __init int svm_hardware_setup(void)
 
 	if (nested) {
 		printk(KERN_INFO "kvm: Nested Virtualization enabled\n");
-		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
+		kvm_enable_efer_bits(EFER_SVME);
+		if (!boot_cpu_has(X86_FEATURE_NO_LMSLE))
+			kvm_enable_efer_bits(EFER_LMSLE);
 	}
 
 	/*
-- 
2.37.3.968.ga6b4b080e4-goog