linux-kernel - [PATCH] KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default value when tsc scaling disabled

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Date:   Wed, 23 Feb 2022 13:56:49 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        stable@...r.kernel.org
Subject: [PATCH] KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default value when tsc scaling disabled

If nested tsc scaling is disabled, MSR_AMD64_TSC_RATIO should
never have non default value.

Due to way nested tsc scaling support was implmented in qemu,
it would set this msr to 0 when nested tsc scaling was disabled.
Ignore that value for now, as it causes no harm.


Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
Cc: stable@...r.kernel.org

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/svm/svm.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 7038c76fa841..b80ad471776f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2705,8 +2705,23 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 	u64 data = msr->data;
 	switch (ecx) {
 	case MSR_AMD64_TSC_RATIO:
-		if (!msr->host_initiated && !svm->tsc_scaling_enabled)
-			return 1;
+
+		if (!svm->tsc_scaling_enabled) {
+
+			if (!msr->host_initiated)
+				return 1;
+			/*
+			 * In case TSC scaling is not enabled, always
+			 * leave this MSR at the default value.
+			 *
+			 * Due to bug in qemu 6.2.0, it would try to set
+			 * this msr to 0 if tsc scaling is not enabled.
+			 * Ignore this value as well.
+			 */
+			if (data != 0 && data != svm->tsc_ratio_msr)
+				return 1;
+			break;
+		}
 
 		if (data & TSC_RATIO_RSVD)
 			return 1;
-- 
2.26.3