[PATCH 5.16 015/164] KVM: x86: nSVM: disallow userspace setting of MSR_AMD64_TSC_RATIO to non default value when tsc scaling disabled

Greg Kroah-Hartman
2022-02-28T18:01:23Z

From: Maxim Levitsky <mlevitsk@redhat.com>

commit e910a53fb4f20aa012e46371ffb4c32c8da259b4 upstream.

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@vger.kernel.org

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220223115649.319134-1-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/svm/svm.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2903,8 +2903,23 @@ static int svm_set_msr(struct kvm_vcpu *
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;