Messages in this thread | | | Date | Tue, 9 Aug 2016 14:23:20 +0200 | From | Radim Krčmář <> | Subject | Re: [PATCH 1/2] KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC |
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2016-08-09 17:32+0800, Yang Zhang: > On 2016/8/9 2:16, Radim Krčmář wrote: >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >> @@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu) >> return 1; >> } >> >> + if (cpu_has_vmx_msr_bitmap()) { >> + vmx->nested.msr_bitmap = >> + (unsigned long *)__get_free_page(GFP_KERNEL); >> + if (!vmx->nested.msr_bitmap) >> + goto out_msr_bitmap; >> + } >> + > > We export msr_bitmap to guest even it is not supported by hardware. So we > need to allocate msr_bitmap for L1 unconditionally.
We do emulate the feature, but the vmx->nested.msr_bitmap is used only if VMX supports it to avoid some VM exits:
>> @@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12) >> } >> >> if (cpu_has_vmx_msr_bitmap() && >> - exec_control & CPU_BASED_USE_MSR_BITMAPS) { >> - nested_vmx_merge_msr_bitmap(vcpu, vmcs12); >> - /* MSR_BITMAP will be set by following vmx_set_efer. */ >> - } else >> + exec_control & CPU_BASED_USE_MSR_BITMAPS && >> + nested_vmx_merge_msr_bitmap(vcpu, vmcs12)) >> + ; /* MSR_BITMAP will be set by following vmx_set_efer. */ >> + else >> exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;
The else branch is taken if !cpu_has_vmx_msr_bitmap() and disables msr bitmaps. Similar check for vmx_set_msr_bitmap(), so the NULL doesn't even get written to VMCS.
KVM always uses L1's msr bitmaps when emulating the feature.
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