Re: [PATCH 1/2] KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC

[Yang Zhang]

On 2016/8/9 20:23, Radim Krčmář wrote:

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.

Yes, you are right. I forget the bitmap only used by hardware.:(

--
Yang
Alibaba Cloud Computing