KVM: x86: handle missing MPX in nested virtualization

When doing nested virtualization, we may be able to read BNDCFGS but
still not be allowed to write to GUEST_BNDCFGS in the VMCS.  Guard
writes to the field with vmx_mpx_supported(), and similarly hide the
MSR from userspace if the processor does not support the field.

We could work around this with the generic MSR save/load machinery,
but there is only a limited number of MSR save/load slots and it is
not really worthwhile to waste one for a scenario that should not
happen except in the nested virtualization case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3f5fb4535f9..aa986959f23 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3937,6 +3937,23 @@ static void kvm_init_msr_list(void)
 	for (i = j = KVM_SAVE_MSRS_BEGIN; i < ARRAY_SIZE(msrs_to_save); i++) {
 		if (rdmsr_safe(msrs_to_save[i], &dummy[0], &dummy[1]) < 0)
 			continue;
+
+		/*
+		 * Even MSRs that are valid in the host may not be exposed
+		 * to the guests in some cases.  We could work around this
+		 * in VMX with the generic MSR save/load machinery, but it
+		 * is not really worthwhile since it will really only
+		 * happen with nested virtualization.
+		 */
+		switch (msrs_to_save[i]) {
+		case MSR_IA32_BNDCFGS:
+			if (!kvm_x86_ops->mpx_supported())
+				continue;
+			break;
+		default:
+			break;
+		}
+
 		if (j < i)
 			msrs_to_save[j] = msrs_to_save[i];
 		j++;