VMX: values written to MSR_IA32_SYSENTER_E[IS]P should be canonical

A recent KVM change by Nadav Amit <namit@xxxxxxxxxxxxxxxxx> helped spot
that we have the same issue as they did.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Tim Deegan <tim@xxxxxxx>

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2273,9 +2273,13 @@ static int vmx_msr_write_intercept(unsig
         __vmwrite(GUEST_SYSENTER_CS, msr_content);
         break;
     case MSR_IA32_SYSENTER_ESP:
+        if ( !is_canonical_address(msr_content) )
+            goto gp_fault;
         __vmwrite(GUEST_SYSENTER_ESP, msr_content);
         break;
     case MSR_IA32_SYSENTER_EIP:
+        if ( !is_canonical_address(msr_content) )
+            goto gp_fault;
         __vmwrite(GUEST_SYSENTER_EIP, msr_content);
         break;
     case MSR_IA32_DEBUGCTLMSR: {