[PATCH v2 2/2] x86: validate VM assist value in arch_set_info_guest()

[Jan Beulich <jbeulich@xxxxxxxx>]

While I can't spot anything that would go wrong, just like the
respective hypercall only permits applicable bits to be set, we should
also do so when loading guest context.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I'd like to note that Arm lacks a field to save/restore vm_assist.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -932,6 +932,9 @@ int arch_set_info_guest(
         }
     }
 
+    if ( v->vcpu_id == 0 && (c(vm_assist) & ~arch_vm_assist_valid(d)) )
+        return -EINVAL;
+
     if ( is_hvm_domain(d) )
     {
         for ( i = 0; i < ARRAY_SIZE(v->arch.dr); ++i )