References: CVE-2012-4538 XSA-23 bnc#786519

xen/mm/shadow: check toplevel pagetables are present before unhooking them.

If the guest has not fully populated its top-level PAE entries when it calls
HVMOP_pagetable_dying, the shadow code could try to unhook entries from
MFN 0.  Add a check to avoid that case.

This issue was introduced by c/s 21239:b9d2db109cf5.

This is a security problem, XSA-23 / CVE-2012-4538.

Signed-off-by: Tim Deegan <tim@xen.org>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

Index: xen-4.2.0-testing/xen/arch/x86/mm/shadow/multi.c
===================================================================
--- xen-4.2.0-testing.orig/xen/arch/x86/mm/shadow/multi.c
+++ xen-4.2.0-testing/xen/arch/x86/mm/shadow/multi.c
@@ -4734,8 +4734,12 @@ static void sh_pagetable_dying(struct vc
         unsigned long gfn;
         mfn_t smfn, gmfn;
 
-        if ( fast_path )
-            smfn = _mfn(pagetable_get_pfn(v->arch.shadow_table[i]));
+        if ( fast_path ) {
+            if ( pagetable_is_null(v->arch.shadow_table[i]) )
+                smfn = _mfn(INVALID_MFN);
+            else
+                smfn = _mfn(pagetable_get_pfn(v->arch.shadow_table[i]));
+        }
         else
         {
             /* retrieving the l2s */