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>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-4.1-testing changeset: 23409:61eb3d030f52
Backport-requested-by: security@xen.org
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index ca7635b4163c4868cc00f4c3fe7cf3cc6bbd2bda..243b9f4d44326f7d5344fe72327946225442db67 100644 (file)
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -4698,8 +4698,12 @@ static void sh_pagetable_dying(struct vcpu *v, paddr_t gpa)
     }
     for ( i = 0; i < 4; i++ )
     {
-        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 */