[Xen-changelog] [xen-4.1-testing] xen/mm/shadow: check toplevel pagetables are present before unhooking them.

[Xen patchbot-4.1-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1352893409 0
# Node ID 61eb3d030f52b3785ed0935090821f4d48d65dc9
# Parent  f635b1447d7e35fb10cf5373ee365111f892c932
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@xxxxxxx>
Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---


diff -r f635b1447d7e -r 61eb3d030f52 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:40:45 2012 +0000
+++ b/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:43:29 2012 +0000
@@ -4737,8 +4737,12 @@ static void sh_pagetable_dying(struct vc
     }
     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 */

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