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

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1352893341 0
# Node ID c7a01b6450e483ca839228bf1e1e44de692e3458
# Parent  6b6a4007a6091610a29b71cc32908c74113b852b
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 6b6a4007a609 -r c7a01b6450e4 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:36:02 2012 +0000
+++ b/xen/arch/x86/mm/shadow/multi.c    Wed Nov 14 11:42:21 2012 +0000
@@ -4478,8 +4478,12 @@ 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 */

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