[Xen-changelog] [xen-4.2-testing] x86/physmap: Prevent incorrect updates of m2p mappings

[Xen patchbot-4.2-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1352893017 0
# Node ID 4cffe28427e0c7dbeaa7c109ed393dde0fe026ba
# Parent  788af5959f692ca16942937055afb09b760f2166
x86/physmap: Prevent incorrect updates of m2p mappings

In certain conditions, such as low memory, set_p2m_entry() can fail.
Currently, the p2m and m2p tables will get out of sync because we still
update the m2p table after the p2m update has failed.

If that happens, subsequent guest-invoked memory operations can cause
BUG()s and ASSERT()s to kill Xen.

This is fixed by only updating the m2p table iff the p2m was
successfully updated.

This is a security problem, XSA-22 / CVE-2012-4537.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

xen-unstable changeset: 26149:6b6a4007a609
Backport-requested-by: security@xxxxxxx
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---


diff -r 788af5959f69 -r 4cffe28427e0 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c     Wed Nov 14 11:33:15 2012 +0000
+++ b/xen/arch/x86/mm/p2m.c     Wed Nov 14 11:36:57 2012 +0000
@@ -654,7 +654,10 @@ guest_physmap_add_entry(struct domain *d
     if ( mfn_valid(_mfn(mfn)) ) 
     {
         if ( !set_p2m_entry(p2m, gfn, _mfn(mfn), page_order, t, 
p2m->default_access) )
+        {
             rc = -EINVAL;
+            goto out; /* Failed to update p2m, bail without updating m2p. */
+        }
         if ( !p2m_is_grant(t) )
         {
             for ( i = 0; i < (1UL << page_order); i++ )
@@ -677,6 +680,7 @@ guest_physmap_add_entry(struct domain *d
         }
     }
 
+out:
     p2m_unlock(p2m);
 
     return rc;

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