References: CVE-2012-5513 XSA-29 bnc#789951

xen: add missing guest address range checks to XENMEM_exchange handlers

Ever since its existence (3.0.3 iirc) the handler for this has been
using non address range checking guest memory accessors (i.e.
the ones prefixed with two underscores) without first range
checking the accessed space (via guest_handle_okay()), allowing
a guest to access and overwrite hypervisor memory.

This is XSA-29 / CVE-2012-5513.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -115,6 +115,12 @@ int compat_memory_op(unsigned int cmd, X
                   (cmp.xchg.out.nr_extents << cmp.xchg.out.extent_order)) )
                 return -EINVAL;
 
+            if ( !compat_handle_okay(cmp.xchg.in.extent_start,
+                                     cmp.xchg.in.nr_extents) ||
+                 !compat_handle_okay(cmp.xchg.out.extent_start,
+                                     cmp.xchg.out.nr_extents) )
+                return -EFAULT;
+
             start_extent = cmp.xchg.nr_exchanged;
             end_extent = (COMPAT_ARG_XLAT_SIZE - sizeof(*nat.xchg)) /
                          (((1U << ABS(order_delta)) + 1) *
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -308,6 +308,13 @@ static long memory_exchange(XEN_GUEST_HA
         goto fail_early;
     }
 
+    if ( !guest_handle_okay(exch.in.extent_start, exch.in.nr_extents) ||
+         !guest_handle_okay(exch.out.extent_start, exch.out.nr_extents) )
+    {
+        rc = -EFAULT;
+        goto fail_early;
+    }
+
     /* Only privileged guests can allocate multi-page contiguous extents. */
     if ( !multipage_allocation_permitted(current->domain,
                                          exch.in.extent_order) ||