xen/physmap: Do not permit a guest to populate PoD pages for itself

PoD is supposed to be entirely transparent to guest, but this interface has
been left exposed for a long time.

The use of PoD requires careful co-ordination by the toolstack with the
XENMEM_{get,set}_pod_target hypercalls, and xenstore ballooning target.  The
best a guest can do without toolstack cooperation crash.

Furthermore, there are combinations of features (e.g. c/s c63868ff "libxl:
disallow PCI device assignment for HVM guest when PoD is enabled") which a
toolstack might wish to explicitly prohibit (in this case, because the two
simply don't function in combination).  In such cases, the guest mustn't be
able to subvert the configuration chosen by the toolstack.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 1ead35c8ab0773ed785c095751ba1cc5d349baf1..f34dd5685f9d336d0a9a3b25a2414c16ee9bea3e 100644 (file)
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -140,14 +140,14 @@ static void populate_physmap(struct memop_args *a)
     struct page_info *page;
     unsigned int i, j;
     xen_pfn_t gpfn, mfn;
-    struct domain *d = a->domain;
+    struct domain *d = a->domain, *curr_d = current->domain;
 
     if ( !guest_handle_subrange_okay(a->extent_list, a->nr_done,
                                      a->nr_extents-1) )
         return;
 
     if ( a->extent_order > (a->memflags & MEMF_populate_on_demand ? MAX_ORDER :
-                            max_order(current->domain)) )
+                            max_order(curr_d)) )
         return;
 
     for ( i = a->nr_done; i < a->nr_extents; i++ )
@@ -163,6 +163,10 @@ static void populate_physmap(struct memop_args *a)
 
         if ( a->memflags & MEMF_populate_on_demand )
         {
+            /* Disallow populating PoD pages on oneself. */
+            if ( d == curr_d )
+                goto out;
+
             if ( guest_physmap_mark_populate_on_demand(d, gpfn,
                                                        a->extent_order) < 0 )
                 goto out;