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>
master commit: 2a99aa99fc84a45f505f84802af56b006d14c52e
master date: 2016-08-19 18:40:11 +0100

diff --git a/xen/common/memory.c b/xen/common/memory.c
index a64b18b2ce92523eeef1153daef5e098d04a9c32..b23d272d5c01c980f4aa145a5fe5bd75f679e8f9 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 long 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;