[Xen-devel] [PATCH v2 2/2] xen/physmap: Do not permit a guest to populate PoD pages for itself

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

v2:
 * Move the exclusion logic into populate_physmap().  No functional change.
---
 xen/common/memory.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 1ead35c..f34dd56 100644
--- 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;
-- 
2.1.4


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