gnttab: fix releasing of memory upon switches between versions

gnttab_unpopulate_status_frames() incompletely freed the pages
previously used as status frame in that they did not get removed from
the domain's xenpage_list, thus causing subsequent list corruption
when those pages did get allocated again for the same or another purpose.

Similarly, grant_table_create() and gnttab_grow_table() both improperly
clean up in the event of an error - pages already shared with the guest
can't be freed by just passing them to free_xenheap_page(). Fix this by
sharing the pages only after all allocations succeeded.

This is CVE-2012-5510 / XSA-26.

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

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index c01ad00..6fb2be9 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1173,12 +1173,13 @@ fault:
 }
 
 static int
-gnttab_populate_status_frames(struct domain *d, struct grant_table *gt)
+gnttab_populate_status_frames(struct domain *d, struct grant_table *gt,
+                              unsigned int req_nr_frames)
 {
     unsigned i;
     unsigned req_status_frames;
 
-    req_status_frames = grant_to_status_frames(gt->nr_grant_frames);
+    req_status_frames = grant_to_status_frames(req_nr_frames);
     for ( i = nr_status_frames(gt); i < req_status_frames; i++ )
     {
         if ( (gt->status[i] = alloc_xenheap_page()) == NULL )
@@ -1209,7 +1210,12 @@ gnttab_unpopulate_status_frames(struct domain *d, struct grant_table *gt)
 
     for ( i = 0; i < nr_status_frames(gt); i++ )
     {
-        page_set_owner(virt_to_page(gt->status[i]), dom_xen);
+        struct page_info *pg = virt_to_page(gt->status[i]);
+
+        BUG_ON(page_get_owner(pg) != d);
+        if ( test_and_clear_bit(_PGC_allocated, &pg->count_info) )
+            put_page(pg);
+        BUG_ON(pg->count_info & ~PGC_xen_heap);
         free_xenheap_page(gt->status[i]);
         gt->status[i] = NULL;
     }
@@ -1247,19 +1253,18 @@ gnttab_grow_table(struct domain *d, unsigned int req_nr_frames)
         clear_page(gt->shared_raw[i]);
     }
 
-    /* Share the new shared frames with the recipient domain */
-    for ( i = nr_grant_frames(gt); i < req_nr_frames; i++ )
-        gnttab_create_shared_page(d, gt, i);
-
-    gt->nr_grant_frames = req_nr_frames;
-
     /* Status pages - version 2 */
     if (gt->gt_version > 1)
     {
-        if ( gnttab_populate_status_frames(d, gt) )
+        if ( gnttab_populate_status_frames(d, gt, req_nr_frames) )
             goto shared_alloc_failed;
     }
 
+    /* Share the new shared frames with the recipient domain */
+    for ( i = nr_grant_frames(gt); i < req_nr_frames; i++ )
+        gnttab_create_shared_page(d, gt, i);
+    gt->nr_grant_frames = req_nr_frames;
+
     return 1;
 
 shared_alloc_failed:
@@ -2157,7 +2162,7 @@ gnttab_set_version(XEN_GUEST_HANDLE(gnttab_set_version_t uop))
 
     if ( op.version == 2 && gt->gt_version < 2 )
     {
-        res = gnttab_populate_status_frames(d, gt);
+        res = gnttab_populate_status_frames(d, gt, nr_grant_frames(gt));
         if ( res < 0)
             goto out_unlock;
     }
@@ -2600,14 +2605,15 @@ grant_table_create(
         clear_page(t->shared_raw[i]);
     }
     
-    for ( i = 0; i < INITIAL_NR_GRANT_FRAMES; i++ )
-        gnttab_create_shared_page(d, t, i);
-
     /* Status pages for grant table - for version 2 */
     t->status = xzalloc_array(grant_status_t *,
                               grant_to_status_frames(max_nr_grant_frames));
     if ( t->status == NULL )
         goto no_mem_4;
+
+    for ( i = 0; i < INITIAL_NR_GRANT_FRAMES; i++ )
+        gnttab_create_shared_page(d, t, i);
+
     t->nr_status_frames = 0;
 
     /* Okay, install the structure. */