gnttab: correct logic to get page references during map requests

The rules for reference counting are somewhat complicated:

* Each of GNTTAB_host_map and GNTTAB_device_map need their own
reference count

* If the mapping is writeable:
 - GNTTAB_host_map needs a type count under only some conditions
 - GNTTAB_device_map always needs a type count

If the mapping succeeds, we need to keep all of these; if the mapping
fails, we need to release whatever references we have acquired so far.

Additionally, the code that does a lot of this calculation "inherits"
a reference as part of the process of finding out who the owner is.

Finally, if the grant is mapped as writeable (without the
GNTMAP_readonly flag), but the hypervisor cannot grab a
PGT_writeable_page type, the entire operation should fail.

Unfortunately, the current code has several logic holes:

* If a grant is mapped only GNTTAB_device_map, and with a writeable
  mapping, but in conditions where a *host* type count is not
  necessary, the code will fail to grab the necessary type count.

* If a grant is mapped both GNTTAB_device_map and GNTTAB_host_map,
  with a writeable mapping, in conditions where the host type count is
  not necessary, *and* where the page cannot be changed to type
  PGT_writeable, the condition will not be detected.

In both cases, this means that on success, the type count will be
erroneously reduced when the grant is unmapped.  In the second case,
the type count will be erroneously reduced on the failure path as
well.  (In the first case the failure path logic has the same hole
as the reference grabbing logic.)

Additionally, the return value of get_page() is not checked; but this
may fail even if the first get_page() succeeded due to a reference
counting overflow.

First of all, simplify the restoration logic by explicitly counting
the reference and type references acquired.

Consider each mapping type separately, explicitly marking the
'incoming' reference as used so we know when we need to grab a second
one.

Finally, always check the return value of get_page[_type]() and go to
the failure path if appropriate.

This is part of XSA-224.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 672582faad77ba244171451565357332f77991c2..34086f90c29570359cbc1df2de9ff3ec5eaf40b6 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -758,12 +758,12 @@ __gnttab_map_grant_ref(
     struct grant_table *lgt, *rgt;
     struct vcpu   *led;
     int            handle;
-    unsigned long  frame = 0, nr_gets = 0;
+    unsigned long  frame = 0;
     struct page_info *pg = NULL;
     int            rc = GNTST_okay;
     u32            old_pin;
     u32            act_pin;
-    unsigned int   cache_flags;
+    unsigned int   cache_flags, refcnt = 0, typecnt = 0;
     struct active_grant_entry *act = NULL;
     struct grant_mapping *mt;
     grant_entry_header_t *shah;
@@ -889,11 +889,17 @@ __gnttab_map_grant_ref(
     else
         owner = page_get_owner(pg);
 
+    if ( owner )
+        refcnt++;
+
     if ( !pg || (owner == dom_io) )
     {
         /* Only needed the reference to confirm dom_io ownership. */
         if ( pg )
+        {
             put_page(pg);
+            refcnt--;
+        }
 
         if ( paging_mode_external(ld) )
         {
@@ -921,27 +927,38 @@ __gnttab_map_grant_ref(
     }
     else if ( owner == rd || owner == dom_cow )
     {
-        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
+        if ( (op->flags & GNTMAP_device_map) && !(op->flags & GNTMAP_readonly) )
         {
             if ( (owner == dom_cow) ||
                  !get_page_type(pg, PGT_writable_page) )
                 goto could_not_pin;
+            typecnt++;
         }
 
-        nr_gets++;
         if ( op->flags & GNTMAP_host_map )
         {
-            rc = create_grant_host_mapping(op->host_addr, frame, op->flags, 0);
-            if ( rc != GNTST_okay )
-                goto undo_out;
-
+            /*
+             * Only need to grab another reference if device_map claimed
+             * the other one.
+             */
             if ( op->flags & GNTMAP_device_map )
             {
-                nr_gets++;
-                (void)get_page(pg, rd);
-                if ( !(op->flags & GNTMAP_readonly) )
-                    get_page_type(pg, PGT_writable_page);
+                if ( !get_page(pg, rd) )
+                    goto could_not_pin;
+                refcnt++;
+            }
+
+            if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
+            {
+                if ( (owner == dom_cow) ||
+                     !get_page_type(pg, PGT_writable_page) )
+                    goto could_not_pin;
+                typecnt++;
             }
+
+            rc = create_grant_host_mapping(op->host_addr, frame, op->flags, 0);
+            if ( rc != GNTST_okay )
+                goto undo_out;
         }
     }
     else
@@ -950,8 +967,6 @@ __gnttab_map_grant_ref(
         if ( !rd->is_dying )
             gdprintk(XENLOG_WARNING, "Could not pin grant frame %lx\n",
                      frame);
-        if ( owner != NULL )
-            put_page(pg);
         rc = GNTST_general_error;
         goto undo_out;
     }
@@ -1014,18 +1029,11 @@ __gnttab_map_grant_ref(
     return;
 
  undo_out:
-    if ( nr_gets > 1 )
-    {
-        if ( !(op->flags & GNTMAP_readonly) )
-            put_page_type(pg);
-        put_page(pg);
-    }
-    if ( nr_gets > 0 )
-    {
-        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
-            put_page_type(pg);
+    while ( typecnt-- )
+        put_page_type(pg);
+
+    while ( refcnt-- )
         put_page(pg);
-    }
 
     grant_read_lock(rgt);