gnttab: correct maptrack table accesses

In order to observe a consistent (limit,pointer-table) pair, the reader
needs to either hold the maptrack lock (in line with documentation) or
both sides need to order their accesses suitably (the writer side
barrier was removed by commit dff515dfea ["gnttab: use per-VCPU
maptrack free lists"], and a read side barrier has never been there).

Make the writer publish a new table page before limit (for bounds
checks to work), and new list head last (for racing maptrack_entry()
invocations to work). At the same time add read barriers to lockless
readers.

Additionally get_maptrack_handle() must not assume ->maptrack_head to
not change behind its back: Another handle may be put (updating only
->maptrack_tail) and then got or stolen (updating ->maptrack_head).

This is part of XSA-218.

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

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 7098af7a8789b195df556be8a976ca83ba9481e3..927fd2b5fac35aed822c0fe8cafea3b8f75bd18a 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -395,7 +395,7 @@ get_maptrack_handle(
     struct grant_table *lgt)
 {
     struct vcpu          *curr = current;
-    int                   i;
+    unsigned int          i, head;
     grant_handle_t        handle;
     struct grant_mapping *new_mt;
 
@@ -451,17 +451,20 @@ get_maptrack_handle(
         new_mt[i].ref = handle + i + 1;
         new_mt[i].vcpu = curr->vcpu_id;
     }
-    new_mt[i - 1].ref = curr->maptrack_head;
 
     /* Set tail directly if this is the first page for this VCPU. */
     if ( curr->maptrack_tail == MAPTRACK_TAIL )
         curr->maptrack_tail = handle + MAPTRACK_PER_PAGE - 1;
 
-    write_atomic(&curr->maptrack_head, handle + 1);
-
     lgt->maptrack[nr_maptrack_frames(lgt)] = new_mt;
+    smp_wmb();
     lgt->maptrack_limit += MAPTRACK_PER_PAGE;
 
+    do {
+        new_mt[i - 1].ref = read_atomic(&curr->maptrack_head);
+        head = cmpxchg(&curr->maptrack_head, new_mt[i - 1].ref, handle + 1);
+    } while ( head != new_mt[i - 1].ref );
+
     spin_unlock(&lgt->maptrack_lock);
 
     return handle;
@@ -727,6 +730,7 @@ static unsigned int mapkind(
     for ( handle = 0; !(kind & MAPKIND_WRITE) &&
                       handle < lgt->maptrack_limit; handle++ )
     {
+        smp_rmb();
         map = &maptrack_entry(lgt, handle);
         if ( !(map->flags & (GNTMAP_device_map|GNTMAP_host_map)) ||
              map->domid != rd->domain_id )
@@ -1094,6 +1098,7 @@ __gnttab_unmap_common(
         return;
     }
 
+    smp_rmb();
     map = &maptrack_entry(lgt, op->handle);
 
     grant_read_lock(lgt);