gnttab: split maptrack lock to make it fulfill its purpose again

The way the lock is currently being used in get_maptrack_handle(), it
protects only the maptrack limit: The function acts on current's list
only, so races on list accesses are impossible even without the lock.

Otoh list access races are possible between __get_maptrack_handle() and
put_maptrack_handle(), due to the invocation of the former for other
than current from steal_maptrack_handle(). Introduce a per-vCPU lock
for list accesses to become race free again. This lock will be
uncontended except when it becomes necessary to take the steal path,
i.e. in the common case there should be no meaningful performance
impact.

When in get_maptrack_handle adds a stolen entry to a fresh, empty,
freelist, we think that there is probably no concurrency.  However,
this is not a fast path and adding the locking there makes the code
clearly correct.

Also, while we are here: the stolen maptrack_entry's tail pointer was
not properly set.  Set it.

This is CVE-2017-12136 / XSA-228.

Reported-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

docs/misc/grant-tables.txt

@@ -87,7 +87,8 @@ is complete.
                                inconsistent grant table state such as current
                                version, partially initialized active table pages,
                                etc.
-  grant_table->maptrack_lock : spinlock used to protect the maptrack free list
+  grant_table->maptrack_lock : spinlock used to protect the maptrack limit
+  v->maptrack_freelist_lock  : spinlock used to protect the maptrack free list
   active_grant_entry->lock   : spinlock used to serialize modifications to
                                active entries
 
@@ -102,6 +103,10 @@ is complete.
  The maptrack free list is protected by its own spinlock. The maptrack
  lock may be locked while holding the grant table lock.
 
+ The maptrack_freelist_lock is an innermost lock.  It may be locked
+ while holding other locks, but no other locks may be acquired within
+ it.
+
  Active entries are obtained by calling active_entry_acquire(gt, ref).
  This function returns a pointer to the active entry after locking its
  spinlock. The caller must hold the grant table read lock before

xen/common/grant_table.c

@@ -304,11 +304,16 @@ __get_maptrack_handle(
 {
     unsigned int head, next, prev_head;
 
+    spin_lock(&v->maptrack_freelist_lock);
+
     do {
         /* No maptrack pages allocated for this VCPU yet? */
         head = read_atomic(&v->maptrack_head);
         if ( unlikely(head == MAPTRACK_TAIL) )
+        {
+            spin_unlock(&v->maptrack_freelist_lock);
             return -1;
+        }
 
         /*
          * Always keep one entry in the free list to make it easier to
@@ -316,12 +321,17 @@ __get_maptrack_handle(
          */
         next = read_atomic(&maptrack_entry(t, head).ref);
         if ( unlikely(next == MAPTRACK_TAIL) )
+        {
+            spin_unlock(&v->maptrack_freelist_lock);
             return -1;
+        }
 
         prev_head = head;
         head = cmpxchg(&v->maptrack_head, prev_head, next);
     } while ( head != prev_head );
 
+    spin_unlock(&v->maptrack_freelist_lock);
+
     return head;
 }
 
@@ -380,6 +390,8 @@ put_maptrack_handle(
     /* 2. Add entry to the tail of the list on the original VCPU. */
     v = currd->vcpu[maptrack_entry(t, handle).vcpu];
 
+    spin_lock(&v->maptrack_freelist_lock);
+
     cur_tail = read_atomic(&v->maptrack_tail);
     do {
         prev_tail = cur_tail;
@@ -388,6 +400,8 @@ put_maptrack_handle(
 
     /* 3. Update the old tail entry to point to the new entry. */
     write_atomic(&maptrack_entry(t, prev_tail).ref, handle);
+
+    spin_unlock(&v->maptrack_freelist_lock);
 }
 
 static inline int
@@ -411,10 +425,6 @@ get_maptrack_handle(
      */
     if ( nr_maptrack_frames(lgt) >= max_maptrack_frames )
     {
-        /*
-         * Can drop the lock since no other VCPU can be adding a new
-         * frame once they've run out.
-         */
         spin_unlock(&lgt->maptrack_lock);
 
         /*
@@ -426,8 +436,12 @@ get_maptrack_handle(
             handle = steal_maptrack_handle(lgt, curr);
             if ( handle == -1 )
                 return -1;
+            spin_lock(&curr->maptrack_freelist_lock);
+            maptrack_entry(lgt, handle).ref = MAPTRACK_TAIL;
             curr->maptrack_tail = handle;
-            write_atomic(&curr->maptrack_head, handle);
+            if ( curr->maptrack_head == MAPTRACK_TAIL )
+                write_atomic(&curr->maptrack_head, handle);
+            spin_unlock(&curr->maptrack_freelist_lock);
         }
         return steal_maptrack_handle(lgt, curr);
     }
@@ -460,12 +474,15 @@ get_maptrack_handle(
     smp_wmb();
     lgt->maptrack_limit += MAPTRACK_PER_PAGE;
 
+    spin_unlock(&lgt->maptrack_lock);
+    spin_lock(&curr->maptrack_freelist_lock);
+
     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);
+    spin_unlock(&curr->maptrack_freelist_lock);
 
     return handle;
 }
@@ -3475,6 +3492,7 @@ grant_table_destroy(
 
 void grant_table_init_vcpu(struct vcpu *v)
 {
+    spin_lock_init(&v->maptrack_freelist_lock);
     v->maptrack_head = MAPTRACK_TAIL;
     v->maptrack_tail = MAPTRACK_TAIL;
 }

xen/include/xen/grant_table.h

@@ -79,7 +79,7 @@ struct grant_table {
     /* Mapping tracking table per vcpu. */
     struct grant_mapping **maptrack;
     unsigned int          maptrack_limit;
-    /* Lock protecting the maptrack page list, head, and limit */
+    /* Lock protecting the maptrack limit */
     spinlock_t            maptrack_lock;
     /* The defined versions are 1 and 2.  Set to 0 if we don't know
        what version to use yet. */

xen/include/xen/sched.h

@@ -230,6 +230,7 @@ struct vcpu
     int              controller_pause_count;
 
     /* Grant table map tracking. */
+    spinlock_t       maptrack_freelist_lock;
     unsigned int     maptrack_head;
     unsigned int     maptrack_tail;