Consider a scenario in which one processor, call it Pt, is performing

vm_object_terminate() on a device-backed object at the same time that
another processor, call it Pa, is performing dev_pager_alloc() on the
same device.  The problem is that vm_pager_object_lookup() should not be
allowed to return a doomed object, i.e., an object with OBJ_DEAD set,
but it does.  In detail, the unfortunate sequence of events is: Pt in
vm_object_terminate() holds the doomed object's lock and sets OBJ_DEAD
on the object.  Pa in dev_pager_alloc() holds dev_pager_sx and calls
vm_pager_object_lookup(), which returns the doomed object.  Next, Pa
calls vm_object_reference(), which requires the doomed object's lock, so
Pa waits for Pt to release the doomed object's lock.  Pt proceeds to the
point in vm_object_terminate() where it releases the doomed object's
lock.  Pa is now able to complete vm_object_reference() because it can
now complete the acquisition of the doomed object's lock.  So, now the
doomed object has a reference count of one!  Pa releases dev_pager_sx
and returns the doomed object from dev_pager_alloc().  Pt now acquires
dev_pager_mtx, removes the doomed object from dev_pager_object_list,
releases dev_pager_mtx, and finally calls uma_zfree with the doomed
object.  However, the doomed object is still in use by Pa.

Repeating my key point, vm_pager_object_lookup() must not return a
doomed object.  Moreover, the test for the object's state, i.e.,
doomed or not, and the increment of the object's reference count
should be carried out atomically.

Reviewed by:	kib
Approved by:	re (kensmith)
MFC after:	3 weeks

1 files changed, 19 insertions, 7 deletions

diff --git a/sys/vm/vm_pager.c b/sys/vm/vm_pager.c
index 28bd135..f03b951 100644
--- a/sys/vm/vm_pager.c
+++ b/sys/vm/vm_pager.c
@@ -261,17 +261,29 @@ vm_pager_deallocate(object)
  * vm_pager_has_page() - inline, see vm/vm_pager.h
  */
 
+/*
+ * Search the specified pager object list for an object with the
+ * specified handle.  If an object with the specified handle is found,
+ * increase its reference count and return it.  Otherwise, return NULL.
+ *
+ * The pager object list must be locked.
+ */
 vm_object_t
-vm_pager_object_lookup(pg_list, handle)
-	struct pagerlst *pg_list;
-	void *handle;
+vm_pager_object_lookup(struct pagerlst *pg_list, void *handle)
 {
 	vm_object_t object;
 
-	TAILQ_FOREACH(object, pg_list, pager_object_list)
-		if (object->handle == handle)
-			return (object);
-	return (NULL);
+	TAILQ_FOREACH(object, pg_list, pager_object_list) {
+		VM_OBJECT_LOCK(object);
+		if (object->handle == handle &&
+		    (object->flags & OBJ_DEAD) == 0) {
+			vm_object_reference_locked(object);
+			VM_OBJECT_UNLOCK(object);
+			break;
+		}
+		VM_OBJECT_UNLOCK(object);
+	}
+	return (object);
 }
 
 /*