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, 0 insertions, 4 deletions

diff --git a/sys/vm/phys_pager.c b/sys/vm/phys_pager.c
index 8f3fd3a..626e888 100644
--- a/sys/vm/phys_pager.c
+++ b/sys/vm/phys_pager.c
@@ -101,10 +101,6 @@ phys_pager_alloc(void *handle, vm_ooffset_t size, vm_prot_t prot,
 			    pager_object_list);
 			mtx_unlock(&phys_pager_mtx);
 		} else {
-			/*
-			 * Gain a reference to the object.
-			 */
-			vm_object_reference(object);
 			if (pindex > object->size)
 				object->size = pindex;
 		}