block, bfq: fix use after free in bfq_bfqq_expire

The function bfq_bfqq_expire() invokes the function
__bfq_bfqq_expire(), and the latter may free the in-service bfq-queue.
If this happens, then no other instruction of bfq_bfqq_expire() must
be executed, or a use-after-free will occur.

Basing on the assumption that __bfq_bfqq_expire() invokes
bfq_put_queue() on the in-service bfq-queue exactly once, the queue is
assumed to be freed if its refcounter is equal to one right before
invoking __bfq_bfqq_expire().

But, since commit 9dee8b3 ("block, bfq: fix queue removal from
weights tree") this assumption is false. __bfq_bfqq_expire() may also
invoke bfq_weights_tree_remove() and, since commit 9dee8b3
("block, bfq: fix queue removal from weights tree"), also
the latter function may invoke bfq_put_queue(). So __bfq_bfqq_expire()
may invoke bfq_put_queue() twice, and this is the actual case where
the in-service queue may happen to be freed.

To address this issue, this commit moves the check on the refcounter
of the queue right around the last bfq_put_queue() that may be invoked
on the queue.

Fixes: 9dee8b3 ("block, bfq: fix queue removal from weights tree")
Reported-by: Dmitrii Tcvetkov <[email protected]>
Reported-by: Douglas Anderson <[email protected]>
Tested-by: Dmitrii Tcvetkov <[email protected]>
Tested-by: Douglas Anderson <[email protected]>
Signed-off-by: Paolo Valente <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

block/bfq-iosched.c

@@ -2822,7 +2822,7 @@ static void bfq_dispatch_remove(struct request_queue *q, struct request *rq)
 	bfq_remove_request(q, rq);
 }
 
-static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq)
+static bool __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 {
 	/*
 	 * If this bfqq is shared between multiple processes, check
@@ -2855,9 +2855,11 @@ static void __bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 	/*
 	 * All in-service entities must have been properly deactivated
 	 * or requeued before executing the next function, which
-	 * resets all in-service entites as no more in service.
+	 * resets all in-service entities as no more in service. This
+	 * may cause bfqq to be freed. If this happens, the next
+	 * function returns true.
 	 */
-	__bfq_bfqd_reset_in_service(bfqd);
+	return __bfq_bfqd_reset_in_service(bfqd);
 }
 
 /**
@@ -3262,7 +3264,6 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
 	bool slow;
 	unsigned long delta = 0;
 	struct bfq_entity *entity = &bfqq->entity;
-	int ref;
 
 	/*
 	 * Check whether the process is slow (see bfq_bfqq_is_slow).
@@ -3347,10 +3348,8 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
 	 * reason.
 	 */
 	__bfq_bfqq_recalc_budget(bfqd, bfqq, reason);
-	ref = bfqq->ref;
-	__bfq_bfqq_expire(bfqd, bfqq);
-
-	if (ref == 1) /* bfqq is gone, no more actions on it */
+	if (__bfq_bfqq_expire(bfqd, bfqq))
+		/* bfqq is gone, no more actions on it */
 		return;
 
 	bfqq->injected_service = 0;

block/bfq-iosched.h

@@ -995,7 +995,7 @@ bool __bfq_deactivate_entity(struct bfq_entity *entity,
 			     bool ins_into_idle_tree);
 bool next_queue_may_preempt(struct bfq_data *bfqd);
 struct bfq_queue *bfq_get_next_queue(struct bfq_data *bfqd);
-void __bfq_bfqd_reset_in_service(struct bfq_data *bfqd);
+bool __bfq_bfqd_reset_in_service(struct bfq_data *bfqd);
 void bfq_deactivate_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 			 bool ins_into_idle_tree, bool expiration);
 void bfq_activate_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq);

block/bfq-wf2q.c

@@ -1605,7 +1605,8 @@ struct bfq_queue *bfq_get_next_queue(struct bfq_data *bfqd)
 	return bfqq;
 }
 
-void __bfq_bfqd_reset_in_service(struct bfq_data *bfqd)
+/* returns true if the in-service queue gets freed */
+bool __bfq_bfqd_reset_in_service(struct bfq_data *bfqd)
 {
 	struct bfq_queue *in_serv_bfqq = bfqd->in_service_queue;
 	struct bfq_entity *in_serv_entity = &in_serv_bfqq->entity;
@@ -1629,8 +1630,20 @@ void __bfq_bfqd_reset_in_service(struct bfq_data *bfqd)
 	 * service tree either, then release the service reference to
 	 * the queue it represents (taken with bfq_get_entity).
 	 */
-	if (!in_serv_entity->on_st)
+	if (!in_serv_entity->on_st) {
+		/*
+		 * If no process is referencing in_serv_bfqq any
+		 * longer, then the service reference may be the only
+		 * reference to the queue. If this is the case, then
+		 * bfqq gets freed here.
+		 */
+		int ref = in_serv_bfqq->ref;
 		bfq_put_queue(in_serv_bfqq);
+		if (ref == 1)
+			return true;
+	}
+
+	return false;
 }
 
 void bfq_deactivate_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,