Re: [PATCH] block, bfq: fix use after free in bfq_bfqq_expire

[Paolo Valente]

This patch causes some checkpatch complain, sorry. Sending a V2 right away.

Paolo

> Il giorno 10 apr 2019, alle ore 10:26, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 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 9dee8b3b057e1 ("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 9dee8b3b057e1, 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.
> 
> Reported-by: Dmitrii Tcvetkov <demfloro@xxxxxxxxxxx>
> Reported-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Tested-by: Dmitrii Tcvetkov <demfloro@xxxxxxxxxxx>
> Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
> ---
> block/bfq-iosched.c | 15 +++++++--------
> block/bfq-iosched.h |  2 +-
> block/bfq-wf2q.c    | 17 +++++++++++++++--
> 3 files changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index fac188dd78fa..30b88ec7ad26 100644
> --- a/block/bfq-iosched.c
> +++ b/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 entites 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;
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 062e1c4787f4..86394e503ca9 100644
> --- a/block/bfq-iosched.h
> +++ b/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);
> diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> index a11bef75483d..ae4d000ac0af 100644
> --- a/block/bfq-wf2q.c
> +++ b/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,
> -- 
> 2.20.1
>