block, bfq: deschedule empty bfq_queues not referred by any process

Since commit 3726112ec731 ("block, bfq: re-schedule empty queues if
they deserve I/O plugging"), to prevent the service guarantees of a
bfq_queue from being violated, the bfq_queue may be left busy, i.e.,
scheduled for service, even if empty (see comments in
__bfq_bfqq_expire() for details). But, if no process will send
requests to the bfq_queue any longer, then there is no point in
keeping the bfq_queue scheduled for service.

In addition, keeping the bfq_queue scheduled for service, but with no
process reference any longer, may cause the bfq_queue to be freed when
descheduled from service. But this is assumed to never happen, and
causes a UAF if it happens. This, in turn, caused crashes [1, 2].

This commit fixes this issue by descheduling an empty bfq_queue when
it remains with not process reference.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1767539
[2] https://bugzilla.kernel.org/show_bug.cgi?id=205447

Fixes: 3726112ec731 ("block, bfq: re-schedule empty queues if they deserve I/O plugging")
Reported-by: Chris Evich <cevich@redhat.com>
Reported-by: Patrick Dung <patdung100@gmail.com>
Reported-by: Thorsten Schubert <tschubert@bafh.org>
Tested-by: Thorsten Schubert <tschubert@bafh.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0319d63..0c62144 100644 (file)
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2713,6 +2713,28 @@ static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
        }
 }
 
+
+static
+void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq)
+{
+       /*
+        * To prevent bfqq's service guarantees from being violated,
+        * bfqq may be left busy, i.e., queued for service, even if
+        * empty (see comments in __bfq_bfqq_expire() for
+        * details). But, if no process will send requests to bfqq any
+        * longer, then there is no point in keeping bfqq queued for
+        * service. In addition, keeping bfqq queued for service, but
+        * with no process ref any longer, may have caused bfqq to be
+        * freed when dequeued from service. But this is assumed to
+        * never happen.
+        */
+       if (bfq_bfqq_busy(bfqq) && RB_EMPTY_ROOT(&bfqq->sort_list) &&
+           bfqq != bfqd->in_service_queue)
+               bfq_del_bfqq_busy(bfqd, bfqq, false);
+
+       bfq_put_queue(bfqq);
+}
+
 static void
 bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
                struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
@@ -2783,8 +2805,7 @@ bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
         */
        new_bfqq->pid = -1;
        bfqq->bic = NULL;
-       /* release process reference to bfqq */
-       bfq_put_queue(bfqq);
+       bfq_release_process_ref(bfqd, bfqq);
 }
 
 static bool bfq_allow_bio_merge(struct request_queue *q, struct request *rq,
@@ -4899,7 +4920,7 @@ static void bfq_exit_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq)
 
        bfq_put_cooperator(bfqq);
 
-       bfq_put_queue(bfqq); /* release process reference */
+       bfq_release_process_ref(bfqd, bfqq);
 }
 
 static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
@@ -5001,8 +5022,7 @@ static void bfq_check_ioprio_change(struct bfq_io_cq *bic, struct bio *bio)
 
        bfqq = bic_to_bfqq(bic, false);
        if (bfqq) {
-               /* release process reference on this queue */
-               bfq_put_queue(bfqq);
+               bfq_release_process_ref(bfqd, bfqq);
                bfqq = bfq_get_queue(bfqd, bio, BLK_RW_ASYNC, bic);
                bic_set_bfqq(bic, bfqq, false);
        }
@@ -5963,7 +5983,7 @@ bfq_split_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq)
 
        bfq_put_cooperator(bfqq);
 
-       bfq_put_queue(bfqq);
+       bfq_release_process_ref(bfqq->bfqd, bfqq);
        return NULL;
 }