[PATCH BUGFIX] block, bfq: deschedule empty bfq_queues not referred by any process

Paolo Valente
2019-11-12T07:49:17Z

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>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
block/bfq-iosched.c | 31 +++++++++++++++++++++++++------
1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0319d6339822..ba68627f7740 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2713,6 +2713,27 @@ 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))
+ 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 +2804,7 @@ 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 +4919,7 @@ 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 +5021,7 @@ 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 +5982,7 @@ 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;
}

--
2.20.1