[PATCH AUTOSEL 5.17 35/43] Revert "Revert "block, bfq: honor already-setup queue merges""

Sasha Levin
2022-03-28T11:22:50Z

From: Paolo Valente <paolo.valente@linaro.org>

[ Upstream commit 15729ff8143f8135b03988a100a19e66d7cb7ecd ]

A crash [1] happened to be triggered in conjunction with commit
2d52c58b9c9b ("block, bfq: honor already-setup queue merges"). The
latter was then reverted by commit ebc69e897e17 ("Revert "block, bfq:
honor already-setup queue merges""). Yet, the reverted commit was not
the one introducing the bug. In fact, it actually triggered a UAF
introduced by a different commit, and now fixed by commit d29bd41428cf
("block, bfq: reset last_bfqq_created on group change").

So, there is no point in keeping commit 2d52c58b9c9b ("block, bfq:
honor already-setup queue merges") out. This commit restores it.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=214503

Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20211125181510.15004-1-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
block/bfq-iosched.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 8735f075230f..1dff82d34b44 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2782,6 +2782,15 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
* are likely to increase the throughput.
*/
bfqq->new_bfqq = new_bfqq;
+ /*
+ * The above assignment schedules the following redirections:
+ * each time some I/O for bfqq arrives, the process that
+ * generated that I/O is disassociated from bfqq and
+ * associated with new_bfqq. Here we increases new_bfqq->ref
+ * in advance, adding the number of processes that are
+ * expected to be associated with new_bfqq as they happen to
+ * issue I/O.
+ */
new_bfqq->ref += process_refs;
return new_bfqq;
}
@@ -2844,6 +2853,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
{
struct bfq_queue *in_service_bfqq, *new_bfqq;

+ /* if a merge has already been setup, then proceed with that first */
+ if (bfqq->new_bfqq)
+ return bfqq->new_bfqq;
+
/*
* Check delayed stable merge for rotational or non-queueing
* devs. For this branch to be executed, bfqq must not be
@@ -2945,9 +2958,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
if (bfq_too_late_for_merging(bfqq))
return NULL;

- if (bfqq->new_bfqq)
- return bfqq->new_bfqq;
-
if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
return NULL;

--
2.34.1