[RFC PATCH v2 1/2] blk-mq: Clean up references to old requests when freeing rqs

[John Garry]

It has been reported many times that a use-after-free can be intermittently
found when iterating busy requests:

- https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@xxxxxxxxxx/
- https://lore.kernel.org/linux-block/5c3ac5af-ed81-11e4-fee3-f92175f14daf@xxxxxxx/T/#m6c1ac11540522716f645d004e2a5a13c9f218908
- https://lore.kernel.org/linux-block/04e2f9e8-79fa-f1cb-ab23-4a15bf3f64cc@xxxxxxxxx/

The issue is that when we switch IO scheduler or change queue depth,
there may be references in the driver tagset to the stale requests; and
since getting a tag and setting the request pointer is not atomic, a
tagset iter may see a stale request for a tag recently allocated.

As a solution, clean up any references to those requests in the driver
tagset. This is done with a cmpxchg to make safe any race with setting the
driver tagset request from another queue.

Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
---
 block/blk-mq-sched.c |  2 +-
 block/blk-mq-tag.c   |  2 +-
 block/blk-mq.c       | 20 ++++++++++++++++++--
 block/blk-mq.h       |  2 ++
 4 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index deff4e826e23..fa1035e1ac6e 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -621,7 +621,7 @@ void blk_mq_sched_free_requests(struct request_queue *q)
 
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (hctx->sched_tags)
-			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
+			blk_mq_free_rqs_ext(q->tag_set, hctx->sched_tags, i, hctx->tags);
 	}
 }
 
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 6cfd6bb00ede..a6df2d5df88a 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -580,7 +580,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 			return -ENOMEM;
 		}
 
-		blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
+		blk_mq_free_rqs_ext(set, *tagsptr, hctx->queue_num, hctx->tags);
 		blk_mq_free_rq_map(*tagsptr, flags);
 		*tagsptr = new;
 	} else {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6f207ec9ef83..8465d7c5ebf0 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2278,8 +2278,8 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 	return BLK_QC_T_NONE;
 }
 
-void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
-		     unsigned int hctx_idx)
+void __blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
+		     unsigned int hctx_idx, struct blk_mq_tags *ref_tags)
 {
 	struct page *page;
 
@@ -2288,10 +2288,14 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 
 		for (i = 0; i < tags->nr_tags; i++) {
 			struct request *rq = tags->static_rqs[i];
+			int j;
 
 			if (!rq)
 				continue;
 			set->ops->exit_request(set, rq, hctx_idx);
+			/* clean up any references which occur in @ref_tags */
+			for (j = 0; ref_tags && j < ref_tags->nr_tags; j++)
+				cmpxchg(&ref_tags->rqs[j], rq, 0);
 			tags->static_rqs[i] = NULL;
 		}
 	}
@@ -2308,6 +2312,18 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 	}
 }
 
+void blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
+		     unsigned int hctx_idx, struct blk_mq_tags *ref_tags)
+{
+	__blk_mq_free_rqs_ext(set, tags, hctx_idx, ref_tags);
+}
+			 
+void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
+		     unsigned int hctx_idx)
+{
+	__blk_mq_free_rqs_ext(set, tags, hctx_idx, NULL);
+}
+
 void blk_mq_free_rq_map(struct blk_mq_tags *tags, unsigned int flags)
 {
 	kfree(tags->rqs);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index c1458d9502f1..487ca30a6509 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -53,6 +53,8 @@ struct request *blk_mq_dequeue_from_ctx(struct blk_mq_hw_ctx *hctx,
  */
 void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx);
+void blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
+		     unsigned int hctx_idx, struct blk_mq_tags *references);
 void blk_mq_free_rq_map(struct blk_mq_tags *tags, unsigned int flags);
 struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
 					unsigned int hctx_idx,
-- 
2.26.2