blk-mq: fix kernel panic during iterating over flush request

For fixing use-after-free during iterating over requests, we grabbed
request's refcount before calling ->fn in commit 2e315dc0 ("blk-mq:
grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter").
Turns out this way may cause kernel panic when iterating over one flush
request:

1) old flush request's tag is just released, and this tag is reused by
one new request, but ->rqs[] isn't updated yet

2) the flush request can be re-used for submitting one new flush command,
so blk_rq_init() is called at the same time

3) meantime blk_mq_queue_tag_busy_iter() is called, and old flush request
is retrieved from ->rqs[tag]; when blk_mq_put_rq_ref() is called,
flush_rq->end_io may not be updated yet, so NULL pointer dereference
is triggered in blk_mq_put_rq_ref().

Fix the issue by calling refcount_set(&flush_rq->ref, 1) after
flush_rq->end_io is set. So far the only other caller of blk_rq_init() is
scsi_ioctl_reset() in which the request doesn't enter block IO stack and
the request reference count isn't used, so the change is safe.

Fixes: 2e315dc0 ("blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter")
Reported-by: N"Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
Tested-by: N"Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJohn Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20210811142624.618598-1-ming.lei@redhat.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -122,7 +122,6 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
 	rq->internal_tag = BLK_MQ_NO_TAG;
 	rq->start_time_ns = ktime_get_ns();
 	rq->part = NULL;
-	refcount_set(&rq->ref, 1);
 	blk_crypto_rq_set_defaults(rq);
 }
 EXPORT_SYMBOL(blk_rq_init);

block/blk-flush.c

@@ -329,6 +329,14 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 	flush_rq->rq_flags |= RQF_FLUSH_SEQ;
 	flush_rq->rq_disk = first_rq->rq_disk;
 	flush_rq->end_io = flush_end_io;
+	/*
+	 * Order WRITE ->end_io and WRITE rq->ref, and its pair is the one
+	 * implied in refcount_inc_not_zero() called from
+	 * blk_mq_find_and_get_req(), which orders WRITE/READ flush_rq->ref
+	 * and READ flush_rq->end_io
+	 */
+	smp_wmb();
+	refcount_set(&flush_rq->ref, 1);
 
 	blk_flush_queue_rq(flush_rq, false);
 }