[PATCH 5.13 05/22] blk-mq: fix kernel panic during iterating over flush request

From: Greg Kroah-Hartman
Date: Fri Sep 10 2021 - 08:33:38 EST

From: Ming Lei <ming.lei@xxxxxxxxxx>

commit c2da19ed50554ce52ecbad3655c98371fe58599f upstream.

For fixing use-after-free during iterating over requests, we grabbed
request's refcount before calling ->fn in commit 2e315dc07df0 ("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

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: 2e315dc07df0 ("blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter")
Reported-by: "Blank-Burian, Markus, Dr." <blankburian@xxxxxxxxxxxxxxx>
Tested-by: "Blank-Burian, Markus, Dr." <blankburian@xxxxxxxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: John Garry <john.garry@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210811142624.618598-1-ming.lei@xxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Cc: Yi Zhang <yi.zhang@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
block/blk-core.c | 1 -
block/blk-flush.c | 8 ++++++++
2 files changed, 8 insertions(+), 1 deletion(-)

--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -122,7 +122,6 @@ void blk_rq_init(struct request_queue *q
rq->internal_tag = BLK_MQ_NO_TAG;
rq->start_time_ns = ktime_get_ns();
rq->part = NULL;
- refcount_set(&rq->ref, 1);
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -329,6 +329,14 @@ static void blk_kick_flush(struct reques
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);