hi,
On Thu, Aug 12, 2021 at 07:47:55PM +0800, Xiaoguang Wang wrote:
hi,
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
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: 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>
---
block/blk-core.c | 1 -
block/blk-flush.c | 8 ++++++++
2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index 0874bc2fcdb4..b5098739f72a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -121,7 +121,6 @@ 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);
diff --git a/block/blk-flush.c b/block/blk-flush.c
index 1002f6c58181..4912c8dbb1d8 100644
--- a/block/blk-flush.c
+++ b/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
Recently we run into similar panic which is a NULL dereference on
rq->mq_hctx in is_flush_rq(), we also
guess there is race bug just what you have fixed.
But I have one question here, for a blk-mq device, before issuing the first
flush req, flush_rq->end_io
is NULL, and for following flush reqs on this blk-mq device,
flush_rq->end_io won't be NULL. I searched
the codes and don't find any place that sets flush_rq->end_io to be NULL
once flush_rq has been completed.
blk_kick_flush():
blk_rq_init
memset(rq, 0, sizeof(*rq));
Oh, thanks.
I searched codes for quite a while...
Regards,
Xiaoguang Wang
Thanks,
Ming
