From: Ming Lei ming.lei@redhat.com
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 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@uni-muenster.de Tested-by: "Blank-Burian, Markus, Dr." blankburian@uni-muenster.de Signed-off-by: Ming Lei ming.lei@redhat.com Reviewed-by: Christoph Hellwig hch@lst.de Reviewed-by: John Garry john.garry@huawei.com Link: https://lore.kernel.org/r/20210811142624.618598-1-ming.lei@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk Cc: Yi Zhang yi.zhang@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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); blk_crypto_rq_set_defaults(rq); } EXPORT_SYMBOL(blk_rq_init); --- 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); }