static int __blk_rq_map_user_iov(struct request *rq,
		struct rq_map_data *map_data, struct iov_iter *iter,
		gfp_t gfp_mask, bool copy)
{
	struct request_queue *q = rq->q;
	struct bio *bio, *orig_bio;
	int ret;

	if (copy)
		bio = bio_copy_user_iov(q, map_data, iter, gfp_mask);
	else
		bio = bio_map_user_iov(q, iter, gfp_mask);

	if (IS_ERR(bio))
		return PTR_ERR(bio);

	if (map_data && map_data->null_mapped)
		bio_set_flag(bio, BIO_NULL_MAPPED);

	iov_iter_advance(iter, bio->bi_iter.bi_size);
	if (map_data)
		map_data->offset += bio->bi_iter.bi_size;

	orig_bio = bio;
	blk_queue_bounce(q, &bio);

	/*
	 * We link the bounce buffer in and could have to traverse it
	 * later so we have to get a ref to prevent it from being freed
	 */
	bio_get(bio);

	ret = blk_rq_append_bio(rq, bio);
	if (ret) {
		bio_endio(bio);
		__blk_rq_unmap_user(orig_bio);
		bio_put(bio);
		return ret;
	}

	return 0;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.