lib/iov_iter.c: initialize bi.bi_idx before iterating over bvec

Initialize bi.bi_idx as 0 before iterating over bvec, otherwise
garbage data can be used as ->bi_idx.

Cc: Christoph Hellwig <[email protected]>
Reported-and-tested-by: Klara Modin <[email protected]>
Fixes: e4e535b ("iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages")
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

lib/iov_iter.c

@@ -1699,6 +1699,7 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
 		i->bvec++;
 		skip = 0;
 	}
+	bi.bi_idx = 0;
 	bi.bi_size = maxsize + skip;
 	bi.bi_bvec_done = skip;