io_uring/rw: check for NULL io_br_sel when putting a buffer

Both the read and write side use kiocb_done() to finish a request, and
kiocb_done() will call io_put_kbuf() in case a provided buffer was used
for the request. Provided buffers are not supported for writes, hence
NULL is being passed in. This normally works fine, as io_put_kbuf()
won't actually use the value unless REQ_F_BUFFER_RING or
REQ_F_BUFFER_SELECTED is set in the request flags. But depending on
compiler (or whether or not CONFIG_CC_OPTIMIZE_FOR_SIZE is set), that
may be done even though the value is never used. This will then cause a
NULL pointer dereference.

Make it a bit more obvious and check for a NULL io_br_sel, and don't
even bother calling io_put_kbuf() for that case.

Fixes: 5fda512 ("io_uring/kbuf: switch to storing struct io_buffer_list locally")
Reported-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/rw.c

@@ -655,13 +655,17 @@ static int kiocb_done(struct io_kiocb *req, ssize_t ret,
 	if (ret >= 0 && req->flags & REQ_F_CUR_POS)
 		req->file->f_pos = rw->kiocb.ki_pos;
 	if (ret >= 0 && !(req->ctx->flags & IORING_SETUP_IOPOLL)) {
+		u32 cflags = 0;
+
 		__io_complete_rw_common(req, ret);
 		/*
 		 * Safe to call io_end from here as we're inline
 		 * from the submission path.
 		 */
 		io_req_io_end(req);
-		io_req_set_res(req, final_ret, io_put_kbuf(req, ret, sel->buf_list));
+		if (sel)
+			cflags = io_put_kbuf(req, ret, sel->buf_list);
+		io_req_set_res(req, final_ret, cflags);
 		io_req_rw_cleanup(req, issue_flags);
 		return IOU_COMPLETE;
 	} else {