io_uring: don't map read/write iovec potentially twice

If we have a read/write that is deferred, we already setup the async IO
context for that request, and mapped it. When we later try and execute
the request and we get -EAGAIN, we don't want to attempt to re-map it.
If we do, we end up with garbage in the iovec, which typically leads
to an -EFAULT or -EINVAL completion.

Cc: stable@vger.kernel.org # 5.5
Reported-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2171,10 +2171,12 @@ static int io_setup_async_rw(struct io_kiocb *req, ssize_t io_size,
 {
 	if (!io_op_defs[req->opcode].async_ctx)
 		return 0;
-	if (!req->io && io_alloc_async_ctx(req))
+	if (!req->io) {
+		if (io_alloc_async_ctx(req))
 			return -ENOMEM;
 
 		io_req_map_rw(req, io_size, iovec, fast_iov, iter);
+	}
 	req->work.func = io_rw_async;
 	return 0;
 }