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

to #26323588

commit 5d204bcfa09330972ad3428a8f81c23f371d3e6d upstream.

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: NDan Melnic <dmm@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

fs/io_uring.c

@@ -2168,10 +2168,12 @@ 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;
 }