io_uring/net: save msghdr->msg_control for retries

Commit cac9e4418f4cbd548ccb065b3adcafe073f7f7d2 upstream.

If the application sets ->msg_control and we have to later retry this
command, or if it got queued with IOSQE_ASYNC to begin with, then we
need to retain the original msg_control value. This is due to the net
stack overwriting this field with an in-kernel pointer, to copy it
in. Hitting that path for the second time will now fail the copy from
user, as it's attempting to copy from a non-user address.

Cc: stable@vger.kernel.org # 5.10+
Link: axboe/liburing#880
Reported-and-tested-by: Marek Majkowski <marek@cloudflare.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

io_uring/io_uring.c

@@ -581,6 +581,7 @@ struct io_sr_msg {
 	size_t				len;
 	size_t				done_io;
 	struct io_buffer		*kbuf;
+	void __user			*msg_control;
 };
 
 struct io_open {
@@ -4864,10 +4865,16 @@ static int io_setup_async_msg(struct io_kiocb *req,
 static int io_sendmsg_copy_hdr(struct io_kiocb *req,
 			       struct io_async_msghdr *iomsg)
 {
+	struct io_sr_msg *sr = &req->sr_msg;
+	int ret;
+
 	iomsg->msg.msg_name = &iomsg->addr;
 	iomsg->free_iov = iomsg->fast_iov;
-	return sendmsg_copy_msghdr(&iomsg->msg, req->sr_msg.umsg,
+	ret = sendmsg_copy_msghdr(&iomsg->msg, req->sr_msg.umsg,
 				   req->sr_msg.msg_flags, &iomsg->free_iov);
+	/* save msg_control as sys_sendmsg() overwrites it */
+	sr->msg_control = iomsg->msg.msg_control;
+	return ret;
 }
 
 static int io_sendmsg_prep_async(struct io_kiocb *req)
@@ -4924,6 +4931,8 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
 		if (ret)
 			return ret;
 		kmsg = &iomsg;
+	} else {
+		kmsg->msg.msg_control = sr->msg_control;
 	}
 
 	flags = req->sr_msg.msg_flags;