media: mc: fix potential use-after-free in media_request_alloc()

Commit 6f504cbf108a ("media: convert media_request_alloc() to
FD_PREPARE()") moved the call to fd_install() (now hidden in
fd_publish()) before the snprintf(), making the later write to
potentially already freed memory, as userland is free to call
close() concurrently right after the call to fd_install() which
may end up in the request_fops.release() handler freeing 'req'.

Fixes: 6f504cbf108a ("media: convert media_request_alloc() to FD_PREPARE()")
Signed-off-by: Mathias Krause <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

drivers/media/mc/mc-request.c

···
 
 	fd_prepare_file(fdf)->private_data = req;
 
-	*alloc_fd = fd_publish(fdf);
-
 	snprintf(req->debug_str, sizeof(req->debug_str), "%u:%d",
-		 atomic_inc_return(&mdev->request_id), *alloc_fd);
+		 atomic_inc_return(&mdev->request_id), fd_prepare_fd(fdf));
 	dev_dbg(mdev->dev, "request: allocated %s\n", req->debug_str);
+
+	*alloc_fd = fd_publish(fdf);
 
 	return 0;