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

Commit 6f504cb ("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: 6f504cb ("media: convert media_request_alloc() to FD_PREPARE()")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://patch.msgid.link/20251209210903.603958-1-minipli@grsecurity.net
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: minipli-oss

drivers/media/mc/mc-request.c

@@ -315,12 +315,12 @@ int media_request_alloc(struct media_device *mdev, int *alloc_fd)
 
 	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;
 
 err_free_req: