fanotify: don't write with size under sizeof(response)

fanotify_write() only aligned copy_from_user size to sizeof(response)
for higher values. This patch avoids all values below as suggested
by Amir Goldstein and set to response size unconditionally.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Fabian Frederick <[email protected]>
Reviewed-by: Amir Goldstein <[email protected]>
Signed-off-by: Jan Kara <[email protected]>

Author: Fabian Frederick

fs/notify/fanotify/fanotify_user.c

@@ -487,8 +487,10 @@ static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t
 
 	group = file->private_data;
 
-	if (count > sizeof(response))
-		count = sizeof(response);
+	if (count < sizeof(response))
+		return -EINVAL;
+
+	count = sizeof(response);
 
 	pr_debug("%s: group=%p count=%zu\n", __func__, group, count);