vcs: prevent write access to vcsu devices

Commit d21b0be ("vt: introduce unicode mode for /dev/vcs") guarded
against using devices containing attributes as this is not yet
implemented. It however failed to guard against writes to any devices
as this is also unimplemented.

Reported-by: Or Cohen <[email protected]>
Signed-off-by: Nicolas Pitre <[email protected]>
Cc: <[email protected]> # v4.19+
Cc: Jiri Slaby <[email protected]>
Fixes: d21b0be ("vt: introduce unicode mode for /dev/vcs")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: npitre

drivers/tty/vt/vc_screen.c

@@ -456,6 +456,9 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 	size_t ret;
 	char *con_buf;
 
+	if (use_unicode(inode))
+		return -EOPNOTSUPP;
+
 	con_buf = (char *) __get_free_page(GFP_KERNEL);
 	if (!con_buf)
 		return -ENOMEM;