vt_kdsetmode: extend console locking

stable inclusion
from linux-4.19.206
commit 0776c1a20babb4ad0b7ce7f2f4e0806a97663187
CVE: CVE-2021-3753

--------------------------------

commit 2287a51b upstream.

As per the long-suffering comment.
Reported-by: NMinh Yuan <yuanmingbuaa@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/tty/vt/vt_ioctl.c

@@ -484,16 +484,19 @@ int vt_ioctl(struct tty_struct *tty,
 			ret = -EINVAL;
 			goto out;
 		}
-		/* FIXME: this needs the console lock extending */
-		if (vc->vc_mode == (unsigned char) arg)
+		console_lock();
+		if (vc->vc_mode == (unsigned char) arg) {
+			console_unlock();
 			break;
+		}
 		vc->vc_mode = (unsigned char) arg;
-		if (console != fg_console)
+		if (console != fg_console) {
+			console_unlock();
 			break;
+		}
 		/*
 		 * explicitly blank/unblank the screen if switching modes
 		 */
-		console_lock();
 		if (arg == KD_TEXT)
 			do_unblank_screen(1);
 		else