tty: Restrict access to TIOCLINUX' copy-and-paste subcommands

TIOCLINUX can be used for privilege escalation on virtual terminals when
code is executed via tools like su/sudo and sandboxing tools.

By abusing the selection features, a lower-privileged application can
write content to the console, select and copy/paste that content and
thereby executing code on the privileged account. See also the poc
here:

  https://www.openwall.com/lists/oss-security/2023/03/14/3

Selection is usually used by tools like gpm that provide mouse features
on the virtual console. gpm already runs as root (due to earlier
changes that restrict access to a user on the current TTY), therefore
it will still work with this change.

With this change, the following TIOCLINUX subcommands require
CAP_SYS_ADMIN:

 * TIOCL_SETSEL - setting the selected region on the terminal
 * TIOCL_PASTESEL - pasting the contents of the selected region into
   the input buffer
 * TIOCL_SELLOADLUT - changing word-by-word selection behaviour

The security problem mitigated is similar to the security risks caused
by TIOCSTI, which, since kernel 6.2, can be disabled with
CONFIG_LEGACY_TIOCSTI=n.

Signed-off-by: Hanno Böck <hanno@hboeck.de>
Signed-off-by: Günther Noack <gnoack@google.com>
Tested-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20230828164117.3608812-2-gnoack@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -3155,16 +3155,22 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 
 	switch (type) {
 	case TIOCL_SETSEL:
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
 		return set_selection_user((struct tiocl_selection
 					 __user *)(p+1), tty);
 	case TIOCL_PASTESEL:
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
 		return paste_selection(tty);
 	case TIOCL_UNBLANKSCREEN:
 		console_lock();
 		unblank_screen();
 		console_unlock();
 		break;
 	case TIOCL_SELLOADLUT:
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
 		console_lock();
 		ret = sel_loadlut(p);
 		console_unlock();