tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN

With this, processes without CAP_SYS_ADMIN are able to use TIOCLINUX with
subcode TIOCL_SETSEL, in the selection modes TIOCL_SETPOINTER,
TIOCL_SELCLEAR and TIOCL_SELMOUSEREPORT.

TIOCL_SETSEL was previously changed to require CAP_SYS_ADMIN, as this IOCTL
let callers change the selection buffer and could be used to simulate
keypresses.  These three TIOCL_SETSEL selection modes, however, are safe to
use, as they do not modify the selection buffer.

This fixes a mouse support regression that affected Emacs (invisible mouse
cursor).

Cc: stable <stable@kernel.org>
Link: https://lore.kernel.org/r/ee3ec63269b43b34e1c90dd8c9743bf8@finder.org
Fixes: 8d1b43f ("tty: Restrict access to TIOCLINUX' copy-and-paste subcommands")
Signed-off-by: Günther Noack <gnoack@google.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250110142122.1013222-1-gnoack@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: gnoack

drivers/tty/vt/selection.c

@@ -192,6 +192,20 @@ int set_selection_user(const struct tiocl_selection __user *sel,
 	if (copy_from_user(&v, sel, sizeof(*sel)))
 		return -EFAULT;
 
+	/*
+	 * TIOCL_SELCLEAR, TIOCL_SELPOINTER and TIOCL_SELMOUSEREPORT are OK to
+	 * use without CAP_SYS_ADMIN as they do not modify the selection.
+	 */
+	switch (v.sel_mode) {
+	case TIOCL_SELCLEAR:
+	case TIOCL_SELPOINTER:
+	case TIOCL_SELMOUSEREPORT:
+		break;
+	default:
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
+	}
+
 	return set_selection_kernel(&v, tty);
 }
 

drivers/tty/vt/vt.c

@@ -3345,8 +3345,6 @@ 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(param, tty);
 	case TIOCL_PASTESEL:
 		if (!capable(CAP_SYS_ADMIN))