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>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index f5004231cb6a927be7e58c4e1b4d0f5ea64a8b8d..e3bb498a7036cf93fcf2ed03257d045d5a2890a8 100644 (file)
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3155,9 +3155,13 @@ 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();
@@ -3165,6 +3169,8 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
                console_unlock();
                break;
        case TIOCL_SELLOADLUT:
+               if (!capable(CAP_SYS_ADMIN))
+                       return -EPERM;
                console_lock();
                ret = sel_loadlut(p);
                console_unlock();