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

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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>
This commit is contained in:
Hanno Böck 2023-08-28 18:41:17 +02:00 committed by Greg Kroah-Hartman
parent 7cda0b9eb6
commit 8d1b43f6a6
1 changed files with 6 additions and 0 deletions
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6
drivers/tty/vt/vt.c
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@ -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();