Re: [PATCH v5 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

From: Kees Cook
Date: Wed May 03 2017 - 15:30:24 EST

On Mon, Apr 24, 2017 at 9:15 PM, Matt Brown <matt@xxxxxxxxx> wrote:
> This patchset introduces the tiocsti_restrict sysctl, whose default is
> controlled via CONFIG_SECURITY_TIOCSTI_RESTRICT. When activated, this
> control restricts all TIOCSTI ioctl calls from non CAP_SYS_ADMIN users.
> This patch was inspired from GRKERNSEC_HARDEN_TTY.
> This patch would have prevented
> under the following
> conditions:
> * non-privileged container
> * container run inside new user namespace
> Possible effects on userland:
> There could be a few user programs that would be effected by this
> change.
> See: <*TIOCSTI>
> notable programs are: agetty, csh, xemacs and tcsh
> However, I still believe that this change is worth it given that the
> Kconfig defaults to n. This will be a feature that is turned on for the
> same reason that people activate it when using grsecurity. Users of this
> opt-in feature will realize that they are choosing security over some OS
> features like unprivileged TIOCSTI ioctls, as should be clear in the
> Kconfig help message.
> Threat Model/Patch Rational:
> From grsecurity's config for GRKERNSEC_HARDEN_TTY.
> | There are very few legitimate uses for this functionality and it
> | has made vulnerabilities in several 'su'-like programs possible in
> | the past. Even without these vulnerabilities, it provides an
> | attacker with an easy mechanism to move laterally among other
> | processes within the same user's compromised session.
> So if one process within a tty session becomes compromised it can follow
> that additional processes, that are thought to be in different security
> boundaries, can be compromised as a result. When using a program like su
> or sudo, these additional processes could be in a tty session where TTY file
> descriptors are indeed shared over privilege boundaries.
> This is also an excellent writeup about the issue:
> <>
> When user namespaces are in use, the check for the capability
> CAP_SYS_ADMIN is done against the user namespace that originally opened
> the tty.

This looks like it's ready to go. Greg, can you include this in your
tree? That seems like the best place, even though it touches a few

Please consider it:

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>



> # Changes since v4:
> * fixed typo
> # Changes since v3:
> * use get_user_ns and put_user_ns to take and drop references to the owner
> user namespace because CONFIG_USER_NS is an option
> # Changes since v2:
> * take/drop reference to user namespace on tty struct alloc/free to prevent
> use-after-free.
> # Changes since v1:
> * added owner_user_ns to tty_struct to enable capability checks against
> the namespace that created the tty.
> * rewording in different places to make patchset purpose clear
> * Added Documentation

Kees Cook
Pixel Security