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Date: Fri,  5 May 2017 19:20:16 -0400
From: Matt Brown <>
Subject: [PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

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
* 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
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.

# Changes since v5:
* added acks/reviews

# 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

# 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

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