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Message-Id: <20170423235417.12809-1-matt@nmatt.com>
Date: Sun, 23 Apr 2017 19:54:15 -0400
From: Matt Brown <matt@...tt.com>
To: serge@...lyn.com,
jmorris@...ei.org,
gregkh@...uxfoundation.org,
jslaby@...e.com,
corbet@....net,
keescook@...omium.org
Cc: akpm@...ux-foundation.org,
jannh@...gle.com,
kernel-hardening@...ts.openwall.com,
linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org
Subject: [PATCH v3 0/2] tiocsti-restrict : 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
https://bugzilla.redhat.com/show_bug.cgi?id=1411256 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: <https://codesearch.debian.net/search?q=ioctl%5C%28.*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:
<http://www.halfdog.net/Security/2012/TtyPushbackPrivilegeEscalation/>
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 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
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