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Date: Mon, 29 May 2017 17:27:30 -0700
From: Casey Schaufler <>
To: Boris Lukashev <>,
 Alan Cox <>
Cc: Matt Brown <>, Greg KH <>,
 "Serge E. Hallyn" <>, Kees Cook <>,,
 linux-security-module <>,
 linux-kernel <>
Subject: Re: Re: [PATCH v7 2/2] security: tty: make TIOCSTI
 ioctl require CAP_SYS_ADMIN

On 5/29/2017 4:51 PM, Boris Lukashev wrote:
> With all due respect sir, i believe your review falls short of the
> purpose of this effort - to harden the kernel against flaws in
> userspace. Comments along the line of "if <userspace> does it right
> then your patch is pointless" are not relevant to the context of
> securing kernel functions/interfaces. What userspace should do has
> little bearing on defensive measures actually implemented in the
> kernel - if we took the approach of "someone else is responsible for
> that" in military operations, the world would be a much darker and
> different place today. Those who have the luxury of standoff from the
> critical impacts of security vulnerabilities may not take into account
> the fact that peoples lives literally depend on Linux getting a lot
> more secure, and quickly.

You are not going to help anyone with a kernel configuration that
breaks agetty, csh, xemacs and tcsh. The opportunities for using
such a configuration are limited.

> If this work were not valuable, it wouldnt be an enabled kernel option
> on a massive number of kernels with attack surfaces reduced by the
> compound protections offered by the grsec patch set.

I'll bet you a beverage that 99-44/100% of the people who have
this enabled have no clue that it's even there. And if they did,
most of them would turn it off.

> I can't speak for
> the grsec people, but having read a small fraction of the commentary
> around the subject of mainline integration, it seems to me that NAKs
> like this are exactly why they had no interest in even trying - this
> review is based on the cultural views of the kernel community, not on
> the security benefits offered by the work in the current state of
> affairs (where userspace is broken).

A security clamp-down that breaks important stuff is going
to have a tough row to hoe going upstream. Same with a performance
enhancement that breaks things.

> The purpose of each of these
> protections (being ported over from grsec) is not to offer carte
> blanche defense against all attackers and vectors, but to prevent
> specific classes of bugs from reducing the security posture of the
> system. By implementing these defenses in a layered manner we can
> significantly reduce our kernel attack surface.

Sure, but they have to work right. That's an important reason to do
small changes. A change that isn't acceptable can be rejected without
slowing the general progress.

> Once userspace catches
> up and does things the right way, and has no capacity for doing them
> the wrong way (aka, nothing attackers can use to bypass the proper
> userspace behavior), then the functionality really does become
> pointless, and can then be removed.

Well, until someone comes along with yet another spiffy feature
like containers and breaks it again. This is why a really good
solution is required, and the one proposed isn't up to snuff.

> >From a practical perspective, can alternative solutions be offered
> along with NAKs?

They often are, but let's face it, not everyone has the time,
desire and/or expertise to solve every problem that comes up.

> Killing things on the vine isnt great, and if a
> security measure is being denied, upstream should provide their
> solution to how they want to address the problem (or just an outline
> to guide the hardened folks).

The impact of a "security measure" can exceed the value provided.
That is, I understand, the basis of the NAK. We need to be careful
to keep in mind that, until such time as there is substantial interest
in the sort of systemic changes that truly remove this class of issue,
we're going to have to justify the risk/reward trade off when we try
to introduce a change.

> On Mon, May 29, 2017 at 6:26 PM, Alan Cox <> wrote:
>> On Mon, 29 May 2017 17:38:00 -0400
>> Matt Brown <> wrote:
>>> This 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.
>> Which is really quite pointless as I keep pointing out and you keep
>> reposting this nonsense.
>>> This patch depends on patch 1/2
>>> 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
>> And assuming no other ioctl could be used in an attack. Only there are
>> rather a lot of ways an app with access to a tty can cause mischief if
>> it's the same controlling tty as the higher privileged context that
>> launched it.
>> Properly written code allocates a new pty/tty pair for the lower
>> privileged session. If the code doesn't do that then your change merely
>> modifies the degree of mayhem it can cause. If it does it right then your
>> patch is pointless.
>>> Possible effects on userland:
>>> There could be a few user programs that would be effected by this
>>> change.
>> In other words, it's yet another weird config option that breaks stuff.
>> NAK v7.
>> Alan

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