[kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

Tue May 30 02:00:41 UTC 2017

Casey Schaufler,

First I must start this off by saying I really appreciate your presentation on
LSMs that is up on youtube. I've got a LSM in the works and your talk has
helped me a bunch.

On 5/29/17 8:27 PM, Casey Schaufler wrote:
> 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.

This patch does not break these programs as you imply. 99% of users of these
programs will not be effected. Its not like the TIOCSTI ioctl is a critical
part of these programs.

Also as I've stated elsewhere, this is not breaking userspace because this
Kconfig/sysctl defaults to n. If someone is using the programs listed above in
a way that does utilize an unprivileged call to the TIOCSTI ioctl, they can
turn this feature off.

> 
>> 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.
> 

First, I don't know how to parse "99-44/100%" and therefore do not wish to
wager a beverage on such confusing odds ;)

Second, as stated above, this feature is off by default. However, I would expect
this sysctl to show up in lists of procedures for hardening linux servers.

>> 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.
> 

Can you please state your reasons for why you believe this solution is not "up
to snuff?" So far myself and others have given what I believe to be sound
responses to any objections to this patch.

>> >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 <gnomes at lxorguk.ukuu.org.uk> wrote:
>>> On Mon, 29 May 2017 17:38:00 -0400
>>> Matt Brown <matt at nmatt.com> 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
>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1411256 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
>>
>>
> 

Thanks,
Matt Brown
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