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Message-ID: <fa62a2e0-cbc0-d3ac-1b7b-77cea32d774c@nmatt.com>
Date: Fri, 2 Jun 2017 15:26:53 -0400
From: Matt Brown <matt@...tt.com>
To: Kees Cook <keescook@...omium.org>
Cc: "Serge E. Hallyn" <serge@...lyn.com>,
Alan Cox <gnomes@...rguk.ukuu.org.uk>,
Casey Schaufler <casey@...aufler-ca.com>,
Boris Lukashev <blukashev@...pervictus.com>,
Greg KH <gregkh@...uxfoundation.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
linux-security-module <linux-security-module@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: Re: [PATCH v7 2/2] security: tty: make TIOCSTI
ioctl require CAP_SYS_ADMIN
On 6/2/17 3:25 PM, Kees Cook wrote:
> On Fri, Jun 2, 2017 at 12:22 PM, Matt Brown <matt@...tt.com> wrote:
>> On 6/2/17 2:18 PM, Serge E. Hallyn wrote:
>>> Quoting Matt Brown (matt@...tt.com):
>>>> On 6/2/17 12:57 PM, Serge E. Hallyn wrote:
>>>>> I'm not quite sure what you're asking for here. Let me offer a precise
>>>>> strawman design. I'm sure there are problems with it, it's just a starting
>>>>> point.
>>>>>
>>>>> system-wide whitelist (for now 'may_push_chars') is full by default.
>>>>>
>>>>
>>>> So is may_push_chars just an alias for TIOCSTI? Or are there some
>>>> potential whitelist members that would map to multiple ioctls?
>>>
>>> <shrug> I'm seeing it as only TIOCSTI right now.
>>>
>>>>> By default, nothing changes - you can use those on your own tty, need
>>>>> CAP_SYS_ADMIN against init_user_ns otherwise.
>>>>>
>>>>> Introduce a new CAP_TTY_PRIVILEGED.
>>>>>
>>>>
>>>> I'm fine with this.
>>>>
>>>>> When may_push_chars is removed from the whitelist, you lose the ability
>>>>> to use TIOCSTI on a tty - even your own - if you do not have CAP_TTY_PRIVILEGED
>>>>> against the tty's user_ns.
>>>>>
>>>>
>>>> How do you propose storing/updating the whitelist? sysctl?
>>>>
>>>> If it is a sysctl, would each whitelist member have a sysctl?
>>>> e.g.: kernel.ioctlwhitelist.may_push_chars = 1
>>>>
>>>> Overall, I'm fine with this idea.
>>>
>>> That sounds reasonable. Or a securityfs file - I guess not everyone
>>> has securityfs, but if it were to become part of YAMA then that would
>>> work.
>>>
>>
>> Yama doesn't depend on securityfs does it?
>>
>> What do other people think? Should this be an addition to YAMA or its
>> own thing?
>>
>> Alan Cox: what do you think of the above ioctl whitelisting scheme?
>
> It's easy to stack LSMs, so since Yama is ptrace-focused, perhaps make
> a separate one for TTY hardening?
>
Sounds good. I also like the idea of them being separate.
Matt Brown
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