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Date: Fri, 2 Jun 2017 15:26:53 -0400
From: Matt Brown <>
To: Kees Cook <>
Cc: "Serge E. Hallyn" <>,
 Alan Cox <>,
 Casey Schaufler <>,
 Boris Lukashev <>,
 Greg KH <>,
 "" <>,
 linux-security-module <>,
 linux-kernel <>,
 Linux API <>
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 <> wrote:
>> On 6/2/17 2:18 PM, Serge E. Hallyn wrote:
>>> Quoting Matt Brown (
>>>> 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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