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Date: Fri, 2 Jun 2017 12:25:25 -0700
From: Kees Cook <>
To: Matt Brown <>
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 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?


Kees Cook
Pixel Security

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