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

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Message-ID: <20170602181822.GA5125@mail.hallyn.com>
Date:   Fri, 2 Jun 2017 13:18:22 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Matt Brown <matt@...tt.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Kees Cook <keescook@...omium.org>,
        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@...r.kernel.org
Subject: Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make
 TIOCSTI ioctl require CAP_SYS_ADMIN

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.

-serge