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

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Message-ID: <4a8e9445-4c3f-e44e-9d84-fef2ca956517@nmatt.com>
Date:   Sat, 3 Jun 2017 18:22:56 -0400
From:   Matt Brown <matt@...tt.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        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>
Subject: Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI
 ioctl require CAP_SYS_ADMIN

On 06/03/2017 06:00 PM, Alan Cox wrote:
>> TIOCSLCKTRMIOS
>
> That one I'm more dubious about
>
>> TIOCSLTC
>> TIOCSSOFTCAR
>
> tty_io.c also has a few and n_tty has a couple we'd want.
>
>>
>> would it be overkill to have a sysctl kernel.ttyioctlwhitelist.X where X
>> is one of the ioctls above?
>
> Why would anyone want to change the entries on that list
>

Did you see Serge's proposed solution? I want us to not be talking past
each other. Serge proposed the following:

| 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.
|
| 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.

The question is how do you add/remove something from this whitelist? I
assume by add/remove we don't mean that you have to recompile your
kernel to change the whitelist!

you earlier said you wanted the check to look like this:

| if (!whitelisted(ioctl) && different_namespace && magic_flag)

I want to know which namespace you are talking about here. Did you mean
user_namespace? (the namespace I added tracking for in the tty_struct)