linux-kernel - Re: [kernel-hardening] Re: [PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS

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Date:   Tue, 30 May 2017 22:52:45 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Peter Dolding <oiaohm@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Micay <danielmicay@...il.com>,
        Matt Brown <matt@...tt.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, James Morris <jmorris@...ei.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 v6 0/2] security: tty: make
 TIOCSTI ioctl require CAP_SYS_ADMIN

> > So tty stuff should under a tty capabilities.  
> 
> (last reply on this)
> 
> Currently capabilities.7 says
> 
>               * employ  the  TIOCSTI ioctl(2) to insert characters into the input queue of a
>                 terminal other than the caller's controlling terminal;
> 
> for CAP_SYS_ADMIN.
> 
> So you can create a new CAP_SYS_TIOCSSTI if you like, and offer a patch where
> *both* CAP_SYS_ADMIN and CAP_SYS_ADMIN suffice.  Again, see CAP_SYSLOG for a
> prior example.

Even then it wouldn't be useful because the attacker can use every other
interface in the tty layer, many of which you can't magic away behind a
capability bit. And the applications would need changing to use the
feature - at which point any theoretical broken apps can instead be fixed
to use a pty/tty pair and actually fix the real problem.

Alan