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 10:52:35 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Peter Dolding <oiaohm@...il.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Micay <danielmicay@...il.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        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

Quoting Peter Dolding (oiaohm@...il.com):
> On Sat, May 20, 2017 at 12:33 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
> > On Fri, May 19, 2017 at 12:48:17PM +1000, Peter Dolding wrote:
> >> Using cap_sys_admin as fix is like removing car windsheld because
> >> vision is being blocked by a rock hitting it.
> >
> > Nonsense.  If the application has cap_sys_admin then it is less contained and
> > more trusted anyway.  If I went to the trouble to run an application in a
> > private user namespace (where it can have cap_sys_admin, but not targeted
> > at my tty) then it should be more contained.  That's the point of targeted
> > capabilities.
> 
> The thing that is missed every time is how much is cap_sys_admin.
> 
> So you are saying a user namespace has to be set up to contain the defect.
> 
> Really no application should have cap_sys_admin.
> 
> The theory of capabilities is that security should be broken down into
> logical blocks.
> 
> 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.

What you may not do is change it so that on an older kernel you must have
CAP_SYS_ADMIN to use TIOCSTI, while on a newer one it does not suffice.

-serge