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Date: Tue, 16 May 2017 19:01:08 +1000
From: Peter Dolding <>
To: Matt Brown <>
Cc: Alan Cox <>,,,,, 
	Jann Horn <>, Kees Cook <>, 
	James Morris <>,, 
	linux-security-module <>, 
	linux-kernel <>
Subject: Re: [PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

> I could see a case being make for CAP_SYS_TTY_CONFIG. However I still
> choose to do with CAP_SYS_ADMIN because it is already in use in the
> TIOCSTI ioctl.
Matt Brown don't give me existing behaviour.    CAP_SYS_ADMIN is
overload.   The documentation tells you that you are not to expand it
and you openly admit you have.

Does anything of TIOSCTI functionally say that it really should be in

If functionality is going to cause security for containers maybe it
should be not in CAP_SYS_ADMIN but in its own capability that can
enabled on file by file base.

> You might be right that CAP_SYS_ADMIN is overloaded, but my patch
> barely adds anything to it since TIOCSTI already falls under its
> control. It seems extreme to say this patch ought to be rejected just
> because it contains CAP_SYS_ADMIN. If we want to fix the state of Linux
> capabilities, then I suggest that should be a separate patchset to
> reorganize them into a more modular set of controls.
We have end up with CAP_SYS_ADMIN a mess by the of a death by a
thousand cuts.   Each person to extend CAP_SYS_ADMIN to it current
mess said the same thing.   My patch barely added anything times that
by a few thousand and you end up with what we have today.   At some
point no more has to be said.

There is no point attempting to tidy it up of the rules are not put in
place so it does not turn into a mess again.

This is not something that is suitable to be done as one large
patchset.   This is better done in the same kind of method that made
it.  So every time people want to alter something associated with
CAP_SYS_ADMIN it has to get assessed and the patch has to be one that
partly corrects the existing mess.    Do this enough times and we will
no longer have a mess on CAP_SYS_ADMIN.
Please note the CapabilityBoundingSet=

Your current patch adds no extra controls for me running a service
under systemd or anything else like it to say I don't want the
processes having the means to-do this even that they are running with
CAP_SYS_ADMIN to perform other tasks..

--employ the TIOCSTI ioctl(2) to insert characters into the input
queue of a terminal other than the caller's controlling terminal;--
This currently under CAP_SYS_ADMIN is vastly more powerful than the
one you are attempt to take away with your patch.  This one can send
messages into other terminals.   This is a vastly more powerful
version of TIOSCTI.

I fact this usage of TIOCSTI I personally think should require two
capabilities flags set.   CAP_SYS_ADMIN section left as it is at this
stage.   With TIOSCTI stuck behind another capability.

If you had added a new capability flag you could set file capabilities
on any of the old applications depending on the now secured behaviour.

Also the general user TIOCSTI issue can be handled a different way as
LXC fix shows.  Where they uses a pty to isolate so meaning in their
fixed setup user TIOSCTI was not harmful but CAP_SYS_ADMIN TIOSCTI
still could be.  You patch as not address this problem because you
shoved everything under CAP_SYS_ADMIN.   But if you add different
capability to use TIOSCIT that is not CAP_SYS_ADMIN  to allow
CAP_SYS_ADMiN TIOSCTI functionality to be disabled.

This is what you see more often than not when you dig into this
patches adding more CAP_SYS_ADMIN.  Adding CAP_SYS_ADMIN is not fixing
a problem in most cases.   Breaking CAP_SYS_ADMIN functionality up
should be the goal not expand it.

Basically you have done something documentation has a note to
developer not to-do.   If you start looking at the problem what you
doing is not helping.   If people end up using CAP_SYS_ADMIN to access
TIOSCTI its giving the program a more powerful version of TIOSCTI to
do more harm with so reduced containment.  Totally anti to what you
are meant to be doing with capabilities..

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