linux-kernel - Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: Kernel 4.3 breaks security in systems using capabilities

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Message-ID: <20151106181820.GB16749@mail.hallyn.com>
Date:	Fri, 6 Nov 2015 12:18:20 -0600
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Klaus Ethgen <Klaus+lkml@...gen.de>
Cc:	Theodore Ts'o <tytso@....edu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Weinberger <richard.weinberger@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Andy Lutomirski <luto@...nel.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re:
 [KERNEL] Re:  Kernel 4.3  breaks security in systems using capabilities

On Fri, Nov 06, 2015 at 06:56:20PM +0100, Klaus Ethgen wrote:
> Am Fr den  6. Nov 2015 um 16:53 schrieb Theodore Ts'o:
> > In the light of that, using things like ambient capabilities, or using
> > setuid binary that immediately drops all caps that it needs, is
> > probably the best we're going to get.
> 
> I do never want that! Even to think about such a way to give any shell
> raised rights is horrible! And that horrible idea is it that makes all
> the ambient capabilities that bad.

I sympathize with your point of view, but Christoph's use case really was
a good one.  A piece of system configuration software needs to do some
networking setup with some privilege, including calling scripts.  It can
either do so as root or not at all - polluting every program that will end
up getting called with fI is not only ugly but simply doesn't work (because
scripts).  Saying that the whole thing must be written as self-contained
executable that never needs to be re-execed is frequently unrealistic.
So this allows a piece of software to run with reduce privilege, and it
is an improvement over the previous state of affairs.

It is not intended for login shells.

I would have been happy if there had been a default-off PR_ENABLE_AMBIENT
prctl which required a new CAP_ENABLE_AMBIENT capability to turn on, but
the current set of rules which removes bits from pA whenever doing an
action which capability-aware software does something which it would have
reasonably expected to drop privilege is a nice safeguard.

-serge
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