linux-kernel - patch to make Linux capabilities into something useful (v 0.3.1)

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Message-ID: <20060905212643.GA13613@clipper.ens.fr>
Date:	Tue, 5 Sep 2006 23:26:43 +0200
From:	David Madore <david.madore@....fr>
To:	Linux Kernel mailing-list <linux-kernel@...r.kernel.org>
Subject: patch to make Linux capabilities into something useful (v 0.3.1)

Hi.

As we all know, capabilities under Linux are currently crippled to the
point of being useless.  Attached is a patch (against 2.6.18-rc6)
which attempts to make them work in a reasonably useful way and at the
same time not break anything.  On top of the "additional" capabilities
that lead up to root, it also adds "regular" capabilities which all
processes have by default and which can be removed from specifically
untrusted programs.

All the gory details as to what it does are explained on this page:
<URL: http://www.madore.org/~david/linux/newcaps/newcaps.html >

In short: currently (i.e., prior to applying this patch), Linux has
capabilities, but they are (deliberately) crippled, and thus,
essentially useless, because nobody could agree on coherent semantics
for them; this patch uncripples them and attempts to give them
reasonable semantics that will, hopefully, neither break legacy Unix
programs nor those that use the current capabilies system
(essentially, Bind9); basically, capabilities are currently useless
because they are never inheritable (=preserved across execve()) and
this patch makes them so (but carefully enough so as not to confuse
existing programs).  Furthermore, whereas the current Linux
capabilities are only "additional" capabilities (meaning that normal,
non-root, processes, have none, and adding capabilities leads up to
root), the patch also suggests (and, to some extent, implements) a new
bunch of "regular" capabilites, which are present on all normal
processes and can be removed so as to provide some measure of
fault-containment for partially untrusted or potentially buggy
programs (thus, these new capabilities can be said to lead down).

Note: Although I believe that this patch will not break anything, it
is still little tested and should be considered alpha quality: it
should on no account be applied on security-critical systems or on a
system were local users are not to be trusted: the security
implications are quite complex and I could quite possibly be wrong in
thinking that it doesn't open any local root hole.

I'd be glad if some people could review this and check my reasoning
attempting to prove that it won't open any security holes (or, on the
contrary, exhibit some).

I'd also be glad if someone had a test suite that could be used to
check that traditional Unix behavior isn't broken after applying the
patch.

Comments are welcome,

-- 
     David A. Madore
    (david.madore@....fr,
     http://www.madore.org/~david/ )

View attachment "caps-0.3.1-linux-2.6.18-rc6.patch" of type "text/plain" (15997 bytes)