linux-kernel - Re: [capabilities] Allow normal inheritance for a configurable set of capabilities

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Message-ID: <20150205003434.GC23013@mail.hallyn.com>
Date:	Thu, 5 Feb 2015 01:34:34 +0100
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Andrew G. Morgan" <morgan@...nel.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Christoph Lameter <cl@...ux.com>, casey@...aufler-ca.com,
	Andy Lutomirski <luto@...capital.net>,
	Jonathan Corbet <corbet@....net>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	linux-security-module@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>, akpm@...uxfoundation.org
Subject: Re: [capabilities] Allow normal inheritance for a configurable set
 of capabilities

Quoting Andrew G. Morgan (morgan@...nel.org):
> I'm not generally in favor of this. Mostly because this seems to be a
> mini-root kind of inheritance that propagates privilege to binaries
> that aren't prepared for privilege.

Earlier in this thread, Casey said:

| One of the holes in the 1003.1e spec is what to do with a program file
| that does not have a capability set attached to it. The two options are
| drop all capabilities and leave the capabilities alone. The latter gives
| you what you're asking for. The former is arguably safer.

and

| It's what we did in Trusted Irix. It made life much easier.

I'm going to need to clear my head a bit before I try to compare that to
the root cause of the sendmail capabilities bug.

>  I don't really buy the mmap code
> concern because the model as it stands says that you trust the binary
> (and all of the various ways it was programmed to execute code) with
> specific privileges. If the binary mmap's some code (PAM modules come
> to mind) then that is part of what it was programmed to/allowed to do.

That's not really the point...  The point is that yes, a mini-root is
exactly what is being asked for :)  I'm not saying I expect an adversary
to do the mmap+jump, but that currently it is a, and the only, way to
do unprivileged userid with retaining some privileges to run legacy
programs.

> That being said, if you really really want this kind of thing, then
> make it a single secure bit (with another lock on/off bit) which, when
> set, makes: fI default to X.
> 
>    pP' = (X & fP) | (pI & fI)
> 
> That way the per-process bounding set still works as advertised and
> you don't need to worry about the existing semantics being violated.

Maybe that is the way to go...

-serge
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