linux-kernel - Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always

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Date:	Thu, 12 Jan 2012 17:17:31 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	oleg@...hat.com, ak@...ux.intel.com, eric.dumazet@...il.com,
	gregkh@...e.de, dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always_unprivileged" patch

On Thu, Jan 12, 2012 at 5:11 PM, Andrew Lutomirski <luto@....edu> wrote:
>
> What if you're a daemon that needs something like CAP_NET_BIND but
> also wants to be able to run other helpers without CAP_NET_BIND?
>
> (Also, preventing dropping of privileges will probably make a patch
> more complicted -- I'll have to find and update all the places that
> allow dropping privileges.)

Hey, if it actually makes it more complicated to say "don't change
privileges", then I guess my argument that it should be simpler is
wrong.

That said, the thing you bring up is *not* the actual use-case for the
suggestion. The use-case is a "run untrusted code". So the use-case
would be to set the flag after you've dropped CAP_NET_BIND, and
*before* you actually run the other helpers. You clearly must have a
fork() or something like that there, since you want to keep the
NET_BIND in the original daemon.

So I don't t think your example is actually the expected situation.
The obvious approach for your example is
 - run deamon with CAP_NET_BIND
 - per connection, fork, drop privileges in child, then set
"restricted" flag, and run the untrusted code.

(where the restricted mode setting may well obviously also do other
things - like limit allowed system calls etc)

                      Linus
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