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

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Date:	Thu, 12 Jan 2012 17:11:39 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 4:57 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jan 12, 2012 at 4:42 PM, Andrew Lutomirski <luto@....edu> wrote:
>>
>> That may prevent another use: set this new flag, chroot, drop
>> privileges, accept network connections.  (The idea being that chroot
>> might work unprivileged if this flag is set.)
>
> Well, if you have privileges, then just do
>
>   chroot();
>   drop privileges
>
> and if you depend on the new flag, then you do
>
>   drop privileges
>   set new flag
>   chroot
>
> and if you want to work either way then you just do
>
>   error = chroot
>   drop privileges
>   set new flag
>   if error
>      chroot
>
> which does the right thing regardless of whether you had privileges
> and/or a new kernel or not.
>
> In any of the three cases I don't see why you'd ever want to drop
> privileges *after* setting the new flag.

Hmm...

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.)

--Andy
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