netdev - Re: [PATCH 2/2] net: Implement SO

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Date:	Wed, 12 Mar 2014 14:12:33 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Vivek Goyal <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	cgroups@...r.kernel.org,
	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>, Tejun Heo <tj@...nel.org>
Cc:	ssorce@...hat.com, jkaluza@...hat.com, lpoetter@...hat.com,
	kay@...hat.com
Subject: Re: [PATCH 2/2] net: Implement SO_PEERCGROUP

On Wed, Mar 12, 2014 at 2:00 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On 03/12/2014 01:46 PM, Vivek Goyal wrote:
>> Implement SO_PEERCGROUP along the lines of SO_PEERCRED. This returns the
>> cgroup of first mounted hierarchy of the task. For the case of client,
>> it represents the cgroup of client at the time of opening the connection.
>> After that client cgroup might change.
>
> Even if people decide that sending cgroups over a unix socket is a good
> idea, this API has my NAK in the strongest possible sense, for whatever
> my NAK is worth.
>
> IMO SO_PEERCRED is a disaster.  Calling send(2) or write(2) should
> *never* imply the use of a credential.  A program should always have to
> *explicitly* request use of a credential.  What you want is SCM_CGROUP.
>
> (I've found privilege escalations before based on this observation, and
> I suspect I'll find them again.)
>
>
> Note that I think that you really want SCM_SOMETHING_ELSE and not
> SCM_CGROUP, but I don't know what the use case is yet.

This might not be quite as awful as I thought.  At least you're
looking up the cgroup at connection time instead of at send time.

OTOH, this is still racy -- the socket could easily outlive the cgroup
that created it.

--Andy
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