linux-kernel - Re: [REVIEW][PATCH 1/2] userns: Better restrictions on when proc and sysfs can be mounted

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Message-ID: <87eh99noa0.fsf@xmission.com>
Date:	Sat, 31 Aug 2013 21:45:11 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [REVIEW][PATCH 1/2] userns: Better restrictions on when proc and sysfs can be mounted

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Andy Lutomirski <luto@...capital.net> writes:
>
>> On Tue, Aug 27, 2013 at 2:44 PM, Eric W. Biederman
>> <ebiederm@...ssion.com> wrote:
>>>
>>> Rely on the fact that another flavor of the filesystem is already
>>> mounted and do not rely on state in the user namespace.
>>
>> Possibly dumb question: does this check whether the pre-existing mount
>> has hidepid set?
>
> Not currently. 
>
> It may be worth doing something with respect to hidepid.  I forget what
> hidepid tries to do, and I need to dash.  But feel free to cook up a
> follow on patch.

So I have thought about this a bit more.

hidepid hides the processes that ptrace_may_access will fail on.

You can only reach the point where an unprivileged mount of a pid
namespace is possible if you have created both a user namespace and a
pid namespace.  Which means the creator of the pid namespace will be
capable of ptracing all of the other processes in the pid namespace
(ignoring setns).

So I don't see a point of worry about hidepid or the hidepid gid on
child pid namespaces.  The cases it is attempting to protecting against
really don't exist.

Eric
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