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From: ebiederm@xmission.com (Eric W. Biederman)
To: Dave Hansen <haveblue@us.ibm.com>
Cc: "Serge E. Hallyn" <serue@us.ibm.com>, Cedric Le Goater <clg@fr.ibm.com>,
       linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
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       Herbert Poetzl <herbert@13thfloor.at>,
       Sam Vilain <sam.vilain@catalyst.net.nz>
Subject: Re: [PATCH -mm 5/7] add user namespace
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Date: Thu, 13 Jul 2006 21:45:49 -0600
In-Reply-To: <1152821011.24925.7.camel@localhost.localdomain> (Dave Hansen's
	message of "Thu, 13 Jul 2006 13:03:31 -0700")
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Dave Hansen <haveblue@us.ibm.com> writes:

> On Thu, 2006-07-13 at 13:02 -0600, Eric W. Biederman wrote:
>> All comparisons of a user equality need to be of the tuple (user namespace,
> user id).
>> Any comparison that does not do that is an optimization.
> ...
>> So my impression was that Cedric's patchset was overoptimized because
>> it did not change most of the uid comparisons, to (user namespace, user id).
>
> I might just be tempted to call them bugs so people understand what I'm
> talking about ;)
>
>> Because you can have access to files created in another user namespace it
>> is very unlikely that optimization will apply very frequently.  The easy
> scenario
>> to get access to a file descriptor from another context is to consider unix
>> domain sockets.
>
> OK, so you're saying that the lack of checks will cause problems rarely,
> and that passing a fd across a unix domain sockets is one of the times
> when you _could_ encounter this problem?

I think for filesystems like /proc and /sys that there will normally
be problems.  However many of those problems can be rationalized away
as a reasonable optimization, or are not immediately apparent.

Passing a file descriptor between process in a unix domain socket is
a case where I can easily construct scenarios where there are
indisputable problems.  It is one of my standard thought experiments
to see if a namespace is sound.

Eric