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From: ebiederm@xmission.com (Eric W. Biederman)
To: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>, Cedric Le Goater <clg@fr.ibm.com>,
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Subject: Re: [PATCH -mm 5/7] add user namespace
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Date: Thu, 13 Jul 2006 21:52:40 -0600
In-Reply-To: <20060713214101.GB2169@sergelap.austin.ibm.com> (Serge
	E. Hallyn's message of "Thu, 13 Jul 2006 16:41:01 -0500")
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"Serge E. Hallyn" <serue@us.ibm.com> writes:

> Quoting Eric W. Biederman (ebiederm@xmission.com):
>> Dave Hansen <haveblue@us.ibm.com> writes:
>> 
>> > On Thu, 2006-07-13 at 12:14 -0600, Eric W. Biederman wrote:
>> >> Maybe.  I really think the sane semantics are in a different uid namespace.
>> >> So you can't assumes uids are the same.  Otherwise you can't handle open
>> >> file descriptors or files passed through unix domain sockets.
>> >
>> > Eric, could you explain this a little bit more?  I'm not sure I
>> > understand the details of why this is a problem?
>> 
>> Very simply.
>> 
>> In the presence of a user namespace.  
>> 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.
>> 
>> 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.
>
> What does that have to do with uids?  If you receive an fd, uids don't
> matter in any case.  The only permission checks which happen are LSM
> hooks, which should be uid-agnostic.

You are guest uid 0.  You get a directory file descriptor from another namespace.
You call fchdir.

If you permission checks are not (user namespace, uid) what can't you do?

Eric