linux-kernel - Re: [PATCH 4/8] user ns: hook permission

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Message-ID: <20070124190633.GB597@sergelap.austin.ibm.com>
Date:	Wed, 24 Jan 2007 13:06:33 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, containers@...ts.osdl.org,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH 4/8] user ns: hook permission

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serue@...ibm.com> writes:
> 
> > From: Serge E. Hallyn <serue@...ibm.com>
> > Subject: [PATCH 4/8] user ns: hook permission
> >
> > Hook permission to check vfsmnt->user_ns against current.
> 
> This looks wrong on several levels.
> - This should ultimately be inside generic_permission instead of
>   permission as there are some distributed filesystems that know how to cope with
>   multiple mount namespaces simultaneous.
> 
> - As implemented the test is not what I would expect.  I would
>   expect comparisons of uid X == uid Y and gid X == gid Y to
>   be replaced by comparing the tuples of uid namesspace and uid.
>   Which would allow access to world readable/writeable files,
>   and it would allow users with CAP_DAC_OVERRIDE to be able to access
>   everything.

Whoa - why on earth would we want that?

> All we are really saying as I understand a user namespace is that 
> instead of uid's uniquely identifying a user the pair the pair uidns,
> uid is uniquely identifies a user.

Ok that would be one way to interpret it, but it is insufficient for
preventing root in one vserver from messing with users in another
vserver.

> Because you didn't pick what I would consider the obvious choice
> you now need an extra mount flag to disable the uid namespace all
> together, so you can transition through the intermediate uid namespace
> state.  That really feels wrong.

Some bit of required bootstrapping seems both acceptable and expected
to me.

> All mounts should have an associated uid namespace and the only

check

> way you should be able to ignore that is to access filesystems
> that can cope with multiple uid namespaces simultaneously.

But it's my fs on my box, why shouldn't i be able to say all uid
namespaces can acces this subtree read-only, just bc you feel the
fs is inadequate?  :)

Note that the tiniest of trees, with just a statically compiled bash,
mount, pivot_mount, and initrc, should suffice, mounted readonly for
all uid namespaces to use to bootstrap.

-serge
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