linux-kernel - Re: [PATCH 30/43] userns: Fail exec for suid and sgid binaries with ids outside our user namespace.

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Date:	Tue, 24 Apr 2012 10:10:40 -0500
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 30/43] userns: Fail exec for suid and sgid binaries with
 ids outside our user namespace.

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serge@...lyn.com> writes:
> 
> > Quoting Eric W. Beiderman (ebiederm@...ssion.com):
> >> From: Eric W. Biederman <ebiederm@...ssion.com>
> >> 
> >
> > Oh, perhaps this is the right place in the thread to discuss the issue of
> > what to do with file capabilities?  I'm ok waiting until the next iteration
> > to even discuss it, so long as we start by refusing setting of fcaps by
> > any task not in init_user_ns.
> 
> For now we do refuse all callers in the init_user_ns because that path
> is protected by a capable and not an ns_capable call.
> 
> And as a general policy I have pushed all of the changes from capable to
> ns_capable out till after we get these other user namespace bits so we
> can get the patches reviewed so hopefully don't enable something that is
> not safe.
> 
> Let's just note here that when we ever get a filesystem mounted in
> something other than the init_user_ns or otherwise allow file
> capabilities that do not belong to the init_user_ns we need to an
> additional exec check to avoid a security issue for processes in the
> init_user_ns using those credentials.
> 
> The other direction the init_user_ns setting file caps on a file and use
> using them in a child namespace seems safe, and practical because of the
> way we handle capabilities.  Aka if you have a capability in an outer
> user namespace you also have it in a child user namespace.  Which means
> a file cap exec today will give you just the capabilities in the child
> user namespace.
> 
> Something else to think about when we reach filesystems mounted in
> different user namespaces (aka unprivileged mounts) are security
> labels on files in different user namespaces.  Not any kind of immediate
> concern but something we may have to handle eventually.

An interesting concern to discuss at the security mini-summit (or just
at the UDS session).

-serge
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