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Date:	Tue, 14 Oct 2014 15:13:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel@...ts.sourceforge.net,
	Miklos Szeredi <miklos@...redi.hu>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Seth Forshee <seth.forshee@...onical.com>
Subject: Re: [PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

On Tue, Oct 14, 2014 at 3:07 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Oct 14, 2014 at 2:57 PM, Eric W. Biederman

>>> Seth, this should address a problem that's related to yours.  If a
>>> userns creates and untrusted fs (by any means, although admittedly fuse
>>> and user namespaces don't work all that well together right now), then
>>> this prevents shenanigans that could happen when the userns passes an fd
>>> pointing at the filesystem out to the root ns.
>>
>> Andy for now I really think we are best not even reading those
>> capabilities into the vfs from unprivileged mounts.
>
> But won't we want to support letting userns containers create setuid
> files and security labels using FUSE and related things for their own
> benefit someday?  This lets us do that without compromising the init
> namespace.

More concretely, root in a userns should be able to have a
setuid-whomever or security-labeled file, and another user in that
userns should be able to exec it and transition.  But, if you're
outside the userns, then:

$ /proc/PID_IN_USERNS/root/path/to/labeled/file

shouldn't transition.

--Andy
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