linux-kernel - Re: [PATCH v5 3/4] fuse: Restrict allow_other to the superblock's namespace or a descendant

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Date:	Tue, 11 Nov 2014 09:37:32 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Andy Lutomirski <luto@...capital.net>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, seth.forshee@...onical.com
Subject: Re: [PATCH v5 3/4] fuse: Restrict allow_other to the superblock's
 namespace or a descendant

On Tue, Nov 11, 2014 at 04:27:37PM +0100, Miklos Szeredi wrote:
> On Wed, Oct 22, 2014 at 04:24:19PM -0500, Seth Forshee wrote:
> > Unprivileged users are normally restricted from mounting with the
> > allow_other option by system policy, but this could be bypassed
> > for a mount done with user namespace root permissions. In such
> > cases allow_oth er should not allow users outside the userns
> > to access the mount as doing so would give the unprivileged user
> > the ability to manipulate processes it would otherwise be unable
> > to manipulate. Therefore access with allow_other should be
> > restricted to users in the userns as the superblock or a
> > descendant of that namespace.
> 
> Fine.
> 
> But aren't this kind of thing supposed to be prevented anyway by having private
> mount namespace coupled with the pid-user-whatever namespace?
> 
> It seems like being a bit too careful (not to say that that's a bad thing).

A userns mount should be in a "private" mount namespace; specifically
the user performing the mount must have CAP_SYS_ADMIN in
mnt_ns->user_ns. The mount may still be accessible via /proc/pid/root
though, and doing this ensures that in any case the user can never use
the mount to manipulate processes that it can't already manipulate.

Thanks,
Seth
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