linux-kernel - Re: [PATCH v3] fs: Treat foreign mounts as nosuid

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Date:	Tue, 21 Oct 2014 11:34:26 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux FS Devel <linux-fsdevel@...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>
Subject: Re: [PATCH v3] fs: Treat foreign mounts as nosuid

On Tue, Oct 14, 2014 at 04:54:47PM -0700, Andy Lutomirski wrote:
> If a process gets access to a mount from a different namespace user
> namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem.
> Technically, trusting mounts created by the same or ancestor user
> namespaces ought to be safe, but it's simpler to distrust all
> foreign mounts.
> 
> This will make it safer to allow more complex filesystems to be
> mounted in non-root user namespaces.
> 
> This does not remove the need for MNT_LOCK_NOSUID.  The setuid,
> setgid, and file capability bits can no longer be abused if code in
> a user namespace were to clear nosuid on an untrusted filesystem,
> but this patch, by itself, is insufficient to protect the system
> from abuse of files that, when execed, would increase MAC privilege.
> 
> As a more concrete explanation, any task that can manipulate a
> vfsmount associated with a given user namespace already has
> capabilities in that namespace and all of its descendents.  If they
> can cause a malicious setuid, setgid, or file-caps executable to
> appear in that mount, then that executable will only allow them to
> elevate privileges in exactly the set of namespaces in which they
> are already privileges.
> 
> On the other hand, if they can cause a malicious executable to
> appear with a dangerous MAC label, running it could change the
> caller's security context in a way that should not have been
> possible, even inside the namespace in which the task is confined.
> 
> As a hardening measure, this would have made CVE-2014-5207 much
> more difficult to exploit.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>

Now that I'm back from plumbers I've finally had a chance to play around
with this and it seems to be working as expected. I'll be considering
this patch a prerequisite for the next round of fuse userns patches.

Acked-by: Seth Forshee <seth.forshee@...onical.com>
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