Re: [PATCH v3 4/7] fs: Treat foreign mounts as nosuid

James Morris
2015-11-18T00:00:53Z

On Tue, 17 Nov 2015, Seth Forshee wrote:

> From: Andy Lutomirski <luto@amacapital.net>
>
> If a process gets access to a mount from a different user
> namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem. Prevent
> this by treating mounts from other mount namespaces and those not
> owned by current_user_ns() or an ancestor as nosuid.
>
> 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@amacapital.net>
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

Acked-by: James Morris <james.l.morris@oracle.com>

--
James Morris
<jmorris@namei.org>