linux-kernel - Re: [kernel-hardening] Re: [PATCH resend 2/2] userns: control capabilities of some user namespaces

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Message-ID: <20171106221418.GA32543@mail.hallyn.com>
Date:   Mon, 6 Nov 2017 16:14:18 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Daniel Micay <danielmicay@...il.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Mahesh Bandewar
         (महेश बंडेवार) <maheshb@...gle.com>,
        Mahesh Bandewar <mahesh@...dewar.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [kernel-hardening] Re: [PATCH resend 2/2] userns: control
 capabilities of some user namespaces

Quoting Daniel Micay (danielmicay@...il.com):
> Substantial added attack surface will never go away as a problem. There
> aren't a finite number of vulnerabilities to be found.

There's varying levels of usefulness and quality.  There is code which I
want to be able to use in a container, and code which I can't ever see a
reason for using there.  The latter, especially if it's also in a
staging driver, would be nice to have a toggle to disable.

You're not advocating dropping the added attack surface, only adding a
way of dealing with an 0day after the fact.  Privilege raising 0days can
exist anywhere, not just in code which only root in a user namespace can
exercise.  So from that point of view, ksplice seems a more complete
solution.  Why not just actually fix the bad code block when we know
about it?

Finally, it has been well argued that you can gain many new caps from
having only a few others.  Given that, how could you ever be sure that,
if an 0day is found which allows root in a user ns to abuse
CAP_NET_ADMIN against the host, just keeping CAP_NET_ADMIN from them
would suffice?  It seems to me that the existing control in
/proc/sys/kernel/unprivileged_userns_clone might be the better duct tape
in that case.

-serge