netdev - Re: [PATCH v17 01/15] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs

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Message-ID: <CAE6n16m+Z242wHZUR07RFJcou4-J=fYtokBb4wXHSXPPad7ffQ@mail.gmail.com>
Date:	Fri, 6 Apr 2012 14:32:09 -0700
From:	Markus Gutschke <markus@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	mcgrathr@...omium.org, tglx@...utronix.de, eparis@...hat.com,
	serge.hallyn@...onical.com, djm@...drot.org, scarybeasts@...il.com,
	indan@....nu, pmoore@...hat.com, corbet@....net,
	eric.dumazet@...il.com, coreyb@...ux.vnet.ibm.com,
	keescook@...omium.org, jmorris@...ei.org,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v17 01/15] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve
 from granting privs

On Fri, Apr 6, 2012 at 14:15, Andrew Lutomirski <luto@....edu> wrote:
> Agreed, but I'm talking about something totally different: if I can
> use CLONE_NEWPID, then I can send an unexpected pid with SCM_CREDS.
> The SCM_CREDS receive code should remap pids.

Yes, I know. It's broken. And so is the view of the /proc filesystem
when inside a pid namespace. And things behave funny if you don't set
up a new "init" process inside of the pid namespace. And I am sure, a
few other things are broken that we just haven't run into.

CLONE_NEWPID is tricky. I can understand, if you want to fix it first.
Looking forward to seeing some patches in the future; please cc me, if
you want feedback from an actual user of this code.

The SCM_CREDS issue is the most serious one of the above, but it
doesn't bother me personally, as I would just set up my sandbox policy
to disallow all of SCM_CREDS (*). But that obviously not a good excuse
for leaving a kernel bug around.

Overall, I like both NO_NEW_PRIVS and BPF filters for seccomp though;
they are a great way to reduce the attack surface of the kernel.
Kernel bugs become a lot less of a headache, if I have a way to filter
out the buggy parts of the kernel. It isn't a panacea, but it's a
great new tool to harden applications.

Markus

*) this is currently difficult to filter SCM_CREDS, if we still want
to allow SCM_RIGHTS. See my earlier complaint about sendmsg().
Currently, filtering of sendmsg() probably requires the use of a
helper process.
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