linux-kernel - Re: General flags to turn things off (getrandom, pid lookup, etc)

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Message-ID: <CALCETrXfzOZEC3mOKkm3_GvitvkEu4kAa3LowRv-fjHnzrQnBg@mail.gmail.com>
Date:	Sun, 27 Jul 2014 15:17:15 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-crypto@...r.kernel.org,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	James Morris <james.l.morris@...cle.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux API <linux-api@...r.kernel.org>,
	Julien Tinnes <jln@...gle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	David Drysdale <drysdale@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: General flags to turn things off (getrandom, pid lookup, etc)

On Jul 27, 2014 5:06 PM, "Theodore Ts'o" <tytso@....edu> wrote:
>
> On Fri, Jul 25, 2014 at 11:30:48AM -0700, Andy Lutomirski wrote:
> >
> > There is recent interest in having a way to turn generally-available
> > kernel features off.  Maybe we should add a good one so we can stop
> > bikeshedding and avoid proliferating dumb interfaces.
>
> I believe the seccomp infrastructure (which is already upstream)
> should be able to do most of what you want, at least with respect to
> features which are exposed via system calls (which was most of your
> list).

Seccomp can't really restrict lookups of non-self pids.  In fact, this
feature idea started out as a response to a patch adding a kind of
nasty seccomp feature to make it sort of possible.

I agree that that seccomp can turn off GRND_RANDOM, but how is it
supposed to do it in such a way that the filtered software will fall
back to something sensible?  -ENOSYS?  -EPERM?  Something else?

I think that -ENOSYS is clearly wrong, but standardizing this would be
nice.  Admittedly, adding something fancy like this for GRND_RANDOM
may not be appropriate.

--Andy

>
> It won't cover x86 specific things like restricting RDTSC or CPUID
> (and as far as I know you can't intercept the CPUID instruction), but
> I'm not sure it matters.  I don't really see the point, myself.
>
>                                          - Ted
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