From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1755370AbeD3VXM (ORCPT <rfc822;w@1wt.eu>);
        Mon, 30 Apr 2018 17:23:12 -0400
Received: from out2-smtp.messagingengine.com ([66.111.4.26]:46435 "EHLO
        out2-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK)
        by vger.kernel.org with ESMTP id S1755266AbeD3VXK (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 30 Apr 2018 17:23:10 -0400
X-ME-Sender: <xms:PonnWv3cDJvU65XL0sSs93ZzIahkRgUs2Bv145E_fTWoIym_TzD3Ew>
Date: Tue, 1 May 2018 07:23:05 +1000
From: "Tobin C. Harding" <me@tobin.cc>
To: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Ted Ts'o" <tytso@mit.edu>, Steven Rostedt <rostedt@goodmis.org>,
        Anna-Maria Gleixner <anna-maria@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: Hashed pointer issues
Message-ID: <20180430212305.GD22100@eros>
References: <alpine.DEB.2.20.1804301741570.3243@hypnos.tec.linutronix.de>
 <CAGXu5jL1W=AC023asgeMRUXAnhU2vd-RJRqGtOOetQ=aXwDcFQ@mail.gmail.com>
 <CA+55aFwieC1-nAs+NFq9RTwaR8ef9hWa4MjNBWL41F-8wM49eA@mail.gmail.com>
 <20180430124135.0cce92e3@gandalf.local.home>
 <CA+55aFzpJC4+tuSopeze72D2sv1urGdap8+Bj2Y9hEYitAkE4Q@mail.gmail.com>
 <CA+55aFyNqnrM=_5pMgEeL20jVKN7MgWKG52pbaR0FayO=5VuXw@mail.gmail.com>
 <CAGXu5jLVJxDJcbhrQU5-wduaP+OvePXP8VvCFMrJ47pYD-zaXg@mail.gmail.com>
 <CA+55aFyreOY8uzOkkDkU=WttNfbH2_6nStfsm9j2T6F5ZyntHg@mail.gmail.com>
 <CAGXu5jKbagUs7cS5y6zRQkgiA0tnD7K0FajwA1Gt+ZCPrc2jfQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAGXu5jKbagUs7cS5y6zRQkgiA0tnD7K0FajwA1Gt+ZCPrc2jfQ@mail.gmail.com>
X-Mailer: Mutt 1.5.24 (2015-08-30)
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Apr 30, 2018 at 12:16:45PM -0700, Kees Cook wrote:
> On Mon, Apr 30, 2018 at 12:00 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> > On Mon, Apr 30, 2018 at 11:38 AM Kees Cook <keescook@chromium.org> wrote:
> >
> >> Something like this? (Untested.)
> >
> > Looks workable.
> >
> >> +       /* If we have hw RNG, start hashing immediately. */
> >> +       if (arch_has_random()) {
> >> +               get_random_bytes_arch(&ptr_key, sizeof(ptr_key));
> >> +               ptr_key_ready();
> >> +               return 0;
> >> +       }
> >
> > Small tweak: you should check the return value of get_random_bytes_arch(),
> > because in theory it can fail.
> >
> > Sadly, that's not actually how get_random_bytes_arch() really works - it
> > falls back on "get_random_bytes()" on failure instead, which is explicitly
> > against the whole point here.
> 
> I just noticed: there are _no_ users of get_random_bytes_arch() ...
> didn't we once use it to feed entropy to the CRNG?
> 
> > So I think it would need some tweaking, with a new function entirely
> > (get_random_bytes_arch() with a failure return for "cannot fill buffer").
> >
> > But that would be just a few more lines, because we could make the existing
> > get_random_bytes_arch() just use the failure-case thing.
> >
> > So add a "get_hw_random_bytes()" that does that same loop in
> > get_random_bytes_arch(), but returns the number of bytes it filled in.
> >
> > Then get_random_bytes_arch() turns into
> >
> >      got = get_hw_random_bytes(p, nbytes);
> >      if (got < nbytes)
> >          get_random_bytes(p+got, nbytes-got);
> >
> > and the initialize_ptr_random() use would be something like
> >
> >      if (get_hw_random_bytes(&ptr_key, sizeof(ptr_key)) == sizeof(ptr_key)) {
> >          ptr_key_ready();
> >          return 0;
> >     }
> >
> > Hmm?
> >
> > Maybe we could call the "get_hw_random_bytes()" something like
> > "get_early_random_bytes()" and the "use HW for it" is purely an
> > implementation detail?
> 
> Yeah, and if we add __must_check, I think this should be fine. Ted,
> any thoughts on this?
> 
> Tobin, is this something you've got time to implement and test?

Sure thing, thanks for the opportunity.

	Tobin