netdev - Re: [PATCH] random: seed random_int_secret at least poorly at core

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Date:	Thu, 14 Nov 2013 06:05:35 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Theodore Ts'o <tytso@....edu>,
	Daniel Borkmann <dborkman@...hat.com>, davem@...emloft.net,
	shemminger@...workplumber.org, fweimer@...hat.com,
	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	linux-wireless@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH] random: seed random_int_secret at least poorly at core_initcall time

On Thu, Nov 14, 2013 at 05:18:30AM +0100, Hannes Frederic Sowa wrote:
> On Wed, Nov 13, 2013 at 09:54:48PM -0500, Theodore Ts'o wrote:
> > On Tue, Nov 12, 2013 at 02:46:03PM +0100, Hannes Frederic Sowa wrote:
> > > > It is needed by fork to set up the stack canary. And this actually gets
> > > > called before the secret is initialized.
> > > 
> > > Maybe we could use this for the time being and use the seeding method
> > > of kaslr as soon as it hits the tree?
> > 
> > Hmm, from what I can tell even early_initcall() is going to be early
> > enough.  The stack canary is set up by boot_init_stack_canary(), which
> > is run very, very early in start_kerne() --- way before
> > early_initcalls, or even before interrupts are enabled.  So adding
> > random_int_secret_init_early() as a core_initcall is still too late.
> 
> Actually I tried to protect the tsk->stack_canary = get_random_int()
> in fork.c. It sets up the per-task canary.
> 
> > I wonder if we need to do something in common with what Kees has been
> > considering for the kaslr code, since it's a similar issue --- we need
> > random number way earlier than we can really afford to initialize
> > /dev/random.
> 
> Definiteley. I would also propose hashing the boot arguments, often
> enough there is a filesystem UUID in there, or even hash the multiboot
> information we are given from grub. Maybe compile-time entropy, at least
> a bit.
> 
> > P.S.  Unless I'm missing something (and I hope I am), it would appear
> > that the stack canary is going to easily predictable by an attacker on
> > non-x86 platforms that don't have RDRAND.  Has someone tested whether
> > or not the stack canary isn't constant across ARM or pre-Sandy Bridge
> > x86 systems?
> 
> In case of protection for interrupt stacks and early cmwq threads,
> it looks pretty bad from a first look at the source (at least for the
> first initialized CPU).

First output on first cpu of get_random_bytes is always identical on amd64
without rdrand. :/

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