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

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Date:	Sat, 16 Nov 2013 08:40:29 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Theodore Ts'o <tytso@....edu>,
	Daniel Borkmann <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	shemminger@...workplumber.org, Florian Weimer <fweimer@...hat.com>,
	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	linux-wireless@...r.kernel.org
Subject: Re: [PATCH] random: seed random_int_secret at least poorly at core_initcall time

On Fri, Nov 15, 2013 at 10:42:28AM -0800, Kees Cook wrote:
> On Wed, Nov 13, 2013 at 8:18 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> 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 haven't looked closely yet at how the stack canary gets plumbed, but
> what do things outside of process context end up using?

Early on boot __stack_chk_guard gets initialized. The address of this symbol
is looked up by gcc to assemble the stack guard checks.

Only on ARM with !CONFIG_SMP the __stack_chk_guard is switched as soon
as a new process context is entered. This is not possible if the kernel
is compiled for CONFIG_SMP and the kernel will fallback to the one global
__stack_chk_guard canary.

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