Re: [PATCH] random: seed random_int_secret at least poorly at core_initcall time

On Wed, Nov 13, 2013 at 8:18 PM, Hannes Frederic Sowa
[off-list ref] wrote:

On Wed, Nov 13, 2013 at 09:54:48PM -0500, Theodore Ts'o wrote:

quoted

On Tue, Nov 12, 2013 at 02:46:03PM +0100, Hannes Frederic Sowa wrote:

quoted

quoted

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?

quoted

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.

Yeah, other suggestions were things ACPI tables, memory layouts, etc.
Basically anything that might be different from system to system. Even
just perturbing by the build strings (which has compiler version,
build date, build host, etc etc) gets us a tiny amount of entropy
(which is still better than 0).

quoted

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).

I'll try to do some tests tomorrow.

Greetings,

  Hannes

-Kees

-- 
Kees Cook
Chrome OS Security