Re: [PATCH] Prefer kASLR over Hibernation

From: James Morse
Date: Tue Apr 12 2016 - 13:52:50 EST

On 11/04/16 19:03, Kees Cook wrote:
> On Mon, Apr 11, 2016 at 1:00 AM, James Morse <james.morse@xxxxxxx> wrote:
>> On 06/04/16 20:44, Kees Cook wrote:
>>> one or the other must be chosen at boot-time. Until now, hibernation
>>> was selected when no choice was made on the command line.
>>> To make the security benefits of kASLR more widely available to end
>>> users (since the use of hibernation is becoming more rare and kASLR,
>>> already available on x86, will be available on arm64 and MIPS soon),
>>> this changes the default to preferring kASLR over hibernation. Users
>>> wanting hibernation can turn off kASLR by adding "nokaslr" to the kernel
>>> command line.
>> While hibernate isn't yet merged for arm64, it does work with kASLR in v4.6-rc*,
>> it would be a shame to have to choose at boot time, (but that's my problem to
>> fix if/when its merged).
> Ah, interesting, so they work together on arm64? (i.e. you've actually
> tested a boot loader that provides the seed for kASLR to operate?)

Almost: due to a lack of firmware support I hacked the efi stub to read a 'seed'
from a system counter.

To check it works I printed the address of 'panic' out during boot:
> [ 0.353712] DEBUG: &panic == ffffff960819a4e8

Then hibernated, and powered the board back on, the resume kernel gives:
> [ 0.353528] DEBUG: &panic == ffffff840819a4e8

But after it has restored the hibernate image, I can dig in /proc/kallsyms to
see the original value:
> root@localhost:~# cat /proc/kallsyms | grep "T panic"
> ffffff960819a4e8 T panic

> Maybe RANDOMIZE_BASE_DEFAULT (default to y) and if x86 && hibernation,
> select =n. and use that for selecting it?

Looks good to me.