Re: [PATCH v2 0/5] split ET_DYN ASLR from mmap ASLR

[Kees Cook <keescook@chromium.org>]

On Mon, Mar 2, 2015 at 11:31 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> To address the "offset2lib" ASLR weakness[1], this separates ET_DYN
>> ASLR from mmap ASLR, as already done on s390. The architectures
>> that are already randomizing mmap (arm, arm64, mips, powerpc, s390,
>> and x86), have their various forms of arch_mmap_rnd() made available
>> via the new CONFIG_ARCH_HAS_ELF_RANDOMIZE. For these architectures,
>> arch_randomize_brk() is collapsed as well.
>>
>> This is an alternative to the solutions in:
>> https://lkml.org/lkml/2015/2/23/442
>
> Looks good so far:
>
> Reviewed-by: Ingo Molnar <mingo@kernel.org>
>
> While reviewing this series I also noticed that the following code
> could be factored out from architecture mmap code as well:
>
>   - arch_pick_mmap_layout() uses very similar patterns across the
>     platforms, with only few variations. Many architectures use
>     the same duplicated mmap_is_legacy() helper as well. There's
>     usually just trivial differences between mmap_legacy_base()
>     approaches as well.

I was nervous to start refactoring this code, but it's true: most of
it is the same.

>   - arch_mmap_rnd(): the PF_RANDOMIZE checks are needlessly
>     exposed to the arch routine - the arch routine should only
>     concentrate on arch details, not generic flags like
>     PF_RANDOMIZE.

Yeah, excellent point. I will send a follow-up patch to move this into
binfmt_elf instead. I'd like to avoid removing it in any of the other
patches since each was attempting a single step in the refactoring.

> In theory the mmap layout could be fully parametrized as well: i.e. no
> callback functions to architectures by default at all: just
> declarations of bits of randomization desired (or, available address
> space bits), and perhaps an arch helper to allow 32-bit vs. 64-bit
> address space distinctions.

Yeah, I was considering that too, since each architecture has a nearly
identical arch_mmap_rnd() at this point. Only the size of the entropy
was changing.

> 'Weird' architectures could provide special routines, but only by
> overriding the default behavior, which should be generic, safe and
> robust.

Yeah, quite true. Should entropy size be a #define like
ELF_ET_DYN_BASE? Something like ASLR_MMAP_ENTROPY and
ASLR_MMAP_ENTROPY_32? Is there a common function for determining a
compat task? That seemed to be per-arch too. Maybe
arch_mmap_entropy()?

-Kees

-- 
Kees Cook
Chrome OS Security