Re: [PATCH v2 04/20] gcc-plugins: Add the randstruct plugin

[Kees Cook <keescook@chromium.org>]

On Fri, Jun 30, 2017 at 1:27 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Fri, Jun 30, 2017 at 9:55 AM, Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
>> On 30 June 2017 at 07:35, Arnd Bergmann <arnd@arndb.de> wrote:
>>> On Fri, Jun 30, 2017 at 12:53 AM, Kees Cook <keescook@chromium.org> wrote:
>>>> The first obviously won't fly. The second just bypasses the problem
>>>> forcing it to be exposed by other people later. The third is likely
>>>> easiest to do now, but reduces the effectiveness of randomization for
>>>> architectures that don't have sensitive immediate values. The fourth
>>>> sounds not generally useful. The fifth may be unacceptable to arm
>>>> maintainers due to performance impacts.
>>>
>>> I was thinking of the fifth solution, but don't know exactly how to
>>> do it. If performance is a concern, I guess we could have separate
>>> implementations for randstruct and traditional builds.
>>>
>>
>> Does this not apply to *all* entries in asm-offsets? If so, I don't
>> see how it is tractable to fix this in the code, unless we add some
>> instrumentation to asm-offsets to whitelist some huge structs and
>> error out on new ones. Or perhaps there's really only a handful?
>
> I think the other structs are all small enough:
>
> * thread_info is at most 720 bytes (including crunch+vfp3, which
>   you wouldn't find in one combined kernel) and not randomized
>   at the moment
> * pt_regs is 72 bytes and I don't see how that would be randomized
> * machine_desc would be a candidate for randomizing, but is only
>   108 bytes
> * proc_info_list is 52 bytes and not currently randomized
> * vm_area_struct is randomized but only 96 bytes.
> * task_struct is clearly large enough, but we only use TSK_ACTIVE_MM
>   and TSK_STACK_CANARY, both can be fixed with your trick.

Yup, that matches what I found. task_struct is the only truly giant struct.

>> In any case, these particular examples are fairly straightforward,
>> since there is no need to preserve the register's value.
>>
>> ldr     r7, [r7, #TSK_STACK_CANARY]
>>
>> could be replaced with
>>
>> .if TSK_STACK_CANARAY >= PAGE_SIZE
>> add r7, r7, #TSK_STACK_CANARY & PAGE_MASK
>> .endif
>> ldr r7, [r7, #TSK_STACK_CANARY & ~PAGE_MASK]
>
> Nice!

Oh, very cool. This'll make it only an asm change in the case where
it's required for randstruct. Perfect. I'll send a patch and carry it
in the randstruct tree.

(In looking at this, it seems tsk_mm is unused in mm/proc-macros.S, so
I'll remove that code unless someone sees something I don't.)

-Kees

-- 
Kees Cook
Pixel Security