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

[Arnd Bergmann]

On Fri, Jun 30, 2017 at 9:55 AM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 30 June 2017 at 07:35, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Fri, Jun 30, 2017 at 12:53 AM, Kees Cook <keescook@xxxxxxxxxxxx> 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.

> 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!

      Arnd