Re: [PATCH v8 5/9] seccomp: split mode set routines

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 25, 2014 at 11:07 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Jun 25, 2014 at 11:00 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Jun 25, 2014 at 10:51 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 06/25, Andy Lutomirski wrote:
>>>>
>>>> On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>> > On 06/25, Andy Lutomirski wrote:
>>>> >>
>>>> >> Write the filter, then smp_mb (or maybe a weaker barrier is okay),
>>>> >> then set the bit.
>>>> >
>>>> > Yes, exactly, this is what I meant. Plas rmb() in __secure_computing().
>>>> >
>>>> > But I still can't understand the rest of your discussion about the
>>>> > ordering we need ;)
>>>>
>>>> Let me try again from scratch.
>>>>
>>>> Currently there are three relevant variables: TIF_SECCOMP,
>>>> seccomp.mode, and seccomp.filter.  __secure_computing needs
>>>> seccomp.mode and seccomp.filter to be in sync, and it wants (but
>>>> doesn't really need) TIF_SECCOMP to be in sync as well.
>>>>
>>>> My suggestion is to rearrange it a bit.  Move mode into seccomp.filter
>>>> (so that filter == NULL implies no seccomp) and don't check
>>
>> This would require that we reimplement mode 1 seccomp via mode 2
>> filters. Which isn't too hard, but may add complexity.
>>
>>>> TIF_SECCOMP in secure_computing.  Then turning on seccomp is entirely
>>>> atomic except for the fact that the seccomp hooks won't be called if
>>>> filter != NULL but !TIF_SECCOMP.  This removes all ordering
>>>> requirements.
>>>
>>> Ah, got it, thanks. Perhaps I missed somehing, but to me this looks like
>>> unnecessary complication at first glance.
>>>
>>> We alredy have TIF_SECCOMP, we need it anyway, and we should only care
>>> about the case when this bit is actually set, so that we can race with
>>> the 1st call of __secure_computing().
>>>
>>> Otherwise we are fine: we can miss the new filter anyway, ->mode can't
>>> be changed it is already nonzero.
>>>
>>>> Alternatively, __secure_computing could still BUG_ON(!seccomp.filter).
>>>> In that case, filter needs to be set before TIF_SECCOMP is set, but
>>>> that's straightforward.
>>>
>>> Yep. And this is how seccomp_assign_mode() already works? It is called
>>> after we change ->filter chain, it changes ->mode before set(TIF_SECCOMP)
>>> just it lacks a barrier.
>>
>> Right, I think the best solution is to add the barrier. I was
>> concerned that adding the read barrier in secure_computing would have
>> a performance impact, though.
>>
>
> I can't speak for ARM, but I think that all of the read barriers are
> essentially free on x86.  (smp_mb is a very different story, but that
> shouldn't be needed here.)

It looks like SMP ARM issues dsb for rmb, which seems a bit expensive.
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204g/CIHJFGFE.html

If I skip the rmb in the secure_computing call before checking mode,
it sounds like I run the risk of racing an out-of-order TIF_SECCOMP vs
mode and filter. This seems unlikely to me, given an addition of the
smp_mb__before_atomic() during the seccomp_assign_mode()? I guess I
don't have a sense of how aggressively ARM might do data caching in
this area. Could the other thread actually see TIF_SECCOMP get set but
still have an out of date copy of seccomp.mode?

I really want to avoid adding anything to the secure_computing()
execution path. :(

-Kees

-- 
Kees Cook
Chrome OS Security