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

[Andy Lutomirski]

On Fri, Jun 27, 2014 at 11:33 AM, Kees Cook <keescook at chromium.org> wrote:
> On Wed, Jun 25, 2014 at 11:07 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>> On Wed, Jun 25, 2014 at 11:00 AM, Kees Cook <keescook at chromium.org> wrote:
>>> On Wed, Jun 25, 2014 at 10:51 AM, Oleg Nesterov <oleg at redhat.com> wrote:
>>>> On 06/25, Andy Lutomirski wrote:
>>>>>
>>>>> On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg at 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. :(

Hence my suggestion to make the ordering not matter.  No ordering
requirement, no barriers.

--Andy

>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC