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

[Andy Lutomirski]

On Fri, Jun 27, 2014 at 11:52 AM, Kees Cook <keescook at chromium.org> wrote:
> On Fri, Jun 27, 2014 at 11:39 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>> 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.
>
> I may be misunderstanding something, but I think there's still an
> ordering problem. We'll have TIF_SECCOMP already, so if we enter
> secure_computing with a NULL filter, we'll kill the process.
>
> Merging .mode and .filter would remove one of the race failure paths:
> having TIF_SECCOMP and not having a mode set (leading to BUG). With
> the merge, we could still race and land in the same place as have
> TIF_SECCOMP and mode==2, but filter==NULL, leading to WARN and kill.

You could just make secure_computing do nothing if filter == NULL.
It's probably faster to test that than TIF_SECCOMP anyway, since you
need to read the filter cacheline regardless, and testing a regular
variable for non-NULLness might be faster than an atomic bit test
operation.  (Or may not -- I don't know.)

>
> I guess the question is how large is the race risk on ARM? Is it
> possible to have TIF_SECCOMP that far out of sync for the thread?

Dunno.  I don't like leaving crashy known races around.

--Andy



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



-- 
Andy Lutomirski
AMA Capital Management, LLC