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

[Andy Lutomirski]

On Fri, Jun 27, 2014 at 12:04 PM, Kees Cook <keescook at chromium.org> wrote:
> On Fri, Jun 27, 2014 at 11:56 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>> 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 am uncomfortable about making filter == NULL be a "fail open"
> condition if TIF_SECCOMP is set.

I'm unconvinced here.  TIF_SECCOMP unset is already a fail-open
condition if filter is set.

--Andy