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

[Andy Lutomirski]

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.)

--Andy