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

[Kees Cook]

On Wed, Jun 25, 2014 at 10:51 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/25, Andy Lutomirski wrote:
>>
>> On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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.


-- 
Kees Cook
Chrome OS Security
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