On Wed, Jun 25, 2014 at 11:00 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > 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. > 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
