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

[Oleg Nesterov <oleg@redhat.com>]

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

Oleg.