Re: [PATCH] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()

[Oleg Nesterov]

On 09/20, Kees Cook wrote:
>
> On Wed, Sep 20, 2017 at 5:56 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > @@ -908,13 +912,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
> >         if (!data)
> >                 goto out;
> >
> > -       get_seccomp_filter(task);
> > +       refcount_inc(&filter->usage);
> >         spin_unlock_irq(&task->sighand->siglock);
> >
> >         if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
> >                 ret = -EFAULT;
> >
> > -       put_seccomp_filter(task);
> > +       __put_seccomp_filter(filter);
> >         return ret;
> 
> Given how reference counting is done for filters, I'd be happier with
> leaving the get_seccomp_filter() as-is,

No, please note that filter != tsk->seccomp.filter, get_seccomp_filter()
won't work.

> (i.e. don't open-code
> the refcount_inc()).

agreed, probably another __get_seccomp_filter(filter) makes sense, especially
if we do other changes like get_nth().

But imo not in this fix.

Oleg.