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

Oleg Nesterov
2017-09-21T10:57:17Z

On 09/20, Kees Cook wrote:
>
> On Wed, Sep 20, 2017 at 5:56 AM, Oleg Nesterov <oleg@redhat.com> 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.