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Date: Thu, 21 Sep 2017 12:57:13 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Chris Salls <chrissalls5@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>, Will Drewry <wad@chromium.org>,
        "security@kernel.org" <security@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, Tycho Andersen <tycho@docker.com>
Subject: Re: [PATCH] seccomp: fix the usage of get/put_seccomp_filter() in
 seccomp_get_filter()
Message-ID: <20170921105713.GA32672@redhat.com>
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 <20170920125621.GA3599@redhat.com>
 <CAGXu5jJKSaGZe2DNup2uGB6rys1nDWFikQRmqp1DNfKb+UfYxw@mail.gmail.com>
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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.