linux-kernel - Re: [PATCH 3/3] seccomp: introduce read protection for struct seccomp

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Date:   Fri, 28 Sep 2018 23:10:48 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 3/3] seccomp: introduce read protection for struct seccomp

On Fri, Sep 28, 2018 at 10:56 PM Tycho Andersen <tycho@...ho.ws> wrote:
>
> On Fri, Sep 28, 2018 at 10:33:34PM +0200, Jann Horn wrote:
> > On Fri, Sep 28, 2018 at 5:47 PM Tycho Andersen <tycho@...ho.ws> wrote:
> > > As Jann pointed out, there is a race between SECCOMP_FILTER_FLAG_TSYNC and
> > > the ptrace code that can inspect a filter of another process. Let's
> > > introduce read locking into the two ptrace accesses so that we don't race.
> >
> > Hmm. Is that true? The ptrace code uses get_nth_filter(), which holds
> > the siglock while grabbing the seccomp filter and bumping its
> > refcount. And TSYNC happens from seccomp_set_mode_filter(), which
> > takes the siglock. So this looks okay to me?
>
> Oh, yes, you're right. So I guess we should just change the comment to
> say we're using siglock to represent the read lock.

Hmm... actually, looking at this closer, I think you only need the
siglock for writing. As far as I can tell, any read (no matter if
current or non-current) can just use READ_ONCE(), because once a
seccomp filter is in a task's seccomp filter chain, it can't be freed
until the task reaches free_task() and calls put_seccomp_filter() from
there. And if the task whose seccomp filter you're trying to read can
reach free_task(), you have bigger problems.