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

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Message-ID: <20180928205652.GC18045@cisco.lan>
Date:   Fri, 28 Sep 2018 14:56:52 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Jann Horn <jannh@...gle.com>
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 10:33:34PM +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.

Tycho