linux-kernel - Re: [RFC 1/3] seccomp: add a return code to trap to userspace

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Message-ID: <20180204200129.2bgq5yfaimg6xdg5@cisco>
Date:   Sun, 4 Feb 2018 21:01:29 +0100
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>
Subject: Re: [RFC 1/3] seccomp: add a return code to trap to userspace

Hi Andy,

On Sun, Feb 04, 2018 at 05:36:33PM +0000, Andy Lutomirski wrote:
> > The actual implementation of this is fairly small, although getting the
> > synchronization right was/is slightly complex. Also worth noting that there
> > is one race still present:
> >
> >   1. a task does a SECCOMP_RET_USER_NOTIF
> >   2. the userspace handler reads this notification
> >   3. the task dies
> >   4. a new task with the same pid starts
> >   5. this new task does a SECCOMP_RET_USER_NOTIF, gets the same cookie id
> >      that the previous one did
> >   6. the userspace handler writes a response
> 
> I'm slightly confused.  I thought the id was never reused for a given
> struct seccomp_filter.  (Also, shouldn't the id be u64, not u32?)

Well, what happens when u32/64 overflows? Eventually it will wrap.

> On very quick reading, I have a question.  What happens if a process
> has two seccomp_filters attached, one of them returns
> SECCOMP_RET_USER_NOTIF, and the *other* one has a listener?

Good question, in seccomp_run_filters(), the first (lowest, last
applied) filter who returns SECCOMP_RET_USER_NOTIF is the one that
gets the notification and the other receives nothing.

I don't really have any reason to prefer this behavior, it's just what
happened without much thought.

Cheers,

Tycho