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From:   Jann Horn <jannh@google.com>
Date:   Mon, 2 Nov 2020 21:04:22 +0100
Message-ID: <CAG48ez2vPUCiZX-swrE2oWx8j-6QCzCRiFGnCPFoGMN+oBFGQw@mail.gmail.com>
Subject: Re: For review: seccomp_user_notif(2) manual page [v2]
To:     Sargun Dhillon <sargun@sargun.me>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Tycho Andersen <tycho@tycho.pizza>,
        Christian Brauner <christian@brauner.io>,
        Kees Cook <keescook@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Robert Sesek <rsesek@google.com>,
        Containers <containers@lists.linux-foundation.org>,
        linux-man <linux-man@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
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On Mon, Nov 2, 2020 at 8:50 PM Sargun Dhillon <sargun@sargun.me> wrote:
> On Mon, Nov 2, 2020 at 11:45 AM Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
> >    Caveats regarding blocking system calls
> >        Suppose that the target performs a blocking system call (e.g.,
> >        accept(2)) that the supervisor should handle.  The supervisor
> >        might then in turn execute the same blocking system call.
> >
> >        In this scenario, it is important to note that if the target's
> >        system call is now interrupted by a signal, the supervisor is not
> >        informed of this.  If the supervisor does not take suitable steps
> >        to actively discover that the target's system call has been
> >        canceled, various difficulties can occur.  Taking the example of
> >        accept(2), the supervisor might remain blocked in its accept(2)
> >        holding a port number that the target (which, after the
> >        interruption by the signal handler, perhaps closed  its listening
> >        socket) might expect to be able to reuse in a bind(2) call.
> >
> >        Therefore, when the supervisor wishes to emulate a blocking system
> >        call, it must do so in such a way that it gets informed if the
> >        target's system call is interrupted by a signal handler.  For
> >        example, if the supervisor itself executes the same blocking
> >        system call, then it could employ a separate thread that uses the
> >        SECCOMP_IOCTL_NOTIF_ID_VALID operation to check if the target is
> >        still blocked in its system call.  Alternatively, in the accept(2)
> >        example, the supervisor might use poll(2) to monitor both the
> >        notification file descriptor (so as as to discover when the
> >        target's accept(2) call has been interrupted) and the listening
> >        file descriptor (so as to know when a connection is available).
> >
> >        If the target's system call is interrupted, the supervisor must
> >        take care to release resources (e.g., file descriptors) that it
> >        acquired on behalf of the target.
> >
> > Does that seem okay?
> >
> This is far clearer than my explanation. The one thing is that *just*
> poll is not good enough, you would poll, with some timeout, and when
> that timeout is hit, check if all the current notifications are valid,
> as poll isn't woken up when an in progress notification goes off
> AFAIK.

Arguably that's so terrible that it qualifies for being in the BUGS
section of the manpage.

If you want this to be fixed properly, I recommend that someone
implements my proposal from
<https://lore.kernel.org/lkml/CAG48ez1O2H5HDikPO-_o-toXTheU8GnZot9woGDsNRNJqSWesA@mail.gmail.com/>,
unless you can come up with something better.