linux-kernel - Re: For review: seccomp_user

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Message-ID: <CAG48ez1VOUEHVQyo-2+uO7J+-jN5rh7=KmrMJiPaFjwCbKR1Sg@mail.gmail.com>
Date:   Thu, 1 Oct 2020 01:11:33 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian@...uner.io>,
        linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Will Drewry <wad@...omium.org>, bpf <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andy Lutomirski <luto@...capital.net>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Robert Sesek <rsesek@...gle.com>
Subject: Re: For review: seccomp_user_notif(2) manual page

On Thu, Oct 1, 2020 at 1:03 AM Tycho Andersen <tycho@...ho.pizza> wrote:
> On Wed, Sep 30, 2020 at 10:34:51PM +0200, Michael Kerrisk (man-pages) wrote:
> > On 9/30/20 5:03 PM, Tycho Andersen wrote:
> > > On Wed, Sep 30, 2020 at 01:07:38PM +0200, Michael Kerrisk (man-pages) wrote:
> > >>        ┌─────────────────────────────────────────────────────┐
> > >>        │FIXME                                                │
> > >>        ├─────────────────────────────────────────────────────┤
> > >>        │From my experiments,  it  appears  that  if  a  SEC‐ │
> > >>        │COMP_IOCTL_NOTIF_RECV   is  done  after  the  target │
> > >>        │process terminates, then the ioctl()  simply  blocks │
> > >>        │(rather than returning an error to indicate that the │
> > >>        │target process no longer exists).                    │
> > >
> > > Yeah, I think Christian wanted to fix this at some point,
> >
> > Do you have a pointer that discussion? I could not find it with a
> > quick search.
> >
> > > but it's a
> > > bit sticky to do.
> >
> > Can you say a few words about the nature of the problem?
>
> I remembered wrong, it's actually in the tree: 99cdb8b9a573 ("seccomp:
> notify about unused filter"). So maybe there's a bug here?

That thing only notifies on ->poll, it doesn't unblock ioctls; and
Michael's sample code uses SECCOMP_IOCTL_NOTIF_RECV to wait. So that
commit doesn't have any effect on this kind of usage.