Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

From: Sargun Dhillon <sargun@sargun.me>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Kees Cook <keescook@chromium.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Aleksa Sarai <cyphar@cyphar.com>, Jann Horn <jannh@google.com>,
	Jeffrey Vander Stoep <jeffv@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Chris Palmer <palmer@google.com>,
	Robert Sesek <rsesek@google.com>, Tycho Andersen <tycho@tycho.ws>,
	Matt Denton <mpdenton@google.com>,
	Kees Cook <keescook@google.com>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier
Date: Fri, 29 May 2020 15:35:45 -0700	[thread overview]
Message-ID: <CAMp4zn_N0CrVzWQzRfcZC3Wip6dxsfp=LYZf=U2ESiAAV55_UA@mail.gmail.com> (raw)
In-Reply-To: <20200529133154.gn2xg6lr7xmkp34p@wittgenstein>

On Fri, May 29, 2020 at 6:31 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> > > +           /* Check if we were woken up by a addfd message */
> > > +           addfd = list_first_entry_or_null(&n.addfd,
> > > +                                            struct seccomp_kaddfd, list);
> > > +           if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) {
> > > +                   seccomp_handle_addfd(addfd);
> > > +                   mutex_unlock(&match->notify_lock);
> > > +                   goto wait;
> > > +           }
> > >             ret = n.val;
> > >             err = n.error;
> > >             flags = n.flags;
> > >     }
> > >
> > > +   /* If there were any pending addfd calls, clear them out */
> > > +   list_for_each_entry_safe(addfd, tmp, &n.addfd, list) {
> > > +           /* The process went away before we got a chance to handle it */
> > > +           addfd->ret = -ESRCH;
> > > +           list_del_init(&addfd->list);
> > > +           complete(&addfd->completion);
> > > +   }
>
> I forgot to ask this in my first review before, don't you need a
> complete(&addfd->completion) call in seccomp_notify_release() before
> freeing it?
>

When complete(&knotif->ready) is called in seccomp_notify_release,
subsequently the notifier (seccomp_do_user_notification) will be woken up and
it'll fail this check:
if (addfd && n.state != SECCOMP_NOTIFY_REPLIED)

Falling through to:
/* If there were any pending addfd calls, clear them out */
list_for_each_entry_safe(addfd, tmp, &n.addfd, list) {
    /* The process went away before we got a chance to handle it */
    addfd->ret = -ESRCH;
    list_del_init(&addfd->list);
    complete(&addfd->completion);
}

Although ESRCH isn't the "right" response, this fall through behaviour
should work.