linux-kernel - Re: [PATCH 1/4] seccomp: don't use semaphore and wait

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANaxB-x=77SDqvoj4vpg8uKvntK+ttHWQyq1gdOhp1LZfAvfsg@mail.gmail.com>
Date:   Tue, 30 Aug 2022 09:07:56 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Oskolkov <posk@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tycho Andersen <tycho@...ho.pizza>,
        Will Drewry <wad@...omium.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 1/4] seccomp: don't use semaphore and wait_queue together

On Tue, Aug 30, 2022 at 2:57 AM Christian Brauner <brauner@...nel.org> wrote:

> >
> > +static bool notify_wakeup(struct seccomp_filter *filter)
> > +{
> > +     bool ret;
> > +
> > +     rcu_read_lock();
> > +     ret = atomic_add_unless(&filter->notif->requests, -1, 0);
>
> Can you please spell out why the change to wait_event_interruptible()
> below that calls notify_wakeup() makes it necessary to rcu protect
> ->notif?

This is my mistake. rcu is used here when I tried to implement notify_wakeup
without introducing notif->request. The idea was to enumerate all elements of
notif->notifications. Now, it doesn't matter. In this context, filter->notif
can be dereferenced without any additional locks. Thanks for catching this.

>
> Given that you're using rcu_read_lock() here and the
> WRITE_ONCE(fitler->notify, NULL) + kfree_rcu() sequence in
> seccomp_notify_free() it seems to imply that filter->notif could be NULL
> here and so would need a non-NULL check on ->notif before incrementing?
>
> > +     rcu_read_unlock();
> > +
> > +     return ret;
> > +}
>
>  static long seccomp_notify_recv(struct seccomp_filter *filter,
>                               void __user *buf)
> @@ -1467,7 +1479,7 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
>
>       memset(&unotif, 0, sizeof(unotif));
>
> -     ret = down_interruptible(&filter->notif->request);
> +     ret = wait_event_interruptible(filter->wqh, notify_wakeup(filter));
>       if (ret < 0)
>               return ret;
>

Thanks,
Andrei