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From:   Jann Horn <jannh@google.com>
Date:   Fri, 11 Sep 2020 02:20:27 +0200
Message-ID: <CAG48ez2=8y7jC9vWSPyYNhwASxGrQaewSBczbr02Ri2YnBJwVA@mail.gmail.com>
Subject: Re: [RFC PATCH 6/6] security/fbfam: Mitigate a fork brute force attack
To:     Kees Cook <keescook@chromium.org>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        John Wood <john.wood@gmx.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Iurii Zaikin <yzaikin@google.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, linux-doc@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>
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On Fri, Sep 11, 2020 at 1:56 AM Kees Cook <keescook@chromium.org> wrote:
> On Thu, Sep 10, 2020 at 01:21:07PM -0700, Kees Cook wrote:
> > From: John Wood <john.wood@gmx.com>
> >
> > In order to mitigate a fork brute force attack it is necessary to kill
> > all the offending tasks. This tasks are all the ones that share the
> > statistical data with the current task (the task that has crashed).
> >
> > Since the attack detection is done in the function fbfam_handle_attack()
> > that is called every time a core dump is triggered, only is needed to
> > kill the others tasks that share the same statistical data, not the
> > current one as this is in the path to be killed.
[...]
> > +     for_each_process(p) {
> > +             if (p == current || p->fbfam_stats != stats)
> > +                     continue;
> > +
> > +             do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_PID);
> > +             pr_warn("fbfam: Offending process with PID %d killed\n",
> > +                     p->pid);
[...]
> > +
> > +             killed += 1;
> > +             if (killed >= to_kill)
> > +                     break;
> > +     }
> > +
> > +     rcu_read_unlock();
>
> Can't newly created processes escape this RCU read lock? I think this
> need alternate locking, or something in the task_alloc hook that will
> block any new process from being created within the stats group.

Good point; the proper way to deal with this would probably be to take
the tasklist_lock in read mode around this loop (with
read_lock(&tasklist_lock) / read_unlock(&tasklist_lock)), which pairs
with the write_lock_irq(&tasklist_lock) in copy_process(). Thanks to
the fatal_signal_pending() check while holding the lock in
copy_process(), that would be race-free - any fork() that has not yet
inserted the new task into the global task list would wait for us to
drop the tasklist_lock, then bail out at the fatal_signal_pending()
check.