linux-kernel - Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack

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Date:   Mon, 14 Sep 2020 21:42:37 +0200
From:   Jann Horn <jannh@...gle.com>
To:     John Wood <john.wood@....com>
Cc:     Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack

On Sun, Sep 13, 2020 at 7:55 PM John Wood <john.wood@....com> wrote:
> On Thu, Sep 10, 2020 at 11:10:38PM +0200, Jann Horn wrote:
> > On Thu, Sep 10, 2020 at 10:22 PM Kees Cook <keescook@...omium.org> wrote:
> > > To detect a fork brute force attack it is necessary to compute the
> > > crashing rate of the application. This calculation is performed in each
> > > fatal fail of a task, or in other words, when a core dump is triggered.
> > > If this rate shows that the application is crashing quickly, there is a
> > > clear signal that an attack is happening.
> > >
> > > Since the crashing rate is computed in milliseconds per fault, if this
> > > rate goes under a certain threshold a warning is triggered.
[...]
> > > +       delta_jiffies = get_jiffies_64() - stats->jiffies;
> > > +       delta_time = jiffies64_to_msecs(delta_jiffies);
> > > +       crashing_rate = delta_time / (u64)stats->faults;
> >
> > Do I see this correctly, is this computing the total runtime of this
> > process hierarchy divided by the total number of faults seen in this
> > process hierarchy? If so, you may want to reconsider whether that's
> > really the behavior you want. For example, if I configure the minimum
> > period between crashes to be 30s (as is the default in the sysctl
> > patch), and I try to attack a server that has been running without any
> > crashes for a month, I'd instantly be able to crash around
> > 30*24*60*60/30 = 86400 times before the detection kicks in. That seems
> > suboptimal.
>
> You are right. This is not the behaviour we want. So, for the next
> version it would be better to compute the crashing period as the time
> between two faults, or the time between the execve call and the first
> fault (first fault case).
>
> However, I am afraid of a premature detection if a child process fails
> twice in a short period.
>
> So, I think it would be a good idea add a new sysctl to setup a
> minimum number of faults before the time between faults starts to be
> computed. And so, the attack detection only will be triggered if the
> application crashes quickly but after a number of crashes.
>
> What do you think?

You could keep a list of the timestamps of the last five crashes or
so, and then take action if the last five crashes happened within
(5-1)*crash_period_limit time.