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

[Jann Horn]

On Sun, Sep 13, 2020 at 7:55 PM John Wood <john.wood at gmx.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 at chromium.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.