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

[John Wood]

On Mon, Sep 14, 2020 at 09:42:37PM +0200, Jann Horn wrote:
> 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:
> > > > +       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.

Ok, your proposed solution seems a more clever one. Anyway I think that a
new sysctl for fine tuning the number of timestamps would be needed.

Thanks,
John Wood