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

[John Wood]

On Thu, Sep 10, 2020 at 04:56:19PM -0700, Kees Cook wrote:
> On Thu, Sep 10, 2020 at 01:21:07PM -0700, Kees Cook wrote:
> >  /**
> > + * fbfam_kill_tasks() - Kill the offending tasks
> > + *
> > + * When a fork brute force attack is detected it is necessary to kill all the
> > + * offending tasks. Since this function is called from fbfam_handle_attack(),
> > + * and so, 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.
> > + *
> > + * When the SIGKILL signal is sent to the offending tasks, this function will be
> > + * called again during the core dump due to the shared statistical data shows a
> > + * quickly crashing rate. So, to avoid kill again the same tasks due to a
> > + * recursive call of this function, it is necessary to disable the attack
> > + * detection setting the jiffies to zero.
> > + *
> > + * To improve the for_each_process loop it is possible to end it when all the
> > + * tasks that shared the same statistics are found.
> > + *
> > + * Return: -EFAULT if the current task doesn't have statistical data. Zero
> > + *         otherwise.
> > + */
> > +static int fbfam_kill_tasks(void)
> > +{
> > +	struct fbfam_stats *stats = current->fbfam_stats;
> > +	struct task_struct *p;
> > +	unsigned int to_kill, killed = 0;
> > +
> > +	if (!stats)
> > +		return -EFAULT;
> > +
> > +	to_kill = refcount_read(&stats->refc) - 1;
> > +	if (!to_kill)
> > +		return 0;
> > +
> > +	/* Disable the attack detection */
> > +	stats->jiffies = 0;
> > +	rcu_read_lock();
> > +
> > +	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);
>
> I'd make this ratelimited (along with Jann's suggestions).

Sorry, but I don't understand what you mean with "make this ratelimited".
A clarification would be greatly appreciated.

> Also, instead of the explicit "fbfam:" prefix, use the regular
> prefixing method:
>
> #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

Understood.

> > +
> > +		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.

I will work on this for the next version. Thanks.

> > +	return 0;
> > +}
>
> --
> Kees Cook

Thanks
John Wood