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

From: John Wood <john.wood@gmx.com>
To: Jann Horn <jannh@google.com>
Cc: John Wood <john.wood@gmx.com>, Kees Cook <keescook@chromium.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.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>
Subject: Re: [RFC PATCH 5/6] security/fbfam: Detect a fork brute force attack
Date: Tue, 15 Sep 2020 20:44:19 +0200	[thread overview]
Message-ID: <20200915175831.GB2900@ubuntu> (raw)
In-Reply-To: <CAG48ez0BcSY0is2LzdkizcOQYkaOJwfa=5ZSwjKb+faRwG9QCA@mail.gmail.com>

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@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