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Message-ID: <CAG48ez0boPBDm=Uh5xHXAxTj0BTRGyGp4uCgPgw7PkOCo47Hdg@mail.gmail.com>
Date: Thu, 10 Sep 2020 22:55:11 +0200
From: Jann Horn <jannh@...gle.com>
To: Kees Cook <keescook@...omium.org>, John Wood <john.wood@....com>
Cc: 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 6/6] security/fbfam: Mitigate a fork brute force attack
On Thu, Sep 10, 2020 at 10:22 PM Kees Cook <keescook@...omium.org> wrote:
> In order to mitigate a fork brute force attack it is necessary to kill
> all the offending tasks. This tasks are all the ones that share the
> statistical data with the current task (the task that has crashed).
>
> Since the attack detection is done in the function fbfam_handle_attack()
> that is called 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 from the function
> fbfam_kill_tasks(), this one 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.
>
> To disable this attack detection, add a condition in the function
> fbfam_handle_attack() to not compute the crashing rate when the jiffies
> stored in the statistical data are set to zero.
[...]
> /**
> - * fbfam_handle_attack() - Fork brute force attack detection.
> + * 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.
This is not a fastpath, there's no need to be clever and optimize
things here, please get rid of that optimization. Especially since
that fastpath looks racy against concurrent execve().
> + * 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)
p->fbfam_stats could change concurrently, you should at least use
READ_ONCE() here.
Also, if this codepath is hit by a non-leader thread, "p == current"
will always be false, and you'll end up killing the caller, too. You
may want to compare with current->group_leader instead.
> + continue;
> +
> + do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_PID);
> + pr_warn("fbfam: Offending process with PID %d killed\n",
> + p->pid);
Normally pr_*() messages about tasks mention not just the pid, but
also the ->comm name of the task.
> + killed += 1;
> + if (killed >= to_kill)
> + break;
> + }
> +
> + rcu_read_unlock();
> + return 0;
> +}
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