Message-Id: <20201025134540.3770-4-john.wood@gmx.com>
Date:   Sun, 25 Oct 2020 14:45:35 +0100
From:   John Wood <john.wood@....com>
To:     Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Cc:     John Wood <john.wood@....com>, Jonathan Corbet <corbet@....net>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: [PATCH v2 3/8] security/brute: Add sysctl attributes to allow detection fine tuning

This is a previous step to add the detection feature.

A fork brute force attack will be detected when an application crashes
quickly. Since, the application crash period is the time between the
execve system call and the first fault or the time between two
consecutives faults add a new sysctl attribute to control the crash
period threshold.

But a detection method based only on this computation has a drawback. If
an application crashes once quickly from the execve system call or
crashes twice in a short period of time for some reason, a false
positive attack will be triggered. To avoid this scenario use a list of
the i last crashes timestamps and compute the application crash period
as follows:

crash_period = (n_last_timestamp - n_minus_i_timestamp) / i;

So, also add a new sysctl attribute to control the size of this list.

This way, each system can tune the detection's sensibility adjusting the
application crash period threshold and the size of the last crashes
timestamps list.

Signed-off-by: John Wood <john.wood@....com>
---
 security/brute/brute.c | 83 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/security/brute/brute.c b/security/brute/brute.c
index 307d07bf9d98..29835fe2f141 100644
--- a/security/brute/brute.c
+++ b/security/brute/brute.c
@@ -4,12 +4,14 @@

 #include <asm/current.h>
 #include <linux/bug.h>
+#include <linux/cache.h>
 #include <linux/compiler.h>
 #include <linux/errno.h>
 #include <linux/gfp.h>
 #include <linux/init.h>
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
+#include <linux/limits.h>
 #include <linux/list.h>
 #include <linux/lsm_hooks.h>
 #include <linux/printk.h>
@@ -17,6 +19,34 @@
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/sysctl.h>
+
+/**
+ * brute_timestamps_list_size - Last crashes timestamps list size.
+ *
+ * The application crash period is the time between the execve system call and
+ * the first fault or the time between two consecutives faults, but this has a
+ * drawback. If an application crashes once quickly from the execve system call
+ * or crashes twice in a short period of time for some reason, a false positive
+ * attack will be triggered. To avoid this scenario use a list of the i last
+ * crashes timestamps and compute the application crash period as follows:
+ *
+ * crash_period = (n_last_timestamp - n_minus_i_timestamp) / i;
+ *
+ * The brute_timestamps_list_size variable sets the size of this list.
+ */
+static unsigned int brute_timestamps_list_size __read_mostly = 5;
+
+/**
+ * brute_crash_period_threshold - Application crash period threshold.
+ *
+ * The units are expressed in milliseconds.
+ *
+ * A fork brute force attack will be detected if the application crash period
+ * falls under this threshold. So, the higher this value, the more sensitive the
+ * detection will be.
+ */
+static unsigned int brute_crash_period_threshold __read_mostly = 30000;

 /**
  * struct brute_stats - Fork brute force attack statistics.
@@ -318,6 +348,58 @@ static struct security_hook_list brute_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(task_free, brute_task_free),
 };

+#ifdef CONFIG_SYSCTL
+static unsigned int uint_one = 1;
+static unsigned int uint_max = UINT_MAX;
+static unsigned int max_brute_timestamps_list_size = 10;
+
+/**
+ * brute_sysctl_path - Sysctl attributes path.
+ */
+static struct ctl_path brute_sysctl_path[] = {
+	{ .procname = "kernel", },
+	{ .procname = "brute", },
+	{ }
+};
+
+/**
+ * brute_sysctl_table - Sysctl attributes.
+ */
+static struct ctl_table brute_sysctl_table[] = {
+	{
+		.procname	= "timestamps_list_size",
+		.data		= &brute_timestamps_list_size,
+		.maxlen		= sizeof(brute_timestamps_list_size),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1		= &uint_one,
+		.extra2		= &max_brute_timestamps_list_size,
+	},
+	{
+		.procname	= "crash_period_threshold",
+		.data		= &brute_crash_period_threshold,
+		.maxlen		= sizeof(brute_crash_period_threshold),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1		= &uint_one,
+		.extra2		= &uint_max,
+	},
+	{ }
+};
+
+/**
+ * brute_init_sysctl() - Initialize the sysctl interface.
+ */
+static void __init brute_init_sysctl(void)
+{
+	if (!register_sysctl_paths(brute_sysctl_path, brute_sysctl_table))
+		panic("Cannot register the sysctl interface\n");
+}
+
+#else
+static inline void brute_init_sysctl(void) { }
+#endif /* CONFIG_SYSCTL */
+
 /**
  * brute_init() - Initialize the brute LSM.
  *
@@ -328,6 +410,7 @@ static int __init brute_init(void)
 	pr_info("Brute initialized\n");
 	security_add_hooks(brute_hooks, ARRAY_SIZE(brute_hooks),
 			   KBUILD_MODNAME);
+	brute_init_sysctl();
 	return 0;
 }

--
2.25.1

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