[RFC PATCH 4/6] security/fbfam: Add a new sysctl to control the crashing rate threshold

[Kees Cook <keescook@chromium.org>]

From: John Wood <john.wood@gmx.com>

This is a previous step to add the detection feature.

A fork brute force attack will be detected when an application crashes
quickly. Since, a rate can be defined as a time per fault, add a new
sysctl to control the crashing rate threshold.

This way, each system can tune the detection's sensibility adjusting the
milliseconds per fault. So, if the application's crashing rate falls
under this threshold an attack will be detected. So, the higher this
value, the faster an attack will be detected.

Signed-off-by: John Wood <john.wood@gmx.com>
---
 include/fbfam/fbfam.h   |  4 ++++
 kernel/sysctl.c         |  9 +++++++++
 security/fbfam/Makefile |  1 +
 security/fbfam/fbfam.c  | 11 +++++++++++
 security/fbfam/sysctl.c | 20 ++++++++++++++++++++
 5 files changed, 45 insertions(+)
 create mode 100644 security/fbfam/sysctl.c

diff --git a/include/fbfam/fbfam.h b/include/fbfam/fbfam.h
index b5b7d1127a52..2cfe51d2b0d5 100644
--- a/include/fbfam/fbfam.h
+++ b/include/fbfam/fbfam.h
@@ -3,8 +3,12 @@
 #define _FBFAM_H_
 
 #include <linux/sched.h>
+#include <linux/sysctl.h>
 
 #ifdef CONFIG_FBFAM
+#ifdef CONFIG_SYSCTL
+extern struct ctl_table fbfam_sysctls[];
+#endif
 int fbfam_fork(struct task_struct *child);
 int fbfam_execve(void);
 int fbfam_exit(void);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 09e70ee2332e..c3b4d737bef3 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -77,6 +77,8 @@
 #include <linux/uaccess.h>
 #include <asm/processor.h>
 
+#include <fbfam/fbfam.h>
+
 #ifdef CONFIG_X86
 #include <asm/nmi.h>
 #include <asm/stacktrace.h>
@@ -2660,6 +2662,13 @@ static struct ctl_table kern_table[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE,
 	},
+#endif
+#ifdef CONFIG_FBFAM
+	{
+		.procname	= "fbfam",
+		.mode		= 0555,
+		.child		= fbfam_sysctls,
+	},
 #endif
 	{ }
 };
diff --git a/security/fbfam/Makefile b/security/fbfam/Makefile
index f4b9f0b19c44..b8d5751ecea4 100644
--- a/security/fbfam/Makefile
+++ b/security/fbfam/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_FBFAM) += fbfam.o
+obj-$(CONFIG_SYSCTL) += sysctl.o
diff --git a/security/fbfam/fbfam.c b/security/fbfam/fbfam.c
index 0387f95f6408..9be4639b72eb 100644
--- a/security/fbfam/fbfam.c
+++ b/security/fbfam/fbfam.c
@@ -7,6 +7,17 @@
 #include <linux/refcount.h>
 #include <linux/slab.h>
 
+/**
+ * sysctl_crashing_rate_threshold - Crashing rate threshold.
+ *
+ * The rate's units are in milliseconds per fault.
+ *
+ * A fork brute force attack will be detected if the application's crashing rate
+ * falls under this threshold. So, the higher this value, the faster an attack
+ * will be detected.
+ */
+unsigned long sysctl_crashing_rate_threshold = 30000;
+
 /**
  * struct fbfam_stats - Fork brute force attack mitigation statistics.
  * @refc: Reference counter.
diff --git a/security/fbfam/sysctl.c b/security/fbfam/sysctl.c
new file mode 100644
index 000000000000..430323ad8e9f
--- /dev/null
+++ b/security/fbfam/sysctl.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/sysctl.h>
+
+extern unsigned long sysctl_crashing_rate_threshold;
+static unsigned long ulong_one = 1;
+static unsigned long ulong_max = ULONG_MAX;
+
+struct ctl_table fbfam_sysctls[] = {
+	{
+		.procname	= "crashing_rate_threshold",
+		.data		= &sysctl_crashing_rate_threshold,
+		.maxlen		= sizeof(sysctl_crashing_rate_threshold),
+		.mode		= 0644,
+		.proc_handler	= proc_doulongvec_minmax,
+		.extra1		= &ulong_one,
+		.extra2		= &ulong_max,
+	},
+	{ }
+};
+
-- 
2.25.1