Hi, more inline.

On Thu, Sep 10, 2020 at 04:14:38PM -0700, Kees Cook wrote:

quoted

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

Instead of doing the extern and adding to sysctl.c, this can all be done
directly (dynamically) from the fbfam.c file instead.

Like Yama do in the yama_init_sysctl() function? As a reference code.

quoted

 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;

I would move the sysctls here, instead. (Also, the above should be
const.)

If the above variable is const how the sysctl interface can modify it?
I think it would be better to declare it as __read_mostly instead. What
do you think?

unsigned long sysctl_crashing_rate_threshold __read_mostly = 30000;

quoted

+
 /**
  * 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,
+	},
+	{ }
+};

I wouldn't bother splitting this into a separate file. (Just leave it in
fbfam.c)

--
Kees Cook

Thanks,
John Wood