security/fbfam: Detect a fork brute force attack

To detect a fork brute force attack it is necessary to compute the
crashing rate of the application. This calculation is performed in each
fatal fail of a task, or in other words, when a core dump is triggered.
If this rate shows that the application is crashing quickly, there is a
clear signal that an attack is happening.

Since the crashing rate is computed in milliseconds per fault, if this
rate goes under a certain threshold a warning is triggered.

Signed-off-by: John Wood <john.wood@gmx.com>

fs/coredump.c

@@ -51,6 +51,7 @@
 #include "internal.h"
 
 #include <trace/events/sched.h>
+#include <fbfam/fbfam.h>
 
 int core_uses_pid;
 unsigned int core_pipe_limit;
@@ -825,6 +826,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 fail_creds:
 	put_cred(cred);
 fail:
+	fbfam_handle_attack(siginfo->si_signo);
 	return;
 }
 

include/fbfam/fbfam.h

@@ -12,10 +12,12 @@ extern struct ctl_table fbfam_sysctls[];
 int fbfam_fork(struct task_struct *child);
 int fbfam_execve(void);
 int fbfam_exit(void);
+int fbfam_handle_attack(int signal);
 #else
 static inline int fbfam_fork(struct task_struct *child) { return 0; }
 static inline int fbfam_execve(void) { return 0; }
 static inline int fbfam_exit(void) { return 0; }
+static inline int fbfam_handle_attack(int signal) { return 0; }
 #endif
 
 #endif /* _FBFAM_H_ */

security/fbfam/fbfam.c

@@ -4,7 +4,9 @@
 #include <linux/errno.h>
 #include <linux/gfp.h>
 #include <linux/jiffies.h>
+#include <linux/printk.h>
 #include <linux/refcount.h>
+#include <linux/signal.h>
 #include <linux/slab.h>
 
 /**
@@ -172,3 +174,40 @@ int fbfam_exit(void)
 	return 0;
 }
 
+/**
+ * fbfam_handle_attack() - Fork brute force attack detection.
+ * @signal: Signal number that causes the core dump.
+ *
+ * The crashing rate of an application is computed in milliseconds per fault in
+ * each crash. So, if this rate goes under a certain threshold there is a clear
+ * signal that the application is crashing quickly. At this moment, a fork brute
+ * force attack is happening.
+ *
+ * Return: -EFAULT if the current task doesn't have statistical data. Zero
+ *         otherwise.
+ */
+int fbfam_handle_attack(int signal)
+{
+	struct fbfam_stats *stats = current->fbfam_stats;
+	u64 delta_jiffies, delta_time;
+	u64 crashing_rate;
+
+	if (!stats)
+		return -EFAULT;
+
+	if (!(signal == SIGILL || signal == SIGBUS || signal == SIGKILL ||
+	      signal == SIGSEGV || signal == SIGSYS))
+		return 0;
+
+	stats->faults += 1;
+
+	delta_jiffies = get_jiffies_64() - stats->jiffies;
+	delta_time = jiffies64_to_msecs(delta_jiffies);
+	crashing_rate = delta_time / (u64)stats->faults;
+
+	if (crashing_rate < (u64)sysctl_crashing_rate_threshold)
+		pr_warn("fbfam: Fork brute force attack detected\n");
+
+	return 0;
+}
+