[PATCH v6 5/8] security/brute: Mitigate a brute force attack

@@ -22,6 +22,7 @@
 #include <linux/math64.h>
 #include <linux/netdevice.h>
 #include <linux/path.h>
+#include <linux/pid.h>
 #include <linux/printk.h>
 #include <linux/refcount.h>
 #include <linux/rwlock.h>

@@ -64,7 +65,7 @@ struct brute_cred {
  * @lock: Lock to protect the brute_stats structure.
  * @refc: Reference counter.
  * @faults: Number of crashes.
- * @jiffies: Last crash timestamp.
+ * @jiffies: Last crash timestamp. If zero, the attack detection is disabled.
  * @period: Crash period's moving average.
  * @saved_cred: Saved credentials.
  * @network: Network activity flag.

@@ -571,6 +572,125 @@ static inline void print_fork_attack_running(void)
 	pr_warn("Fork brute force attack detected [%s]\n", current->comm);
 }

+/**
+ * brute_disabled() - Test if the brute force attack detection is disabled.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * The brute force attack detection enabling/disabling is based on the last
+ * crash timestamp. A zero timestamp indicates that this feature is disabled. A
+ * timestamp greater than zero indicates that the attack detection is enabled.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * It's mandatory to disable interrupts before acquiring the brute_stats::lock
+ * since the task_free hook can be called from an IRQ context during the
+ * execution of the task_fatal_signal hook.
+ *
+ * Context: Must be called with interrupts disabled and brute_stats_ptr_lock
+ *          held.
+ * Return: True if the brute force attack detection is disabled. False
+ *         otherwise.
+ */
+static bool brute_disabled(struct brute_stats *stats)
+{
+	bool disabled;
+
+	spin_lock(&stats->lock);
+	disabled = !stats->jiffies;
+	spin_unlock(&stats->lock);
+
+	return disabled;
+}
+
+/**
+ * brute_disable() - Disable the brute force attack detection.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * To disable the brute force attack detection it is only necessary to set the
+ * last crash timestamp to zero. A zero timestamp indicates that this feature is
+ * disabled. A timestamp greater than zero indicates that the attack detection
+ * is enabled.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * Context: Must be called with interrupts disabled and brute_stats_ptr_lock
+ *          and brute_stats::lock held.
+ */
+static inline void brute_disable(struct brute_stats *stats)
+{
+	stats->jiffies = 0;
+}
+
+/**
+ * enum brute_attack_type - Brute force attack type.
+ * @BRUTE_ATTACK_TYPE_FORK: Attack that happens through the fork system call.
+ * @BRUTE_ATTACK_TYPE_EXEC: Attack that happens through the execve system call.
+ */
+enum brute_attack_type {
+	BRUTE_ATTACK_TYPE_FORK,
+	BRUTE_ATTACK_TYPE_EXEC,
+};
+
+/**
+ * brute_kill_offending_tasks() - Kill the offending tasks.
+ * @attack_type: Brute force attack type.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * When a brute force attack is detected all the offending tasks involved in the
+ * attack must be killed. In other words, it is necessary to kill all the tasks
+ * that share the same statistical data. Moreover, if the attack happens through
+ * the fork system call, the processes that have the same group_leader that the
+ * current task must be avoided since they are in the path to be killed.
+ *
+ * When the SIGKILL signal is sent to the offending tasks, this function will be
+ * called again from the task_fatal_signal hook due to a small crash period. So,
+ * to avoid kill again the same tasks due to a recursive call of this function,
+ * it is necessary to disable the attack detection for this fork hierarchy.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * It's mandatory to disable interrupts before acquiring the brute_stats::lock
+ * since the task_free hook can be called from an IRQ context during the
+ * execution of the task_fatal_signal hook.
+ *
+ * Context: Must be called with interrupts disabled and tasklist_lock and
+ *          brute_stats_ptr_lock held.
+ */
+static void brute_kill_offending_tasks(enum brute_attack_type attack_type,
+				       struct brute_stats *stats)
+{
+	struct task_struct *p;
+	struct brute_stats **p_stats;
+
+	spin_lock(&stats->lock);
+
+	if (attack_type == BRUTE_ATTACK_TYPE_FORK &&
+	    refcount_read(&stats->refc) == 1) {
+		spin_unlock(&stats->lock);
+		return;
+	}
+
+	brute_disable(stats);
+	spin_unlock(&stats->lock);
+
+	for_each_process(p) {
+		if (attack_type == BRUTE_ATTACK_TYPE_FORK &&
+		    p->group_leader == current->group_leader)
+			continue;
+
+		p_stats = brute_stats_ptr(p);
+		if (*p_stats != stats)
+			continue;
+
+		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_PID);
+		pr_warn_ratelimited("Offending process %d [%s] killed\n",
+				    p->pid, p->comm);
+	}
+}
+
 /**
  * brute_manage_fork_attack() - Manage a fork brute force attack.
  * @stats: Statistical data shared by all the fork hierarchy processes.

@@ -586,8 +706,8 @@ static inline void print_fork_attack_running(void)
  * since the task_free hook can be called from an IRQ context during the
  * execution of the task_fatal_signal hook.
  *
- * Context: Must be called with interrupts disabled and brute_stats_ptr_lock
- *          held.
+ * Context: Must be called with interrupts disabled and tasklist_lock and
+ *          brute_stats_ptr_lock held.
  * Return: The last crash timestamp before updating it.
  */
 static u64 brute_manage_fork_attack(struct brute_stats *stats, u64 now)

@@ -595,8 +715,10 @@ static u64 brute_manage_fork_attack(struct brute_stats *stats, u64 now)
 	u64 last_fork_crash;

 	last_fork_crash = brute_update_crash_period(stats, now);
-	if (brute_attack_running(stats))
+	if (brute_attack_running(stats)) {
 		print_fork_attack_running();
+		brute_kill_offending_tasks(BRUTE_ATTACK_TYPE_FORK, stats);
+	}

 	return last_fork_crash;
 }

@@ -783,8 +905,10 @@ static void brute_manage_exec_attack(struct brute_stats *stats, u64 now,
 	if (fork_period == exec_period)
 		return;

-	if (brute_attack_running(exec_stats))
+	if (brute_attack_running(exec_stats)) {
 		print_exec_attack_running(exec_stats);
+		brute_kill_offending_tasks(BRUTE_ATTACK_TYPE_EXEC, exec_stats);
+	}
 }

 /**

@@ -900,10 +1024,9 @@ static void brute_task_fatal_signal(const kernel_siginfo_t *siginfo)
 	read_lock(&tasklist_lock);
 	read_lock_irqsave(&brute_stats_ptr_lock, flags);

-	if (WARN(!*stats, "No statistical data\n"))
-		goto unlock;
-
-	if (!brute_threat_model_supported(siginfo, *stats))
+	if (WARN(!*stats, "No statistical data\n") ||
+	    brute_disabled(*stats) ||
+	    !brute_threat_model_supported(siginfo, *stats))
 		goto unlock;

 	last_fork_crash = brute_manage_fork_attack(*stats, now);
--