[v3,2/8] security/brute: Define a LSM and manage statistical data

diff --git a/security/Kconfig b/security/Kconfig
index 7561f6f99f1d..204bb311b1f1 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -240,6 +240,7 @@  source "security/safesetid/Kconfig"
 source "security/lockdown/Kconfig"

 source "security/integrity/Kconfig"
+source "security/brute/Kconfig"

 choice
 	prompt "First legacy 'major LSM' to be initialized"
@@ -277,11 +278,11 @@  endchoice

 config LSM
 	string "Ordered list of enabled LSMs"
-	default "lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
-	default "lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
-	default "lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
-	default "lockdown,yama,loadpin,safesetid,integrity,bpf" if DEFAULT_SECURITY_DAC
-	default "lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor,bpf"
+	default "brute,lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
+	default "brute,lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
+	default "brute,lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
+	default "brute,lockdown,yama,loadpin,safesetid,integrity,bpf" if DEFAULT_SECURITY_DAC
+	default "brute,lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor,bpf"
 	help
 	  A comma-separated list of LSMs, in initialization order.
 	  Any LSMs left off this list will be ignored. This can be
diff --git a/security/Makefile b/security/Makefile
index 3baf435de541..1236864876da 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -36,3 +36,7 @@  obj-$(CONFIG_BPF_LSM)			+= bpf/
 # Object integrity file lists
 subdir-$(CONFIG_INTEGRITY)		+= integrity
 obj-$(CONFIG_INTEGRITY)			+= integrity/
+
+# Object brute file lists
+subdir-$(CONFIG_SECURITY_FORK_BRUTE)	+= brute
+obj-$(CONFIG_SECURITY_FORK_BRUTE)	+= brute/
diff --git a/security/brute/Kconfig b/security/brute/Kconfig
new file mode 100644
index 000000000000..1bd2df1e2dec
--- /dev/null
+++ b/security/brute/Kconfig
@@ -0,0 +1,12 @@ 
+# SPDX-License-Identifier: GPL-2.0
+config SECURITY_FORK_BRUTE
+	bool "Fork brute force attack detection and mitigation"
+	depends on SECURITY
+	help
+	  This is an LSM that stops any fork brute force attack against
+	  vulnerable userspace processes. The detection method is based on
+	  the application crash period and as a mitigation procedure all the
+	  offending tasks are killed. Like capabilities, this security module
+	  stacks with other LSMs.
+
+	  If you are unsure how to answer this question, answer N.
diff --git a/security/brute/Makefile b/security/brute/Makefile
new file mode 100644
index 000000000000..d3f233a132a9
--- /dev/null
+++ b/security/brute/Makefile
@@ -0,0 +1,2 @@ 
+# SPDX-License-Identifier: GPL-2.0
+obj-$(CONFIG_SECURITY_FORK_BRUTE) += brute.o
diff --git a/security/brute/brute.c b/security/brute/brute.c
new file mode 100644
index 000000000000..70f812bb7763
--- /dev/null
+++ b/security/brute/brute.c
@@ -0,0 +1,252 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <asm/current.h>
+#include <linux/bug.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/lsm_hooks.h>
+#include <linux/printk.h>
+#include <linux/refcount.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/types.h>
+
+/**
+ * struct brute_stats - Fork brute force attack statistics.
+ * @lock: Lock to protect the brute_stats structure.
+ * @refc: Reference counter.
+ * @faults: Number of crashes.
+ * @jiffies: Last crash timestamp.
+ * @period: Crash period's moving average.
+ *
+ * This structure holds the statistical data shared by all the fork hierarchy
+ * processes.
+ */
+struct brute_stats {
+	spinlock_t lock;
+	refcount_t refc;
+	unsigned char faults;
+	u64 jiffies;
+	u64 period;
+};
+
+/**
+ * brute_blob_sizes - LSM blob sizes.
+ *
+ * To share statistical data among all the fork hierarchy processes, define a
+ * pointer to the brute_stats structure as a part of the task_struct's security
+ * blob.
+ */
+static struct lsm_blob_sizes brute_blob_sizes __lsm_ro_after_init = {
+	.lbs_task = sizeof(struct brute_stats *),
+};
+
+/**
+ * brute_stats_ptr() - Get the pointer to the brute_stats structure.
+ * @task: Task that holds the statistical data.
+ *
+ * Return: A pointer to a pointer to the brute_stats structure.
+ */
+static inline struct brute_stats **brute_stats_ptr(struct task_struct *task)
+{
+	return task->security + brute_blob_sizes.lbs_task;
+}
+
+/**
+ * brute_new_stats() - Allocate a new statistics structure.
+ *
+ * If the allocation is successful the reference counter is set to one to
+ * indicate that there will be one task that points to this structure. Also, the
+ * last crash timestamp is set to now. This way, it is possible to compute the
+ * application crash period at the first fault.
+ *
+ * Return: NULL if the allocation fails. A pointer to the new allocated
+ *         statistics structure if it success.
+ */
+static struct brute_stats *brute_new_stats(void)
+{
+	struct brute_stats *stats;
+
+	stats = kmalloc(sizeof(struct brute_stats), GFP_KERNEL);
+	if (!stats)
+		return NULL;
+
+	spin_lock_init(&stats->lock);
+	refcount_set(&stats->refc, 1);
+	stats->faults = 0;
+	stats->jiffies = get_jiffies_64();
+	stats->period = 0;
+
+	return stats;
+}
+
+/**
+ * brute_share_stats() - Share the statistical data between processes.
+ * @src: Source of statistics to be shared.
+ * @dst: Destination of statistics to be shared.
+ *
+ * Copy the src's pointer to the statistical data structure to the dst's pointer
+ * to the same structure. Since there is a new process that shares the same
+ * data, increase the reference counter. The src's pointer 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_alloc hook.
+ */
+static void brute_share_stats(struct brute_stats *src,
+			      struct brute_stats **dst)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&src->lock, flags);
+	refcount_inc(&src->refc);
+	*dst = src;
+	spin_unlock_irqrestore(&src->lock, flags);
+}
+
+/**
+ * brute_task_alloc() - Target for the task_alloc hook.
+ * @task: Task being allocated.
+ * @clone_flags: Contains the flags indicating what should be shared.
+ *
+ * For a correct management of a fork brute force attack it is necessary that
+ * all the tasks hold statistical data. The same statistical data needs to be
+ * shared between all the tasks that hold the same memory contents or in other
+ * words, between all the tasks that have been forked without any execve call.
+ *
+ * To ensure this, if the current task doesn't have statistical data when forks,
+ * it is mandatory to allocate a new statistics structure and share it between
+ * this task and the new one being allocated. Otherwise, share the statistics
+ * that the current task already has.
+ *
+ * Return: -ENOMEM if the allocation of the new statistics structure fails. Zero
+ *         otherwise.
+ */
+static int brute_task_alloc(struct task_struct *task, unsigned long clone_flags)
+{
+	struct brute_stats **stats, **p_stats;
+
+	stats = brute_stats_ptr(task);
+	p_stats = brute_stats_ptr(current);
+
+	if (likely(*p_stats)) {
+		brute_share_stats(*p_stats, stats);
+		return 0;
+	}
+
+	*stats = brute_new_stats();
+	if (!*stats)
+		return -ENOMEM;
+
+	brute_share_stats(*stats, p_stats);
+	return 0;
+}
+
+/**
+ * brute_task_execve() - Target for the bprm_committing_creds hook.
+ * @bprm: Points to the linux_binprm structure.
+ *
+ * When a forked task calls the execve system call, the memory contents are set
+ * with new values. So, in this scenario the parent's statistical data no need
+ * to be shared. Instead, a new statistical data structure must be allocated to
+ * start a new hierarchy. This condition is detected when the statistics
+ * reference counter holds a value greater than or equal to two (a fork always
+ * sets the statistics reference counter to a minimum of two since the parent
+ * and the child task are sharing the same data).
+ *
+ * However, if the execve function is called immediately after another execve
+ * call, althought the memory contents are reset, there is no need to allocate
+ * a new statistical data structure. This is possible because at this moment
+ * only one task (the task that calls the execve function) points to the data.
+ * In this case, the previous allocation is used but the statistics are reset.
+ *
+ * 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 bprm_committing_creds hook.
+ */
+static void brute_task_execve(struct linux_binprm *bprm)
+{
+	struct brute_stats **stats;
+	unsigned long flags;
+
+	stats = brute_stats_ptr(current);
+	if (WARN(!*stats, "No statistical data\n"))
+		return;
+
+	spin_lock_irqsave(&(*stats)->lock, flags);
+
+	if (!refcount_dec_not_one(&(*stats)->refc)) {
+		/* execve call after an execve call */
+		(*stats)->faults = 0;
+		(*stats)->jiffies = get_jiffies_64();
+		(*stats)->period = 0;
+		spin_unlock_irqrestore(&(*stats)->lock, flags);
+		return;
+	}
+
+	/* execve call after a fork call */
+	spin_unlock_irqrestore(&(*stats)->lock, flags);
+	*stats = brute_new_stats();
+	WARN(!*stats, "Cannot allocate statistical data\n");
+}
+
+/**
+ * brute_task_free() - Target for the task_free hook.
+ * @task: Task about to be freed.
+ *
+ * The statistical data that is shared between all the fork hierarchy processes
+ * needs to be freed when this hierarchy disappears.
+ */
+static void brute_task_free(struct task_struct *task)
+{
+	struct brute_stats **stats;
+	bool refc_is_zero;
+
+	stats = brute_stats_ptr(task);
+	if (WARN(!*stats, "No statistical data\n"))
+		return;
+
+	spin_lock(&(*stats)->lock);
+	refc_is_zero = refcount_dec_and_test(&(*stats)->refc);
+	spin_unlock(&(*stats)->lock);
+
+	if (refc_is_zero) {
+		kfree(*stats);
+		*stats = NULL;
+	}
+}
+
+/**
+ * brute_hooks - Targets for the LSM's hooks.
+ */
+static struct security_hook_list brute_hooks[] __lsm_ro_after_init = {
+	LSM_HOOK_INIT(task_alloc, brute_task_alloc),
+	LSM_HOOK_INIT(bprm_committing_creds, brute_task_execve),
+	LSM_HOOK_INIT(task_free, brute_task_free),
+};
+
+/**
+ * brute_init() - Initialize the brute LSM.
+ *
+ * Return: Always returns zero.
+ */
+static int __init brute_init(void)
+{
+	pr_info("Brute initialized\n");
+	security_add_hooks(brute_hooks, ARRAY_SIZE(brute_hooks),
+			   KBUILD_MODNAME);
+	return 0;
+}
+
+DEFINE_LSM(brute) = {
+	.name = KBUILD_MODNAME,
+	.init = brute_init,
+	.blobs = &brute_blob_sizes,
+};