loadpin: Implement custom proc_handler for enforce

Add a new static variable (loadpin_root_writable) to keep the
write-ability state of enforce. Remove set_sysctl and const qualify
loadpin_sysctl_table (moves into .rodata) as there is no longer need to
change the value of extra1. The new proc_handler_loadpin returns -EINVAL
when loadpin_root_writable is false and the kernel var (enforce) is
being written. The old way of modifying the write-ability of enforce
stays in loadpin_check and is still set by calling sb_is_writable.

Signed-off-by: Joel Granados <[e-mail]>

security/loadpin/loadpin.c

···
 static bool deny_reading_verity_digests;
 #endif
 
+// initialized to false
+static bool loadpin_root_writable;
 #ifdef CONFIG_SYSCTL
-static struct ctl_table loadpin_sysctl_table[] = {
+
+static int proc_handler_loadpin(const struct ctl_table *table, int dir,
+				void *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (!loadpin_root_writable && SYSCTL_USER_TO_KERN(dir))
+		return -EINVAL;
+	return proc_dointvec_minmax(table, dir, buffer, lenp, ppos);
+}
+
+static const struct ctl_table loadpin_sysctl_table[] = {
 	{
 		.procname       = "enforce",
 		.data           = &enforce,
 		.maxlen         = sizeof(int),
 		.mode           = 0644,
-		.proc_handler   = proc_dointvec_minmax,
-		.extra1         = SYSCTL_ONE,
+		.proc_handler   = proc_handler_loadpin,
+		.extra1         = SYSCTL_ZERO,
 		.extra2         = SYSCTL_ONE,
 	},
 };
-
-static void set_sysctl(bool is_writable)
-{
-	/*
-	 * If load pinning is not enforced via a read-only block
-	 * device, allow sysctl to change modes for testing.
-	 */
-	if (is_writable)
-		loadpin_sysctl_table[0].extra1 = SYSCTL_ZERO;
-	else
-		loadpin_sysctl_table[0].extra1 = SYSCTL_ONE;
-}
-#else
-static inline void set_sysctl(bool is_writable) { }
 #endif
 
 static void report_writable(struct super_block *mnt_sb, bool writable)
···
 	struct super_block *load_root;
 	const char *origin = kernel_read_file_id_str(id);
 	bool first_root_pin = false;
-	bool load_root_writable;
 
 	/* If the file id is excluded, ignore the pinning. */
 	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
···
 	}
 
 	load_root = file->f_path.mnt->mnt_sb;
-	load_root_writable = sb_is_writable(load_root);
 
 	/* First loaded module/firmware defines the root for all others. */
 	spin_lock(&pinned_root_spinlock);
···
 	spin_unlock(&pinned_root_spinlock);
 
 	if (first_root_pin) {
-		report_writable(pinned_root, load_root_writable);
-		set_sysctl(load_root_writable);
+		loadpin_root_writable = sb_is_writable(pinned_root);
+		report_writable(pinned_root, loadpin_root_writable);
 		report_load(origin, file, "pinned");
 	}