lsm: get rid of the lsm_names list and do some cleanup

The LSM currently has a lot of code to maintain a list of the currently
active LSMs in a human readable string, with the only user being the
"/sys/kernel/security/lsm" code.  Let's drop all of that code and
generate the string on first use and then cache it for subsequent use.

Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: pcmoore

include/linux/lsm_hooks.h

@@ -172,7 +172,6 @@ struct lsm_info {
 
 
 /* DO NOT tamper with these variables outside of the LSM framework */
-extern char *lsm_names;
 extern struct lsm_static_calls_table static_calls_table __ro_after_init;
 
 /**

security/inode.c

@@ -22,6 +22,8 @@
 #include <linux/lsm_hooks.h>
 #include <linux/magic.h>
 
+#include "lsm.h"
+
 static struct vfsmount *mount;
 static int mount_count;
 
@@ -315,12 +317,49 @@ void securityfs_remove(struct dentry *dentry)
 EXPORT_SYMBOL_GPL(securityfs_remove);
 
 #ifdef CONFIG_SECURITY
+#include <linux/spinlock.h>
+
 static struct dentry *lsm_dentry;
+
 static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
 			loff_t *ppos)
 {
-	return simple_read_from_buffer(buf, count, ppos, lsm_names,
-		strlen(lsm_names));
+	int i;
+	static char *str;
+	static size_t len;
+	static DEFINE_SPINLOCK(lock);
+
+	/* NOTE: we never free or modify the string once it is set */
+
+	if (unlikely(!str || !len)) {
+		char *str_tmp;
+		size_t len_tmp = 0;
+
+		for (i = 0; i < lsm_active_cnt; i++)
+			/* the '+ 1' accounts for either a comma or a NUL */
+			len_tmp += strlen(lsm_idlist[i]->name) + 1;
+
+		str_tmp = kmalloc(len_tmp, GFP_KERNEL);
+		if (!str_tmp)
+			return -ENOMEM;
+		str_tmp[0] = '\0';
+
+		for (i = 0; i < lsm_active_cnt; i++) {
+			if (i > 0)
+				strcat(str_tmp, ",");
+			strcat(str_tmp, lsm_idlist[i]->name);
+		}
+
+		spin_lock(&lock);
+		if (!str) {
+			str = str_tmp;
+			len = len_tmp - 1;
+		} else
+			kfree(str_tmp);
+		spin_unlock(&lock);
+	}
+
+	return simple_read_from_buffer(buf, count, ppos, str, len);
 }
 
 static const struct file_operations lsm_ops = {

security/lsm_init.c

@@ -10,8 +10,6 @@
 
 #include "lsm.h"
 
-char *lsm_names;
-
 /* Pointers to LSM sections defined in include/asm-generic/vmlinux.lds.h */
 extern struct lsm_info __start_lsm_info[], __end_lsm_info[];
 extern struct lsm_info __start_early_lsm_info[], __end_early_lsm_info[];
@@ -371,42 +369,6 @@ static void __init lsm_init_ordered(void)
 	}
 }
 
-static bool match_last_lsm(const char *list, const char *lsm)
-{
-	const char *last;
-
-	if (WARN_ON(!list || !lsm))
-		return false;
-	last = strrchr(list, ',');
-	if (last)
-		/* Pass the comma, strcmp() will check for '\0' */
-		last++;
-	else
-		last = list;
-	return !strcmp(last, lsm);
-}
-
-static int lsm_append(const char *new, char **result)
-{
-	char *cp;
-
-	if (*result == NULL) {
-		*result = kstrdup(new, GFP_KERNEL);
-		if (*result == NULL)
-			return -ENOMEM;
-	} else {
-		/* Check if it is the last registered name */
-		if (match_last_lsm(*result, new))
-			return 0;
-		cp = kasprintf(GFP_KERNEL, "%s,%s", *result, new);
-		if (cp == NULL)
-			return -ENOMEM;
-		kfree(*result);
-		*result = cp;
-	}
-	return 0;
-}
-
 static void __init lsm_static_call_init(struct security_hook_list *hl)
 {
 	struct lsm_static_call *scall = hl->scalls;
@@ -443,15 +405,6 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
 		hooks[i].lsmid = lsmid;
 		lsm_static_call_init(&hooks[i]);
 	}
-
-	/*
-	 * Don't try to append during early_security_init(), we'll come back
-	 * and fix this up afterwards.
-	 */
-	if (slab_is_available()) {
-		if (lsm_append(lsmid->name, &lsm_names) < 0)
-			panic("%s - Cannot get early memory.\n", __func__);
-	}
 }
 
 int __init early_security_init(void)
@@ -488,8 +441,6 @@ int __init security_init(void)
 	lsm_early_for_each_raw(lsm) {
 		init_debug("  early started: %s (%s)\n", lsm->id->name,
 			   is_enabled(lsm) ? "enabled" : "disabled");
-		if (lsm->enabled)
-			lsm_append(lsm->id->name, &lsm_names);
 	}
 
 	/* Load LSMs in specified order. */