selinux: fix a missing-check bug in selinux_sb_eat_lsm_opts()

In selinux_sb_eat_lsm_opts(), 'arg' is allocated by kmemdup_nul(). It
returns NULL when fails. So 'arg' should be checked. And 'mnt_opts'
should be freed when error.

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Fixes: 99dbbb593fe6 ("selinux: rewrite selinux_sb_eat_lsm_opts()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index b7db84c..fea66f6 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2625,10 +2625,11 @@
 	char *from = options;
 	char *to = options;
 	bool first = true;
+	int rc;
 
 	while (1) {
 		int len = opt_len(from);
-		int token, rc;
+		int token;
 		char *arg = NULL;
 
 		token = match_opt_prefix(from, len, &arg);
@@ -2644,15 +2645,15 @@
 						*q++ = c;
 				}
 				arg = kmemdup_nul(arg, q - arg, GFP_KERNEL);
+				if (!arg) {
+					rc = -ENOMEM;
+					goto free_opt;
+				}
 			}
 			rc = selinux_add_opt(token, arg, mnt_opts);
 			if (unlikely(rc)) {
 				kfree(arg);
-				if (*mnt_opts) {
-					selinux_free_mnt_opts(*mnt_opts);
-					*mnt_opts = NULL;
-				}
-				return rc;
+				goto free_opt;
 			}
 		} else {
 			if (!first) {	// copy with preceding comma
@@ -2670,6 +2671,13 @@
 	}
 	*to = '\0';
 	return 0;
+
+free_opt:
+	if (*mnt_opts) {
+		selinux_free_mnt_opts(*mnt_opts);
+		*mnt_opts = NULL;
+	}
+	return rc;
 }
 
 static int selinux_sb_remount(struct super_block *sb, void *mnt_opts)