Creds: creds->security can be NULL is selinux is disabled

__validate_process_creds should check if selinux is actually enabled before
running tests on the selinux portion of the credentials struct.
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

include/linux/cred.h

@@ -15,6 +15,7 @@
 #include <linux/capability.h>
 #include <linux/init.h>
 #include <linux/key.h>
+#include <linux/selinux.h>
 #include <asm/atomic.h>
 
 struct user_struct;
@@ -182,11 +183,13 @@ static inline bool creds_are_invalid(const struct cred *cred)
 	if (atomic_read(&cred->usage) < atomic_read(&cred->subscribers))
 		return true;
 #ifdef CONFIG_SECURITY_SELINUX
-	if ((unsigned long) cred->security < PAGE_SIZE)
-		return true;
-	if ((*(u32*)cred->security & 0xffffff00) ==
-	    (POISON_FREE << 24 | POISON_FREE << 16 | POISON_FREE << 8))
-		return true;
+	if (selinux_is_enabled()) {
+		if ((unsigned long) cred->security < PAGE_SIZE)
+			return true;
+		if ((*(u32 *)cred->security & 0xffffff00) ==
+		    (POISON_FREE << 24 | POISON_FREE << 16 | POISON_FREE << 8))
+			return true;
+	}
 #endif
 	return false;
 }

include/linux/selinux.h

@@ -61,6 +61,11 @@ void selinux_secmark_refcount_inc(void);
  *     existing SECMARK targets has been removed/flushed.
  */
 void selinux_secmark_refcount_dec(void);
+
+/**
+ * selinux_is_enabled - is SELinux enabled?
+ */
+bool selinux_is_enabled(void);
 #else
 
 static inline int selinux_string_to_sid(const char *str, u32 *sid)
@@ -84,6 +89,10 @@ static inline void selinux_secmark_refcount_dec(void)
 	return;
 }
 
+static bool selinux_is_enabled(void)
+{
+	return false;
+}
 #endif	/* CONFIG_SECURITY_SELINUX */
 
 #endif /* _LINUX_SELINUX_H */

security/selinux/exports.c

@@ -63,3 +63,9 @@ void selinux_secmark_refcount_dec(void)
 	atomic_dec(&selinux_secmark_refcount);
 }
 EXPORT_SYMBOL_GPL(selinux_secmark_refcount_dec);
+
+bool selinux_is_enabled(void)
+{
+	return selinux_enabled;
+}
+EXPORT_SYMBOL_GPL(selinux_is_enabled);