audit: audit feature to set loginuid immutable

This adds a new 'audit_feature' bit which allows userspace to set it
such that the loginuid is absolutely immutable, even if you have
CAP_AUDIT_CONTROL.
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

include/uapi/linux/audit.h

@@ -387,7 +387,8 @@ struct audit_features {
 };
 
 #define AUDIT_FEATURE_ONLY_UNSET_LOGINUID	0
-#define AUDIT_LAST_FEATURE			AUDIT_FEATURE_ONLY_UNSET_LOGINUID
+#define AUDIT_FEATURE_LOGINUID_IMMUTABLE	1
+#define AUDIT_LAST_FEATURE			AUDIT_FEATURE_LOGINUID_IMMUTABLE
 
 #define audit_feature_valid(x)		((x) >= 0 && (x) <= AUDIT_LAST_FEATURE)
 #define AUDIT_FEATURE_TO_MASK(x)	(1 << ((x) & 31)) /* mask for __u32 */

kernel/audit.c

@@ -144,8 +144,9 @@ static struct audit_features af = {.vers = AUDIT_FEATURE_VERSION,
 				   .features = 0,
 				   .lock = 0,};
 
-static char *audit_feature_names[1] = {
+static char *audit_feature_names[2] = {
 	"only_unset_loginuid",
+	"loginuid_immutable",
 };
 
 

kernel/auditsc.c

@@ -1971,6 +1971,9 @@ static int audit_set_loginuid_perm(kuid_t loginuid)
 	/* if we are unset, we don't need privs */
 	if (!audit_loginuid_set(current))
 		return 0;
+	/* if AUDIT_FEATURE_LOGINUID_IMMUTABLE means never ever allow a change*/
+	if (is_audit_feature_set(AUDIT_FEATURE_LOGINUID_IMMUTABLE))
+		return -EPERM;
 	/* it is set, you need permission */
 	if (!capable(CAP_AUDIT_CONTROL))
 		return -EPERM;