audit: audit feature to only allow unsetting the loginuid

This is a new audit feature which only grants processes with
CAP_AUDIT_CONTROL the ability to unset their loginuid.  They cannot
directly set it from a valid uid to another valid uid.  The ability to
unset the loginuid is nice because a priviledged task, like that of
container creation, can unset the loginuid and then priv is not needed
inside the container when a login daemon needs to set the loginuid.
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

@@ -386,7 +386,8 @@ struct audit_features {
 	__u32	lock;		/* which features to lock */
 };
 
-#define AUDIT_LAST_FEATURE	-1
+#define AUDIT_FEATURE_ONLY_UNSET_LOGINUID	0
+#define AUDIT_LAST_FEATURE			AUDIT_FEATURE_ONLY_UNSET_LOGINUID
 
 #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

@@ -144,7 +144,8 @@ static struct audit_features af = {.vers = AUDIT_FEATURE_VERSION,
 				   .features = 0,
 				   .lock = 0,};
 
-static char *audit_feature_names[0] = {
+static char *audit_feature_names[1] = {
+	"only_unset_loginuid",
 };
 
 

kernel/auditsc.c

@@ -1974,6 +1974,9 @@ static int audit_set_loginuid_perm(kuid_t loginuid)
 	/* it is set, you need permission */
 	if (!capable(CAP_AUDIT_CONTROL))
 		return -EPERM;
+	/* reject if this is not an unset and we don't allow that */
+	if (is_audit_feature_set(AUDIT_FEATURE_ONLY_UNSET_LOGINUID) && uid_valid(loginuid))
+		return -EPERM;
 	return 0;
 }