libsepol: Improve the validation of type aliases

A maliciously crafted binary policy could have a type alias that
refers to an attribute or that ultimately refers back to itself
in a loop.

Validate that type aliases ultimately refer to a primary type
and not to an attribute or themselves.

Signed-off-by: James Carter <jwcart2@gmail.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>

Author: jwcart2

libsepol/src/policydb_validate.c

@@ -7,6 +7,7 @@
 #include <sepol/policydb/services.h>
 
 #include "debug.h"
+#include "private.h"
 #include "kernel_to_common.h"
 #include "policydb_validate.h"
 
@@ -660,19 +661,23 @@ static int validate_type_datum(sepol_handle_t *handle, const type_datum_t *type,
 {
 	if (validate_value(type->s.value, &flavors[SYM_TYPES]))
 		goto bad;
-	if (type->primary && validate_value(type->primary, &flavors[SYM_TYPES]))
+
+	switch (type->flags) {
+	case 0:
+	case TYPE_FLAGS_NEVERAUDIT:
+	case TYPE_FLAGS_PERMISSIVE:
+	case TYPE_FLAGS_NEVERAUDIT | TYPE_FLAGS_PERMISSIVE:
+	case TYPE_FLAGS_EXPAND_ATTR_TRUE:
+	case TYPE_FLAGS_EXPAND_ATTR_FALSE:
+	case TYPE_FLAGS_EXPAND_ATTR:
+		break;
+	default:
 		goto bad;
+	}
 
-	switch (type->flavor) {
-	case TYPE_TYPE:
-	case TYPE_ALIAS:
-		if (!ebitmap_is_empty(&type->types))
-			goto bad;
-		if (type->bounds &&
-		    validate_simpletype(type->bounds, p, flavors))
+	if (type->flavor == TYPE_ATTRIB) {
+		if (type->primary != 1)
 			goto bad;
-		break;
-	case TYPE_ATTRIB:
 		if (p->policy_type == POLICY_KERN) {
 			if (!ebitmap_is_empty(&type->types))
 				goto bad;
@@ -683,21 +688,37 @@ static int validate_type_datum(sepol_handle_t *handle, const type_datum_t *type,
 		}
 		if (type->bounds)
 			goto bad;
-		break;
-	default:
-		goto bad;
-	}
-
-	switch (type->flags) {
-	case 0:
-	case TYPE_FLAGS_NEVERAUDIT:
-	case TYPE_FLAGS_PERMISSIVE:
-	case TYPE_FLAGS_NEVERAUDIT | TYPE_FLAGS_PERMISSIVE:
-	case TYPE_FLAGS_EXPAND_ATTR_TRUE:
-	case TYPE_FLAGS_EXPAND_ATTR_FALSE:
-	case TYPE_FLAGS_EXPAND_ATTR:
-		break;
-	default:
+	} else if ((type->flavor == TYPE_TYPE) ||
+		   (type->flavor == TYPE_ALIAS)) {
+		if (!ebitmap_is_empty(&type->types))
+			goto bad;
+		if (type->bounds &&
+		    validate_simpletype(type->bounds, p, flavors))
+			goto bad;
+		if ((type->flavor == TYPE_TYPE) && type->primary) {
+			if (type->primary > 1)
+				goto bad;
+		} else {
+			const type_datum_t *t = type;
+			uint32_t v;
+			int repeats = 0;
+			while ((t->flavor == TYPE_ALIAS) || (t->primary == 0)) {
+				if (repeats >= MAX_ALIAS_REPEATS)
+					break;
+				v = (t->primary) ? t->primary : t->s.value;
+				if (validate_value(v, &flavors[SYM_TYPES]))
+					break;
+				t = p->type_val_to_struct[v - 1];
+				if (t == type)
+					break;
+				repeats++;
+			}
+			if ((t->flavor != TYPE_TYPE) || (t->primary != 1)) {
+				ERR(handle, "Alias validation failed");
+				goto bad;
+			}
+		}
+	} else {
 		goto bad;
 	}