KEYS: reject NULL restriction string when type is specified

keyctl_restrict_keyring() allows through a NULL restriction when the
"type" is non-NULL, which causes a NULL pointer dereference in
asymmetric_lookup_restriction() when it calls strcmp() on the
restriction string.

But no key types actually use a "NULL restriction" to mean anything, so
update keyctl_restrict_keyring() to reject it with EINVAL.

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 97d3aa0f3134 ("KEYS: Add a lookup_restriction function for the asymmetric key type")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/keyctl.c

@@ -1588,9 +1588,8 @@ error_keyring:
  * The caller must have Setattr permission to change keyring restrictions.
  *
  * The requested type name may be a NULL pointer to reject all attempts
- * to link to the keyring. If _type is non-NULL, _restriction can be
- * NULL or a pointer to a string describing the restriction. If _type is
- * NULL, _restriction must also be NULL.
+ * to link to the keyring.  In this case, _restriction must also be NULL.
+ * Otherwise, both _type and _restriction must be non-NULL.
  *
  * Returns 0 if successful.
  */
@@ -1598,7 +1597,6 @@ long keyctl_restrict_keyring(key_serial_t id, const char __user *_type,
 			     const char __user *_restriction)
 {
 	key_ref_t key_ref;
-	bool link_reject = !_type;
 	char type[32];
 	char *restriction = NULL;
 	long ret;
@@ -1607,31 +1605,29 @@ long keyctl_restrict_keyring(key_serial_t id, const char __user *_type,
 	if (IS_ERR(key_ref))
 		return PTR_ERR(key_ref);
 
+	ret = -EINVAL;
 	if (_type) {
-		ret = key_get_type_from_user(type, _type, sizeof(type));
-		if (ret < 0)
+		if (!_restriction)
 			goto error;
-	}
 
-	if (_restriction) {
-		if (!_type) {
-			ret = -EINVAL;
+		ret = key_get_type_from_user(type, _type, sizeof(type));
+		if (ret < 0)
 			goto error;
-		}
 
 		restriction = strndup_user(_restriction, PAGE_SIZE);
 		if (IS_ERR(restriction)) {
 			ret = PTR_ERR(restriction);
 			goto error;
 		}
+	} else {
+		if (_restriction)
+			goto error;
 	}
 
-	ret = keyring_restrict(key_ref, link_reject ? NULL : type, restriction);
+	ret = keyring_restrict(key_ref, _type ? type : NULL, restriction);
 	kfree(restriction);
-
 error:
 	key_ref_put(key_ref);
-
 	return ret;
 }