userdb: Reference-count DBusUserInfo, DBusGroupInfo

Previously, the hash table indexed by uid (or gid) took ownership of the
single reference to the heap-allocated struct, and the hash table
indexed by username (or group name) had a borrowed pointer to the same
struct that exists in the other hash table.

However, this can break down if you have two or more distinct usernames
that share a numeric identifier. This is generally a bad idea, because
the user-space model in such situations does not match the kernel-space
reality, and in particular there is no effective kernel-level security
boundary between such users, but it is sometimes done anyway.

In this case, when the second username is looked up in the userdb, it
overwrites (replaces) the entry in the hash table that is indexed by
uid, freeing the DBusUserInfo. This results in both the key and the
value in the hash table that is indexed by username becoming dangling
pointers (use-after-free), leading to undefined behaviour, which is
certainly not what we want to see when doing access control.

An equivalent situation can occur with groups, in the rare case where
a numeric group ID has two names (although I have not heard of this
being done in practice).

Solve this by reference-counting the data structure. There are up to
three references in practice: one held temporarily while the lookup
function is populating and storing it, one held by the hash table that
is indexed by uid, and one held by the hash table that is indexed by
name.

Closes: dbus#305
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 2b7948ef907669e844b52c4fa2268d6e3162a70c)

4 files changed, 86 insertions, 27 deletions

diff --git a/dbus/dbus-sysdeps-unix.h b/dbus/dbus-sysdeps-unix.h
index 279cae27..91b4c606 100644
--- a/dbus/dbus-sysdeps-unix.h
+++ b/dbus/dbus-sysdeps-unix.h
@@ -105,6 +105,7 @@ typedef struct DBusGroupInfo DBusGroupInfo;
  */
 struct DBusUserInfo
 {
+  size_t      refcount;       /**< Reference count */
   dbus_uid_t  uid;            /**< UID */
   dbus_gid_t  primary_gid;    /**< GID */
   dbus_gid_t *group_ids;      /**< Groups IDs, *including* above primary group */
@@ -118,6 +119,7 @@ struct DBusUserInfo
  */
 struct DBusGroupInfo
 {
+  size_t      refcount;       /**< Reference count */
   dbus_gid_t  gid;            /**< GID */
   char       *groupname;      /**< Group name */
 };
diff --git a/dbus/dbus-userdb-util.c b/dbus/dbus-userdb-util.c
index 8353e81f..5fba282d 100644
--- a/dbus/dbus-userdb-util.c
+++ b/dbus/dbus-userdb-util.c
@@ -38,6 +38,15 @@
  * @{
  */
 
+static DBusGroupInfo *
+_dbus_group_info_ref (DBusGroupInfo *info)
+{
+  _dbus_assert (info->refcount > 0);
+  _dbus_assert (info->refcount < SIZE_MAX);
+  info->refcount++;
+  return info;
+}
+
 /**
  * Checks to see if the UID sent in is the console user
  *
@@ -287,13 +296,14 @@ _dbus_user_database_lookup_group (DBusUserDatabase *db,
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
           return NULL;
         }
+      info->refcount = 1;
 
       if (gid != DBUS_GID_UNSET)
         {
           if (!_dbus_group_info_fill_gid (info, gid, error))
             {
               _DBUS_ASSERT_ERROR_IS_SET (error);
-              _dbus_group_info_free_allocated (info);
+              _dbus_group_info_unref (info);
               return NULL;
             }
         }
@@ -302,7 +312,7 @@ _dbus_user_database_lookup_group (DBusUserDatabase *db,
           if (!_dbus_group_info_fill (info, groupname, error))
             {
               _DBUS_ASSERT_ERROR_IS_SET (error);
-              _dbus_group_info_free_allocated (info);
+              _dbus_group_info_unref (info);
               return NULL;
             }
         }
@@ -311,23 +321,35 @@ _dbus_user_database_lookup_group (DBusUserDatabase *db,
       gid = DBUS_GID_UNSET;
       groupname = NULL;
 
-      if (!_dbus_hash_table_insert_uintptr (db->groups, info->gid, info))
+      if (_dbus_hash_table_insert_uintptr (db->groups, info->gid, info))
+        {
+          _dbus_group_info_ref (info);
+        }
+      else
         {
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
-          _dbus_group_info_free_allocated (info);
+          _dbus_group_info_unref (info);
           return NULL;
         }
 
 
-      if (!_dbus_hash_table_insert_string (db->groups_by_name,
-                                           info->groupname,
-                                           info))
+      if (_dbus_hash_table_insert_string (db->groups_by_name,
+                                          info->groupname,
+                                          info))
+        {
+          _dbus_group_info_ref (info);
+        }
+      else
         {
           _dbus_hash_table_remove_uintptr (db->groups, info->gid);
+          _dbus_group_info_unref (info);
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
           return NULL;
         }
-      
+
+      /* Release the original reference */
+      _dbus_group_info_unref (info);
+
       /* Return a borrowed reference to the DBusGroupInfo owned by the
        * two hash tables */
       return info;
diff --git a/dbus/dbus-userdb.c b/dbus/dbus-userdb.c
index b4fa35c8..19fc32e4 100644
--- a/dbus/dbus-userdb.c
+++ b/dbus/dbus-userdb.c
@@ -35,34 +35,57 @@
  * @{
  */
 
+static DBusUserInfo *
+_dbus_user_info_ref (DBusUserInfo *info)
+{
+  _dbus_assert (info->refcount > 0);
+  _dbus_assert (info->refcount < SIZE_MAX);
+  info->refcount++;
+  return info;
+}
+
 /**
- * Frees the given #DBusUserInfo's members with _dbus_user_info_free()
+ * Decrements the reference count. If it reaches 0,
+ * frees the given #DBusUserInfo's members with _dbus_user_info_free()
  * and also calls dbus_free() on the block itself
  *
  * @param info the info
  */
 void
-_dbus_user_info_free_allocated (DBusUserInfo *info)
+_dbus_user_info_unref (DBusUserInfo *info)
 {
   if (info == NULL) /* hash table will pass NULL */
     return;
 
+  _dbus_assert (info->refcount > 0);
+  _dbus_assert (info->refcount < SIZE_MAX);
+
+  if (--info->refcount > 0)
+    return;
+
   _dbus_user_info_free (info);
   dbus_free (info);
 }
 
 /**
- * Frees the given #DBusGroupInfo's members with _dbus_group_info_free()
+ * Decrements the reference count. If it reaches 0,
+ * frees the given #DBusGroupInfo's members with _dbus_group_info_free()
  * and also calls dbus_free() on the block itself
  *
  * @param info the info
  */
 void
-_dbus_group_info_free_allocated (DBusGroupInfo *info)
+_dbus_group_info_unref (DBusGroupInfo *info)
 {
   if (info == NULL) /* hash table will pass NULL */
     return;
 
+  _dbus_assert (info->refcount > 0);
+  _dbus_assert (info->refcount < SIZE_MAX);
+
+  if (--info->refcount > 0)
+    return;
+
   _dbus_group_info_free (info);
   dbus_free (info);
 }
@@ -170,13 +193,14 @@ _dbus_user_database_lookup (DBusUserDatabase *db,
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
           return NULL;
         }
+      info->refcount = 1;
 
       if (uid != DBUS_UID_UNSET)
         {
           if (!_dbus_user_info_fill_uid (info, uid, error))
             {
               _DBUS_ASSERT_ERROR_IS_SET (error);
-              _dbus_user_info_free_allocated (info);
+              _dbus_user_info_unref (info);
               return NULL;
             }
         }
@@ -185,7 +209,7 @@ _dbus_user_database_lookup (DBusUserDatabase *db,
           if (!_dbus_user_info_fill (info, username, error))
             {
               _DBUS_ASSERT_ERROR_IS_SET (error);
-              _dbus_user_info_free_allocated (info);
+              _dbus_user_info_unref (info);
               return NULL;
             }
         }
@@ -195,22 +219,33 @@ _dbus_user_database_lookup (DBusUserDatabase *db,
       username = NULL;
 
       /* insert into hash */
-      if (!_dbus_hash_table_insert_uintptr (db->users, info->uid, info))
+      if (_dbus_hash_table_insert_uintptr (db->users, info->uid, info))
+        {
+          _dbus_user_info_ref (info);
+        }
+      else
         {
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
-          _dbus_user_info_free_allocated (info);
+          _dbus_user_info_unref (info);
           return NULL;
         }
 
-      if (!_dbus_hash_table_insert_string (db->users_by_name,
-                                           info->username,
-                                           info))
+      if (_dbus_hash_table_insert_string (db->users_by_name,
+                                          info->username,
+                                          info))
+        {
+          _dbus_user_info_ref (info);
+        }
+      else
         {
           _dbus_hash_table_remove_uintptr (db->users, info->uid);
           dbus_set_error (error, DBUS_ERROR_NO_MEMORY, NULL);
+          _dbus_user_info_unref (info);
           return NULL;
         }
-      
+
+      _dbus_user_info_unref (info);
+
       /* Return a borrowed pointer to the DBusUserInfo owned by the
        * hash tables */
       return info;
@@ -560,24 +595,24 @@ _dbus_user_database_new (void)
   db->refcount = 1;
 
   db->users = _dbus_hash_table_new (DBUS_HASH_UINTPTR,
-                                    NULL, (DBusFreeFunction) _dbus_user_info_free_allocated);
+                                    NULL, (DBusFreeFunction) _dbus_user_info_unref);
   
   if (db->users == NULL)
     goto failed;
 
   db->groups = _dbus_hash_table_new (DBUS_HASH_UINTPTR,
-                                     NULL, (DBusFreeFunction) _dbus_group_info_free_allocated);
+                                     NULL, (DBusFreeFunction) _dbus_group_info_unref);
   
   if (db->groups == NULL)
     goto failed;
 
   db->users_by_name = _dbus_hash_table_new (DBUS_HASH_STRING,
-                                            NULL, NULL);
+                                            NULL, (DBusFreeFunction) _dbus_user_info_unref);
   if (db->users_by_name == NULL)
     goto failed;
   
   db->groups_by_name = _dbus_hash_table_new (DBUS_HASH_STRING,
-                                             NULL, NULL);
+                                             NULL, (DBusFreeFunction) _dbus_group_info_unref);
   if (db->groups_by_name == NULL)
     goto failed;
   
diff --git a/dbus/dbus-userdb.h b/dbus/dbus-userdb.h
index 9e9ed88a..b38e3d18 100644
--- a/dbus/dbus-userdb.h
+++ b/dbus/dbus-userdb.h
@@ -85,10 +85,10 @@ const DBusGroupInfo* _dbus_user_database_lookup_group (DBusUserDatabase *db,
                                                        dbus_gid_t        gid,
                                                        const DBusString *groupname,
                                                        DBusError        *error);
+
+void           _dbus_user_info_unref            (DBusUserInfo     *info);
 DBUS_PRIVATE_EXPORT
-void           _dbus_user_info_free_allocated   (DBusUserInfo     *info);
-DBUS_PRIVATE_EXPORT
-void           _dbus_group_info_free_allocated  (DBusGroupInfo    *info);
+void           _dbus_group_info_unref           (DBusGroupInfo    *info);
 #endif /* DBUS_USERDB_INCLUDES_PRIVATE */
 
 DBUS_PRIVATE_EXPORT