Commit dc94fe3d authored by Simon McVittie's avatar Simon McVittie
Browse files

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's avatarSimon McVittie <smcv@collabora.com>
(cherry picked from commit 2b7948ef)
parent 4ed1b320
......@@ -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 */
};
......
......@@ -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;
......
......@@ -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;
......
......@@ -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
......
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