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Split proxy/client serials based on lower bit (#57)
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Separation between messages created by client and proxy was ensured by
requiring the client to use monotonically increasing serials and adding
an offset to distinguish the client message from proxy messages.

The requirement to only send messages with increasing serials cannot be
ensured by libraries godbus or zbus.

This commit instead reserves the lower_bit=0 space for client messages
and the lower_bit=1 for messages created by the proxy. This gets rid of
any serial translation mechanism and the requirement for increasing
serials.
However, it limits the possible values of serials available to the
client
to about half of the usual maximum value.

Closes #46
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@alexlarsson
alexlarsson authored May 7, 2024
2 parents 03bec4a + 4e7f20e commit 1bcfaea
Showing 1 changed file with 30 additions and 49 deletions.
79 changes: 30 additions & 49 deletions flatpak-proxy.c
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Original file line number Diff line number Diff line change
Expand Up @@ -127,16 +127,19 @@
* reply_serials, i.e. that a reply can only be sent once and by the real
* recipient of an previously sent method call.
*
* We don't however trust the serials from the client. We verify that
* they are strictly increasing to make sure the code is not confused
* by serials being reused.
* Serial numbers larger than MAX_CLIENT_SERIAL reserved for messages created by the
* proxy itself (fake messages). This limits the possible values of serials
* available to the client to the value of MAX_CLIENT_SERIAL. Versions
* older than 0.1.6 required monotonically increasing serials instead. This
* mechanism was dropped since it caused regular issues with multiple D-Bus
* clients.
*
* In order to track the ownership of the allowed names we hijack the
* connection after the initial Hello message, sending AddMatch,
* ListNames and GetNameOwner messages to get a proper view of who
* owns the names atm. Then we listen to NameOwnerChanged events for
* further updates. This causes a slight offset between serials in the
* client and serials as seen by the bus.
* further updates. This causes some serials abow MAX_CLIENT_SERIAL to be
* used for "fake messages".
*
* After that the filter is strictly passive, in that we never
* construct our own requests. For each message received from the
Expand Down Expand Up @@ -192,6 +195,9 @@ typedef struct FlatpakProxyClient FlatpakProxyClient;
#define AUTH_LINE_SENTINEL "\r\n"
#define AUTH_BEGIN "BEGIN"

// Use a relatively hight number since there are not a lot of fake requests we need to do
#define MAX_CLIENT_SERIAL (G_MAXUINT32 - 65536)

typedef enum {
EXPECTED_REPLY_NONE,
EXPECTED_REPLY_NORMAL,
Expand Down Expand Up @@ -292,9 +298,8 @@ struct FlatpakProxyClient
ProxySide bus_side;

/* Filtering data: */
guint32 serial_offset;
guint32 hello_serial;
guint32 last_serial;
guint32 last_fake_serial;
GHashTable *rewrite_reply;
GHashTable *get_owner_reply;

Expand Down Expand Up @@ -442,6 +447,7 @@ flatpak_proxy_client_init (FlatpakProxyClient *client)
init_side (client, &client->client_side);
init_side (client, &client->bus_side);

client->last_fake_serial = MAX_CLIENT_SERIAL;
client->auth_buffer = g_byte_array_new ();
client->rewrite_reply = g_hash_table_new_full (g_direct_hash, g_direct_equal, NULL, g_object_unref);
client->get_owner_reply = g_hash_table_new_full (g_direct_hash, g_direct_equal, NULL, g_free);
Expand Down Expand Up @@ -1068,15 +1074,6 @@ read_uint32 (Header *header, guint8 *ptr)
return GUINT32_FROM_LE (*(guint32 *) ptr);
}

static void
write_uint32 (Header *header, guint8 *ptr, guint32 val)
{
if (header->big_endian)
*(guint32 *) ptr = GUINT32_TO_BE (val);
else
*(guint32 *) ptr = GUINT32_TO_LE (val);
}

static inline guint32
align_by_8 (guint32 offset)
{
Expand Down Expand Up @@ -1185,12 +1182,11 @@ header_debug_str (GString *s, Header *header)
}

static Header *
parse_header (Buffer *buffer, guint32 serial_offset, guint32 reply_serial_offset, guint32 hello_serial, GError **error)
parse_header (Buffer *buffer, GError **error)
{
guint32 array_len, header_len;
guint32 offset, end_offset;
guint8 header_type;
guint32 reply_serial_pos = 0;
const char *signature;
g_autoptr(GError) str_error = NULL;
g_autoptr(GString) header_str = NULL;
Expand Down Expand Up @@ -1417,7 +1413,6 @@ parse_header (Buffer *buffer, guint32 serial_offset, guint32 reply_serial_offset
}

header->has_reply_serial = TRUE;
reply_serial_pos = offset;
header->reply_serial = read_uint32 (header, &buffer->data[offset]);
offset += 4;
break;
Expand Down Expand Up @@ -1593,17 +1588,6 @@ parse_header (Buffer *buffer, guint32 serial_offset, guint32 reply_serial_offset
return NULL;
}

if (serial_offset > 0)
{
header->serial += serial_offset;
write_uint32 (header, &buffer->data[8], header->serial);
}

if (reply_serial_offset > 0 &&
header->has_reply_serial &&
header->reply_serial > hello_serial + reply_serial_offset)
write_uint32 (header, &buffer->data[reply_serial_pos], header->reply_serial - reply_serial_offset);

return g_steal_pointer (&header);
}

Expand Down Expand Up @@ -1850,7 +1834,7 @@ get_error_for_header (FlatpakProxyClient *client, Header *header, const char *er
reply = g_dbus_message_new ();
g_dbus_message_set_message_type (reply, G_DBUS_MESSAGE_TYPE_ERROR);
g_dbus_message_set_flags (reply, G_DBUS_MESSAGE_FLAGS_NO_REPLY_EXPECTED);
g_dbus_message_set_reply_serial (reply, header->serial - client->serial_offset);
g_dbus_message_set_reply_serial (reply, header->serial);
g_dbus_message_set_error_name (reply, error);
g_dbus_message_set_body (reply, g_variant_new ("(s)", error));

Expand All @@ -1865,7 +1849,7 @@ get_bool_reply_for_header (FlatpakProxyClient *client, Header *header, gboolean
reply = g_dbus_message_new ();
g_dbus_message_set_message_type (reply, G_DBUS_MESSAGE_TYPE_METHOD_RETURN);
g_dbus_message_set_flags (reply, G_DBUS_MESSAGE_FLAGS_NO_REPLY_EXPECTED);
g_dbus_message_set_reply_serial (reply, header->serial - client->serial_offset);
g_dbus_message_set_reply_serial (reply, header->serial);
g_dbus_message_set_body (reply, g_variant_new ("(b)", val));

return reply;
Expand Down Expand Up @@ -2293,14 +2277,14 @@ queue_fake_message (FlatpakProxyClient *client, GDBusMessage *message, ExpectedR
{
Buffer *buffer;

client->last_serial++;
client->serial_offset++;
g_dbus_message_set_serial (message, client->last_serial);
client->last_fake_serial++;
g_assert (client->last_fake_serial > MAX_CLIENT_SERIAL);
g_dbus_message_set_serial (message, client->last_fake_serial);
buffer = message_to_buffer (message);
g_object_unref (message);

queue_outgoing_buffer (&client->bus_side, buffer);
queue_expected_reply (&client->client_side, client->last_serial, reply_type);
queue_expected_reply (&client->client_side, client->last_fake_serial, reply_type);
}

/* After the first Hello message we need to synthesize a bunch of messages to synchronize the
Expand Down Expand Up @@ -2346,18 +2330,18 @@ queue_initial_name_ops (FlatpakProxyClient *client)
queue_fake_message (client, message, EXPECTED_REPLY_FILTER);

if (client->proxy->log_messages)
g_print ("C%d: -> org.freedesktop.DBus fake %sAddMatch for %s\n", client->last_serial, name_needs_subtree ? "wildcarded " : "", name);
g_print ("C%d: -> org.freedesktop.DBus fake %sAddMatch for %s\n", client->last_fake_serial, name_needs_subtree ? "wildcarded " : "", name);

if (!name_needs_subtree)
{
/* Get the current owner of the name (if any) so we can apply policy to it */
message = g_dbus_message_new_method_call ("org.freedesktop.DBus", "/", "org.freedesktop.DBus", "GetNameOwner");
g_dbus_message_set_body (message, g_variant_new ("(s)", name));
queue_fake_message (client, message, EXPECTED_REPLY_FAKE_GET_NAME_OWNER);
g_hash_table_replace (client->get_owner_reply, GINT_TO_POINTER (client->last_serial), g_strdup (name));
g_hash_table_replace (client->get_owner_reply, GINT_TO_POINTER (client->last_fake_serial), g_strdup (name));

if (client->proxy->log_messages)
g_print ("C%d: -> org.freedesktop.DBus fake GetNameOwner for %s\n", client->last_serial, name);
g_print ("C%d: -> org.freedesktop.DBus fake GetNameOwner for %s\n", client->last_fake_serial, name);
}
else
has_wildcards = TRUE; /* Send ListNames below */
Expand All @@ -2375,7 +2359,7 @@ queue_initial_name_ops (FlatpakProxyClient *client)
queue_fake_message (client, message, EXPECTED_REPLY_FAKE_LIST_NAMES);

if (client->proxy->log_messages)
g_print ("C%d: -> org.freedesktop.DBus fake ListNames\n", client->last_serial);
g_print ("C%d: -> org.freedesktop.DBus fake ListNames\n", client->last_fake_serial);

/* Stop reading from the client, to avoid incoming messages fighting with the ListNames roundtrip.
We will start it again once we have handled the ListNames reply */
Expand Down Expand Up @@ -2417,10 +2401,10 @@ queue_wildcard_initial_name_ops (FlatpakProxyClient *client, Header *header, Buf
GDBusMessage *message = g_dbus_message_new_method_call ("org.freedesktop.DBus", "/", "org.freedesktop.DBus", "GetNameOwner");
g_dbus_message_set_body (message, g_variant_new ("(s)", name));
queue_fake_message (client, message, EXPECTED_REPLY_FAKE_GET_NAME_OWNER);
g_hash_table_replace (client->get_owner_reply, GINT_TO_POINTER (client->last_serial), g_strdup (name));
g_hash_table_replace (client->get_owner_reply, GINT_TO_POINTER (client->last_fake_serial), g_strdup (name));

if (client->proxy->log_messages)
g_print ("C%d: -> org.freedesktop.DBus fake GetNameOwner for %s\n", client->last_serial, name);
g_print ("C%d: -> org.freedesktop.DBus fake GetNameOwner for %s\n", client->last_fake_serial, name);
}
}
}
Expand All @@ -2439,7 +2423,7 @@ got_buffer_from_client (FlatpakProxyClient *client, ProxySide *side, Buffer *buf

/* Filter and rewrite outgoing messages as needed */

header = parse_header (buffer, client->serial_offset, 0, 0, &error);
header = parse_header (buffer, &error);
if (header == NULL)
{
g_warning ("Invalid message header format from client: %s",
Expand All @@ -2452,16 +2436,13 @@ got_buffer_from_client (FlatpakProxyClient *client, ProxySide *side, Buffer *buf
if (!update_socket_messages (side, buffer, header))
return;

/* Make sure the client is not playing games with the serials, as that
could confuse us. */
if (header->serial <= client->last_serial)
if (header->serial > MAX_CLIENT_SERIAL)
{
g_warning ("Invalid client serial");
g_warning ("Invalid client serial: Exceeds maximum value of %u", MAX_CLIENT_SERIAL);
side_closed (side);
buffer_unref (buffer);
return;
}
client->last_serial = header->serial;

if (client->proxy->log_messages)
print_outgoing_header (header);
Expand Down Expand Up @@ -2611,7 +2592,7 @@ got_buffer_from_bus (FlatpakProxyClient *client, ProxySide *side, Buffer *buffer

/* Filter and rewrite incoming messages as needed */

header = parse_header (buffer, 0, client->serial_offset, client->hello_serial, &error);
header = parse_header (buffer, &error);
if (header == NULL)
{
g_warning ("Invalid message header format from bus: %s",
Expand Down

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