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glib/gio/gdbusauthmechanismexternal.c
Simon McVittie e0a0749268 gdbusauthmechanismexternal: Optionally send empty authorization identity 
When using a GDBus client in a non-trivial user namespace, the result of
geteuid() can differ from the uid in the namespace where the server is
running. This would result in connection attempts being rejected, because
the identity that the client claims to have does not match the identity
that the server derives from its credentials.

RFC 4422 allows us to send an empty authorization identity, which means we
want to authenticate as whatever identity the server can derive from our
out-of-band credentials. In particular, this resolves the authentication
failure when crossing between different Linux user namespaces.

Because D-Bus does not have a way to represent an empty initial response
as distinct from the absence of an initial response, we cannot use the
initial-response optimization (RFC 4422 §4.3.a) in this case, and must
fall back to waiting for the server to send a challenge.

Unfortunately, GDBus versions older than glib!2826 did not implement
the server side of this protocol correctly, and would respond to the
missing initial response in a way that breaks the SASL state machine
(expecting a response without sending a challenge), causing client and
server to deadlock with each waiting for the other to respond. Until
fixed versions of GDBus are widespread, we can't rely on having a server
that can cope with this, so gate it behind a flag, which can be set for
connections that are known to cross non-trivial namespace boundaries.

Originally inspired by
<1ed4723d38>,
and based on earlier work by Giuseppe Scrivano (in which the
cross-namespace behaviour was unconditional, rather than gated by a
flag).

Co-authored-by: Giuseppe Scrivano <giuseppe@scrivano.org>
Signed-off-by: Simon McVittie <smcv@collabora.com>
2022-07-24 13:46:26 +01:00

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/* GDBus - GLib D-Bus Library
*
* Copyright (C) 2008-2010 Red Hat, Inc.
*
* SPDX-License-Identifier: LGPL-2.1-or-later
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General
* Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
*
* Author: David Zeuthen <davidz@redhat.com>
*/
#include "config.h"
#include <string.h>
#include "gdbusauthmechanismexternal.h"
#include "gcredentials.h"
#include "gdbuserror.h"
#include "gioenumtypes.h"
#include "glibintl.h"
#ifdef G_OS_WIN32
#include "gwin32sid.h"
#endif
struct _GDBusAuthMechanismExternalPrivate
{
gboolean is_client;
gboolean is_server;
GDBusAuthMechanismState state;
gboolean empty_data_sent;
};
static gint mechanism_get_priority (void);
static const gchar *mechanism_get_name (void);
static gboolean mechanism_is_supported (GDBusAuthMechanism *mechanism);
static gchar *mechanism_encode_data (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len,
gsize *out_data_len);
static gchar *mechanism_decode_data (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len,
gsize *out_data_len);
static GDBusAuthMechanismState mechanism_server_get_state (GDBusAuthMechanism *mechanism);
static void mechanism_server_initiate (GDBusAuthMechanism *mechanism,
const gchar *initial_response,
gsize initial_response_len);
static void mechanism_server_data_receive (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len);
static gchar *mechanism_server_data_send (GDBusAuthMechanism *mechanism,
gsize *out_data_len);
static gchar *mechanism_server_get_reject_reason (GDBusAuthMechanism *mechanism);
static void mechanism_server_shutdown (GDBusAuthMechanism *mechanism);
static GDBusAuthMechanismState mechanism_client_get_state (GDBusAuthMechanism *mechanism);
static gchar *mechanism_client_initiate (GDBusAuthMechanism *mechanism,
GDBusConnectionFlags conn_flags,
gsize *out_initial_response_len);
static void mechanism_client_data_receive (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len);
static gchar *mechanism_client_data_send (GDBusAuthMechanism *mechanism,
gsize *out_data_len);
static void mechanism_client_shutdown (GDBusAuthMechanism *mechanism);
/* ---------------------------------------------------------------------------------------------------- */
G_DEFINE_TYPE_WITH_PRIVATE (GDBusAuthMechanismExternal, _g_dbus_auth_mechanism_external, G_TYPE_DBUS_AUTH_MECHANISM)
/* ---------------------------------------------------------------------------------------------------- */
static void
_g_dbus_auth_mechanism_external_finalize (GObject *object)
{
//GDBusAuthMechanismExternal *mechanism = G_DBUS_AUTH_MECHANISM_EXTERNAL (object);
if (G_OBJECT_CLASS (_g_dbus_auth_mechanism_external_parent_class)->finalize != NULL)
G_OBJECT_CLASS (_g_dbus_auth_mechanism_external_parent_class)->finalize (object);
}
static void
_g_dbus_auth_mechanism_external_class_init (GDBusAuthMechanismExternalClass *klass)
{
GObjectClass *gobject_class;
GDBusAuthMechanismClass *mechanism_class;
gobject_class = G_OBJECT_CLASS (klass);
gobject_class->finalize = _g_dbus_auth_mechanism_external_finalize;
mechanism_class = G_DBUS_AUTH_MECHANISM_CLASS (klass);
mechanism_class->get_name = mechanism_get_name;
mechanism_class->get_priority = mechanism_get_priority;
mechanism_class->is_supported = mechanism_is_supported;
mechanism_class->encode_data = mechanism_encode_data;
mechanism_class->decode_data = mechanism_decode_data;
mechanism_class->server_get_state = mechanism_server_get_state;
mechanism_class->server_initiate = mechanism_server_initiate;
mechanism_class->server_data_receive = mechanism_server_data_receive;
mechanism_class->server_data_send = mechanism_server_data_send;
mechanism_class->server_get_reject_reason = mechanism_server_get_reject_reason;
mechanism_class->server_shutdown = mechanism_server_shutdown;
mechanism_class->client_get_state = mechanism_client_get_state;
mechanism_class->client_initiate = mechanism_client_initiate;
mechanism_class->client_data_receive = mechanism_client_data_receive;
mechanism_class->client_data_send = mechanism_client_data_send;
mechanism_class->client_shutdown = mechanism_client_shutdown;
}
static void
_g_dbus_auth_mechanism_external_init (GDBusAuthMechanismExternal *mechanism)
{
mechanism->priv = _g_dbus_auth_mechanism_external_get_instance_private (mechanism);
}
/* ---------------------------------------------------------------------------------------------------- */
static gboolean
mechanism_is_supported (GDBusAuthMechanism *mechanism)
{
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), FALSE);
#if defined(G_OS_WIN32)
/* all that is required is current process SID */
return TRUE;
#else
/* This mechanism is only available if credentials has been exchanged */
if (_g_dbus_auth_mechanism_get_credentials (mechanism) != NULL)
return TRUE;
else
return FALSE;
#endif
}
static gint
mechanism_get_priority (void)
{
/* We prefer EXTERNAL to most other mechanism (DBUS_COOKIE_SHA1 and ANONYMOUS) */
return 100;
}
static const gchar *
mechanism_get_name (void)
{
return "EXTERNAL";
}
static gchar *
mechanism_encode_data (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len,
gsize *out_data_len)
{
return NULL;
}
static gchar *
mechanism_decode_data (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len,
gsize *out_data_len)
{
return NULL;
}
/* ---------------------------------------------------------------------------------------------------- */
static GDBusAuthMechanismState
mechanism_server_get_state (GDBusAuthMechanism *mechanism)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), G_DBUS_AUTH_MECHANISM_STATE_INVALID);
g_return_val_if_fail (m->priv->is_server && !m->priv->is_client, G_DBUS_AUTH_MECHANISM_STATE_INVALID);
return m->priv->state;
}
static gboolean
data_matches_credentials (const gchar *data,
gsize data_len,
GCredentials *credentials)
{
gboolean match;
match = FALSE;
if (credentials == NULL)
goto out;
#if defined(G_OS_UNIX)
{
gint64 alleged_uid;
gchar *endp;
/* If we were unable to find out the uid, then nothing
* can possibly match it. */
if (g_credentials_get_unix_user (credentials, NULL) == (uid_t) -1)
goto out;
/* An empty authorization identity means we want to be
* whatever identity the out-of-band credentials say we have
* (RFC 4422 appendix A.1). This effectively matches any uid. */
if (data == NULL || data_len == 0)
{
match = TRUE;
goto out;
}
/* on UNIX, this is the uid as a string in base 10 */
alleged_uid = g_ascii_strtoll (data, &endp, 10);
if (*endp == '\0')
{
if (g_credentials_get_unix_user (credentials, NULL) == alleged_uid)
{
match = TRUE;
}
}
}
#else
/* TODO: Dont know how to compare credentials on this OS. Please implement. */
#endif
out:
return match;
}
static void
mechanism_server_initiate (GDBusAuthMechanism *mechanism,
const gchar *initial_response,
gsize initial_response_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism));
g_return_if_fail (!m->priv->is_server && !m->priv->is_client);
m->priv->is_server = TRUE;
if (initial_response != NULL)
{
if (data_matches_credentials (initial_response,
initial_response_len,
_g_dbus_auth_mechanism_get_credentials (mechanism)))
{
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_ACCEPTED;
}
else
{
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_REJECTED;
}
}
else
{
/* The initial-response optimization was not used, so we need to
* send an empty challenge to prompt the client to respond. */
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_HAVE_DATA_TO_SEND;
}
}
static void
mechanism_server_data_receive (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism));
g_return_if_fail (m->priv->is_server && !m->priv->is_client);
g_return_if_fail (m->priv->state == G_DBUS_AUTH_MECHANISM_STATE_WAITING_FOR_DATA);
if (data_matches_credentials (data,
data_len,
_g_dbus_auth_mechanism_get_credentials (mechanism)))
{
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_ACCEPTED;
}
else
{
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_REJECTED;
}
}
static gchar *
mechanism_server_data_send (GDBusAuthMechanism *mechanism,
gsize *out_data_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), NULL);
g_return_val_if_fail (m->priv->is_server && !m->priv->is_client, NULL);
if (out_data_len)
*out_data_len = 0;
if (m->priv->empty_data_sent)
{
/* We have already sent an empty data response.
Reject the connection. */
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_REJECTED;
return NULL;
}
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_WAITING_FOR_DATA;
m->priv->empty_data_sent = TRUE;
return g_strdup ("");
}
static gchar *
mechanism_server_get_reject_reason (GDBusAuthMechanism *mechanism)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), NULL);
g_return_val_if_fail (m->priv->is_server && !m->priv->is_client, NULL);
g_return_val_if_fail (m->priv->state == G_DBUS_AUTH_MECHANISM_STATE_REJECTED, NULL);
/* can never end up here because we are never in the REJECTED state */
g_assert_not_reached ();
return NULL;
}
static void
mechanism_server_shutdown (GDBusAuthMechanism *mechanism)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism));
g_return_if_fail (m->priv->is_server && !m->priv->is_client);
m->priv->is_server = FALSE;
}
/* ---------------------------------------------------------------------------------------------------- */
static GDBusAuthMechanismState
mechanism_client_get_state (GDBusAuthMechanism *mechanism)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), G_DBUS_AUTH_MECHANISM_STATE_INVALID);
g_return_val_if_fail (m->priv->is_client && !m->priv->is_server, G_DBUS_AUTH_MECHANISM_STATE_INVALID);
return m->priv->state;
}
static gchar *
mechanism_client_initiate (GDBusAuthMechanism *mechanism,
GDBusConnectionFlags conn_flags,
gsize *out_initial_response_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
gchar *initial_response = NULL;
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), NULL);
g_return_val_if_fail (!m->priv->is_server && !m->priv->is_client, NULL);
m->priv->is_client = TRUE;
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_WAITING_FOR_DATA;
*out_initial_response_len = 0;
if (conn_flags & G_DBUS_CONNECTION_FLAGS_CROSS_NAMESPACE)
{
/* If backwards-compatibility with GDBus servers < 2.73.3 is not a
* concern, we do not send an initial response, because there is
* no way to express an empty authorization identity this way.
* Instead, we'll reply to the server's first (empty) challenge
* with an empty authorization identity in our first response. */
g_debug ("Using cross-namespace EXTERNAL authentication (this will deadlock if server is GDBus < 2.73.3)");
}
else
{
/* Send the Unix uid or Windows SID as an initial response.
* This is the only thing that is interoperable with GDBus 2.73.3
* servers. */
#if defined(G_OS_UNIX)
GCredentials *credentials;
credentials = _g_dbus_auth_mechanism_get_credentials (mechanism);
g_assert (credentials != NULL);
initial_response = g_strdup_printf ("%" G_GINT64_FORMAT, (gint64) g_credentials_get_unix_user (credentials, NULL));
#elif defined(G_OS_WIN32)
initial_response = _g_win32_current_process_sid_string (NULL);
#else
/* GDBus < 2.73.3 servers can't have worked on this platform anyway,
* so it isn't a regression to behave as though
* G_DBUS_CONNECTION_FLAGS_CROSS_NAMESPACE had been set. */
g_debug ("Unknown platform, cannot use initial response in EXTERNAL");
#endif
}
if (initial_response)
{
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_ACCEPTED;
*out_initial_response_len = strlen (initial_response);
}
return initial_response;
}
static void
mechanism_client_data_receive (GDBusAuthMechanism *mechanism,
const gchar *data,
gsize data_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism));
g_return_if_fail (m->priv->is_client && !m->priv->is_server);
g_return_if_fail (m->priv->state == G_DBUS_AUTH_MECHANISM_STATE_WAITING_FOR_DATA);
/* The server sent us a challenge, which should normally
* be empty. We respond with our authorization identity. */
m->priv->state = G_DBUS_AUTH_MECHANISM_STATE_HAVE_DATA_TO_SEND;
}
static gchar *
mechanism_client_data_send (GDBusAuthMechanism *mechanism,
gsize *out_data_len)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_val_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism), NULL);
g_return_val_if_fail (m->priv->is_client && !m->priv->is_server, NULL);
g_return_val_if_fail (m->priv->state == G_DBUS_AUTH_MECHANISM_STATE_HAVE_DATA_TO_SEND, NULL);
/* We respond to the server's challenge by sending our
* authorization identity, which is the empty string, meaning
* whoever the out-of-band credentials say we are. */
*out_data_len = 0;
return g_strdup ("");
}
static void
mechanism_client_shutdown (GDBusAuthMechanism *mechanism)
{
GDBusAuthMechanismExternal *m = G_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism);
g_return_if_fail (G_IS_DBUS_AUTH_MECHANISM_EXTERNAL (mechanism));
g_return_if_fail (m->priv->is_client && !m->priv->is_server);
m->priv->is_client = FALSE;
}
/* ---------------------------------------------------------------------------------------------------- */
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