CVE-2015-4625: Bind use of cookies to specific uids

http://lists.freedesktop.org/archives/polkit-devel/2015-June/000425.html

The "cookie" value that Polkit hands out is global to all polkit
users.  And when `AuthenticationAgentResponse` is invoked, we
previously only received the cookie and *target* identity, and
attempted to find an agent from that.

The problem is that the current cookie is just an integer
counter, and if it overflowed, it would be possible for
an successful authorization in one session to trigger a response
in another session.

The overflow and ability to guess the cookie were fixed by the
previous patch.

This patch is conceptually further hardening on top of that.  Polkit
currently treats uids as equivalent from a security domain
perspective; there is no support for
SELinux/AppArmor/etc. differentiation.

We can retrieve the uid from `getuid()` in the setuid helper, which
allows us to ensure the uid invoking `AuthenticationAgentResponse2`
matches that of the agent.

Then the authority only looks at authentication sessions matching the
cookie that were created by a matching uid, thus removing the ability
for different uids to interfere with each other entirely.

Several fixes to this patch were contributed by:
Miloslav Trmač <mitr@redhat.com>

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=90837
CVE: CVE-2015-4625
Reported-by: Tavis Ormandy <taviso@google.com>
Reviewed-by: Miloslav Trmač <mitr@redhat.com>
Signed-off-by: Colin Walters <walters@redhat.com>

8 files changed, 198 insertions, 19 deletions

diff --git a/data/org.freedesktop.PolicyKit1.AuthenticationAgent.xml b/data/org.freedesktop.PolicyKit1.AuthenticationAgent.xml
index 3b519c2..5beef7d 100644
--- a/data/org.freedesktop.PolicyKit1.AuthenticationAgent.xml
+++ b/data/org.freedesktop.PolicyKit1.AuthenticationAgent.xml
@@ -8,7 +8,19 @@
     <annotation name="org.gtk.EggDBus.DocString" value="<para>This D-Bus interface is used for communication between the system-wide PolicyKit daemon and one or more authentication agents each running in a user session.</para><para>An authentication agent must implement this interface and register (passing the object path of the object implementing the interface) using the org.freedesktop.PolicyKit1.Authority.RegisterAuthenticationAgent() and org.freedesktop.PolicyKit1.Authority.UnregisterAuthenticationAgent() methods on the #org.freedesktop.PolicyKit1.Authority interface of the PolicyKit daemon.</para>"/>
 
     <method name="BeginAuthentication">
-      <annotation name="org.gtk.EggDBus.DocString" value="<para>Called by the PolicyKit daemon when the authentication agent needs the user to authenticate as one of the identities in @identities for the action with the identifier @action_id.</para><para>Upon succesful authentication, the authentication agent must invoke the org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse() method on the #org.freedesktop.PolicyKit1.Authority interface of the PolicyKit daemon before returning.</para><para>If the user dismisses the authentication dialog, the authentication agent should return an error.</para>"/>
+      <annotation name="org.gtk.EggDBus.DocString" value="<para>Called
+      by the PolicyKit daemon when the authentication agent needs the
+      user to authenticate as one of the identities in @identities for
+      the action with the identifier @action_id.</para><para>This
+      authentication is normally achieved via the
+      polkit_agent_session_response() API, which invokes a private
+      setuid helper process to verify the authentication.  When
+      successful, it calls the
+      org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse2()
+      method on the #org.freedesktop.PolicyKit1.Authority interface of
+      the PolicyKit daemon before returning.  If the user dismisses the
+      authentication dialog, the authentication agent should call
+      polkit_agent_session_cancel().</para>"/>
 
       <arg name="action_id" direction="in" type="s">
         <annotation name="org.gtk.EggDBus.DocString" value="The identifier for the action that the user is authentication for."/>
diff --git a/data/org.freedesktop.PolicyKit1.Authority.xml b/data/org.freedesktop.PolicyKit1.Authority.xml
index fbfb9cd..f9021ee 100644
--- a/data/org.freedesktop.PolicyKit1.Authority.xml
+++ b/data/org.freedesktop.PolicyKit1.Authority.xml
@@ -313,7 +313,29 @@
     </method>
 
     <method name="AuthenticationAgentResponse">
-      <annotation name="org.gtk.EggDBus.DocString" value="Method for authentication agents to invoke on successful authentication. This method will fail unless a sufficiently privileged caller invokes it."/>
+      <annotation name="org.gtk.EggDBus.DocString" value="Method for authentication agents to invoke on successful
+authentication, intended only for use by a privileged helper process
+internal to polkit."/>
+
+      <arg name="cookie" direction="in" type="s">
+        <annotation name="org.gtk.EggDBus.DocString" value="The cookie identifying the authentication request that was passed to the authentication agent."/>
+      </arg>
+
+      <arg name="identity" direction="in" type="(sa{sv})">
+        <annotation name="org.gtk.EggDBus.Type" value="Identity"/>
+        <annotation name="org.gtk.EggDBus.DocString" value="A #Identity struct describing what identity was authenticated."/>
+      </arg>
+    </method>
+
+    <method name="AuthenticationAgentResponse2">
+      <annotation name="org.gtk.EggDBus.DocString" value="Method for authentication agents to invoke on successful
+authentication, intended only for use by a privileged helper process
+internal to polkit.   Note this method was added in 0.114, and should be preferred over AuthenticationAgentResponse
+as it fixes a security issue."/>
+
+      <arg name="uid" direction="in" type="u">
+        <annotation name="org.gtk.EggDBus.DocString" value="The real uid of the agent.  Normally set by the setuid helper program."/>
+      </arg>
 
       <arg name="cookie" direction="in" type="s">
         <annotation name="org.gtk.EggDBus.DocString" value="The cookie identifying the authentication request that was passed to the authentication agent."/>
diff --git a/docs/polkit/docbook-interface-org.freedesktop.PolicyKit1.Authority.xml b/docs/polkit/docbook-interface-org.freedesktop.PolicyKit1.Authority.xml
index 6525e25..e66bf53 100644
--- a/docs/polkit/docbook-interface-org.freedesktop.PolicyKit1.Authority.xml
+++ b/docs/polkit/docbook-interface-org.freedesktop.PolicyKit1.Authority.xml
@@ -42,6 +42,8 @@ Structure    <link linkend="eggdbus-struct-TemporaryAuthorization">TemporaryAuth
                                   IN  String                         object_path)
 <link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse">AuthenticationAgentResponse</link>      (IN  String                         cookie,
                                   IN  <link linkend="eggdbus-struct-Identity">Identity</link>                       identity)
+<link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse">AuthenticationAgentResponse2</link>      (IN uint32 uid, IN  String                         cookie,
+                                  IN  <link linkend="eggdbus-struct-Identity">Identity</link>                       identity)
 <link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.EnumerateTemporaryAuthorizations">EnumerateTemporaryAuthorizations</link> (IN  <link linkend="eggdbus-struct-Subject">Subject</link>                        subject,
                                   OUT Array&lt;<link linkend="eggdbus-struct-TemporaryAuthorization">TemporaryAuthorization</link>&gt;  temporary_authorizations)
 <link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.RevokeTemporaryAuthorizations">RevokeTemporaryAuthorizations</link>    (IN  <link linkend="eggdbus-struct-Subject">Subject</link>                        subject)
@@ -777,10 +779,52 @@ AuthenticationAgentResponse (IN  String    cookie,
                              IN  <link linkend="eggdbus-struct-Identity">Identity</link>  identity)
     </programlisting>
     <para>
-Method for authentication agents to invoke on successful authentication. This method will fail unless a sufficiently privileged caller invokes it.
+Method for authentication agents to invoke on successful
+authentication, intended only for use by a privileged helper process
+internal to polkit.  Deprecated in favor of AuthenticationAgentResponse2.
+    </para>
+<variablelist role="params">
+  <varlistentry>
+    <term><literal>IN  String <parameter>cookie</parameter></literal>:</term>
+    <listitem>
+      <para>
+The cookie identifying the authentication request that was passed to the authentication agent.
+      </para>
+    </listitem>
+  </varlistentry>
+  <varlistentry>
+    <term><literal>IN  <link linkend="eggdbus-struct-Identity">Identity</link> <parameter>identity</parameter></literal>:</term>
+    <listitem>
+      <para>
+A <link linkend="eggdbus-struct-Identity">Identity</link> struct describing what identity was authenticated.
+      </para>
+    </listitem>
+  </varlistentry>
+</variablelist>
+    </refsect2>
+    <refsect2 role="function" id="eggdbus-method-org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse2">
+      <title>AuthenticationAgentResponse2 ()</title>
+    <programlisting>
+AuthenticationAgentResponse2 (IN  uint32 uid,
+                              IN  String cookie,
+                              IN  <link linkend="eggdbus-struct-Identity">Identity</link>  identity)
+    </programlisting>
+    <para>
+Method for authentication agents to invoke on successful
+authentication, intended only for use by a privileged helper process
+internal to polkit.  Note this method was introduced in 0.114 to fix a security issue.
     </para>
 <variablelist role="params">
   <varlistentry>
+    <term><literal>IN  uint32 <parameter>uid</parameter></literal>:</term>
+    <listitem>
+      <para>
+The user id of the agent; normally this is the owner of the parent pid
+of the process that invoked the internal setuid helper.
+      </para>
+    </listitem>
+  </varlistentry>
+  <varlistentry>
     <term><literal>IN  String <parameter>cookie</parameter></literal>:</term>
     <listitem>
       <para>
diff --git a/docs/polkit/overview.xml b/docs/polkit/overview.xml
index 150a7bc..176d2ea 100644
--- a/docs/polkit/overview.xml
+++ b/docs/polkit/overview.xml
@@ -314,16 +314,18 @@
     <para>
       Authentication agents are provided by desktop environments. When
       an user session starts, the agent registers with the polkit
-      Authority using
-      the <link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.RegisterAuthenticationAgent">RegisterAuthenticationAgent()</link>
+      Authority using the <link
+      linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.RegisterAuthenticationAgent">RegisterAuthenticationAgent()</link>
       method. When services are needed, the authority will invoke
-      methods on
-      the <link linkend="eggdbus-interface-org.freedesktop.PolicyKit1.AuthenticationAgent">org.freedesktop.PolicyKit1.AuthenticationAgent</link>
+      methods on the <link
+      linkend="eggdbus-interface-org.freedesktop.PolicyKit1.AuthenticationAgent">org.freedesktop.PolicyKit1.AuthenticationAgent</link>
       D-Bus interface. Once the user is authenticated, (a privileged
-      part of) the agent invokes
-      the <link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse">AuthenticationAgentResponse()</link>
-      method.  Note that the polkit Authority itself does not care
-      how the agent authenticates the user.
+      part of) the agent invokes the <link
+      linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.AuthenticationAgentResponse">AuthenticationAgentResponse()</link>
+      method.  This method should be treated as an internal
+      implementation detail, and callers should use the public shared
+      library API to invoke it, which currently uses a setuid helper
+      program.
     </para>
     <para>
       The <link linkend="ref-authentication-agent-api">libpolkit-agent-1</link>
diff --git a/src/polkit/polkitauthority.c b/src/polkit/polkitauthority.c
index ab6d3cd..6bd684a 100644
--- a/src/polkit/polkitauthority.c
+++ b/src/polkit/polkitauthority.c
@@ -1492,6 +1492,14 @@ polkit_authority_authentication_agent_response (PolkitAuthority      *authority,
                                                 gpointer              user_data)
 {
   GVariant *identity_value;
+  /* Note that in reality, this API is only accessible to root, and
+   * only called from the setuid helper `polkit-agent-helper-1`.
+   *
+   * However, because this is currently public API, we avoid
+   * triggering warnings from ABI diff type programs by just grabbing
+   * the real uid of the caller here.
+   */
+  uid_t uid = getuid ();
 
   g_return_if_fail (POLKIT_IS_AUTHORITY (authority));
   g_return_if_fail (cookie != NULL);
@@ -1501,8 +1509,9 @@ polkit_authority_authentication_agent_response (PolkitAuthority      *authority,
   identity_value = polkit_identity_to_gvariant (identity);
   g_variant_ref_sink (identity_value);
   g_dbus_proxy_call (authority->proxy,
-                     "AuthenticationAgentResponse",
-                     g_variant_new ("(s@(sa{sv}))",
+                     "AuthenticationAgentResponse2",
+                     g_variant_new ("(us@(sa{sv}))",
+                                    (guint32)uid,
                                     cookie,
                                     identity_value),
                      G_DBUS_CALL_FLAGS_NONE,
diff --git a/src/polkitbackend/polkitbackendauthority.c b/src/polkitbackend/polkitbackendauthority.c
index 601a974..03a4e84 100644
--- a/src/polkitbackend/polkitbackendauthority.c
+++ b/src/polkitbackend/polkitbackendauthority.c
@@ -355,6 +355,7 @@ polkit_backend_authority_unregister_authentication_agent (PolkitBackendAuthority
 gboolean
 polkit_backend_authority_authentication_agent_response (PolkitBackendAuthority    *authority,
                                                         PolkitSubject             *caller,
+                                                        uid_t                      uid,
                                                         const gchar               *cookie,
                                                         PolkitIdentity            *identity,
                                                         GError                   **error)
@@ -373,7 +374,7 @@ polkit_backend_authority_authentication_agent_response (PolkitBackendAuthority
     }
   else
     {
-      return klass->authentication_agent_response (authority, caller, cookie, identity, error);
+      return klass->authentication_agent_response (authority, caller, uid, cookie, identity, error);
     }
 }
 
@@ -587,6 +588,11 @@ static const gchar *server_introspection_data =
   "      <arg type='s' name='cookie' direction='in'/>"
   "      <arg type='(sa{sv})' name='identity' direction='in'/>"
   "    </method>"
+  "    <method name='AuthenticationAgentResponse2'>"
+  "      <arg type='u' name='uid' direction='in'/>"
+  "      <arg type='s' name='cookie' direction='in'/>"
+  "      <arg type='(sa{sv})' name='identity' direction='in'/>"
+  "    </method>"
   "    <method name='EnumerateTemporaryAuthorizations'>"
   "      <arg type='(sa{sv})' name='subject' direction='in'/>"
   "      <arg type='a(ss(sa{sv})tt)' name='temporary_authorizations' direction='out'/>"
@@ -1035,6 +1041,57 @@ server_handle_authentication_agent_response (Server                 *server,
   error = NULL;
   if (!polkit_backend_authority_authentication_agent_response (server->authority,
                                                                caller,
+                                                               (uid_t)-1,
+                                                               cookie,
+                                                               identity,
+                                                               &error))
+    {
+      g_dbus_method_invocation_return_gerror (invocation, error);
+      g_error_free (error);
+      goto out;
+    }
+
+  g_dbus_method_invocation_return_value (invocation, g_variant_new ("()"));
+
+ out:
+  if (identity != NULL)
+    g_object_unref (identity);
+}
+
+static void
+server_handle_authentication_agent_response2 (Server                 *server,
+                                              GVariant               *parameters,
+                                              PolkitSubject          *caller,
+                                              GDBusMethodInvocation  *invocation)
+{
+  const gchar *cookie;
+  GVariant *identity_gvariant;
+  PolkitIdentity *identity;
+  GError *error;
+  guint32 uid;
+
+  identity = NULL;
+
+  g_variant_get (parameters,
+                 "(u&s@(sa{sv}))",
+                 &uid,
+                 &cookie,
+                 &identity_gvariant);
+
+  error = NULL;
+  identity = polkit_identity_new_for_gvariant (identity_gvariant, &error);
+  if (identity == NULL)
+    {
+      g_prefix_error (&error, "Error getting identity: ");
+      g_dbus_method_invocation_return_gerror (invocation, error);
+      g_error_free (error);
+      goto out;
+    }
+
+  error = NULL;
+  if (!polkit_backend_authority_authentication_agent_response (server->authority,
+                                                               caller,
+                                                               (uid_t)uid,
                                                                cookie,
                                                                identity,
                                                                &error))
@@ -1222,6 +1279,8 @@ server_handle_method_call (GDBusConnection        *connection,
     server_handle_unregister_authentication_agent (server, parameters, caller, invocation);
   else if (g_strcmp0 (method_name, "AuthenticationAgentResponse") == 0)
     server_handle_authentication_agent_response (server, parameters, caller, invocation);
+  else if (g_strcmp0 (method_name, "AuthenticationAgentResponse2") == 0)
+    server_handle_authentication_agent_response2 (server, parameters, caller, invocation);
   else if (g_strcmp0 (method_name, "EnumerateTemporaryAuthorizations") == 0)
     server_handle_enumerate_temporary_authorizations (server, parameters, caller, invocation);
   else if (g_strcmp0 (method_name, "RevokeTemporaryAuthorizations") == 0)
diff --git a/src/polkitbackend/polkitbackendauthority.h b/src/polkitbackend/polkitbackendauthority.h
index f9f7385..88df82e 100644
--- a/src/polkitbackend/polkitbackendauthority.h
+++ b/src/polkitbackend/polkitbackendauthority.h
@@ -147,6 +147,7 @@ struct _PolkitBackendAuthorityClass
 
   gboolean (*authentication_agent_response) (PolkitBackendAuthority   *authority,
                                              PolkitSubject            *caller,
+                                             uid_t                     uid,
                                              const gchar              *cookie,
                                              PolkitIdentity           *identity,
                                              GError                  **error);
@@ -249,6 +250,7 @@ gboolean polkit_backend_authority_unregister_authentication_agent (PolkitBackend
 
 gboolean polkit_backend_authority_authentication_agent_response (PolkitBackendAuthority    *authority,
                                                                  PolkitSubject             *caller,
+                                                                 uid_t                      uid,
                                                                  const gchar               *cookie,
                                                                  PolkitIdentity            *identity,
                                                                  GError                   **error);
diff --git a/src/polkitbackend/polkitbackendinteractiveauthority.c b/src/polkitbackend/polkitbackendinteractiveauthority.c
index 15adc6a..96725f7 100644
--- a/src/polkitbackend/polkitbackendinteractiveauthority.c
+++ b/src/polkitbackend/polkitbackendinteractiveauthority.c
@@ -108,8 +108,9 @@ static AuthenticationAgent *get_authentication_agent_for_subject (PolkitBackendI
                                                                   PolkitSubject *subject);
 
 
-static AuthenticationSession *get_authentication_session_for_cookie (PolkitBackendInteractiveAuthority *authority,
-                                                                     const gchar *cookie);
+static AuthenticationSession *get_authentication_session_for_uid_and_cookie (PolkitBackendInteractiveAuthority *authority,
+                                                                             uid_t                              uid,
+                                                                             const gchar                       *cookie);
 
 static GList *get_authentication_sessions_initiated_by_system_bus_unique_name (PolkitBackendInteractiveAuthority *authority,
                                                                                const gchar *system_bus_unique_name);
@@ -169,6 +170,7 @@ static gboolean polkit_backend_interactive_authority_unregister_authentication_a
 
 static gboolean polkit_backend_interactive_authority_authentication_agent_response (PolkitBackendAuthority   *authority,
                                                                               PolkitSubject            *caller,
+                                                                              uid_t                     uid,
                                                                               const gchar              *cookie,
                                                                               PolkitIdentity           *identity,
                                                                               GError                  **error);
@@ -440,6 +442,7 @@ struct AuthenticationAgent
 {
   volatile gint ref_count;
 
+  uid_t creator_uid;
   PolkitSubject *scope;
   guint64 serial;
 
@@ -1603,6 +1606,7 @@ authentication_agent_unref (AuthenticationAgent *agent)
 static AuthenticationAgent *
 authentication_agent_new (guint64      serial,
                           PolkitSubject *scope,
+                          PolkitIdentity *creator,
                           const gchar *unique_system_bus_name,
                           const gchar *locale,
                           const gchar *object_path,
@@ -1611,6 +1615,10 @@ authentication_agent_new (guint64      serial,
 {
   AuthenticationAgent *agent;
   GDBusProxy *proxy;
+  PolkitUnixUser *creator_user;
+
+  g_assert (POLKIT_IS_UNIX_USER (creator));
+  creator_user = POLKIT_UNIX_USER (creator);
 
   if (!g_variant_is_object_path (object_path))
     {
@@ -1638,6 +1646,7 @@ authentication_agent_new (guint64      serial,
   agent->ref_count = 1;
   agent->serial = serial;
   agent->scope = g_object_ref (scope);
+  agent->creator_uid = (uid_t)polkit_unix_user_get_uid (creator_user);
   agent->object_path = g_strdup (object_path);
   agent->unique_system_bus_name = g_strdup (unique_system_bus_name);
   agent->locale = g_strdup (locale);
@@ -1736,8 +1745,9 @@ get_authentication_agent_for_subject (PolkitBackendInteractiveAuthority *authori
 }
 
 static AuthenticationSession *
-get_authentication_session_for_cookie (PolkitBackendInteractiveAuthority *authority,
-                                       const gchar *cookie)
+get_authentication_session_for_uid_and_cookie (PolkitBackendInteractiveAuthority *authority,
+                                               uid_t                              uid,
+                                               const gchar                       *cookie)
 {
   PolkitBackendInteractiveAuthorityPrivate *priv;
   GHashTableIter hash_iter;
@@ -1755,6 +1765,23 @@ get_authentication_session_for_cookie (PolkitBackendInteractiveAuthority *author
     {
       GList *l;
 
+      /* We need to ensure that if somehow we have duplicate cookies
+       * due to wrapping, that the cookie used is matched to the user
+       * who called AuthenticationAgentResponse2.  See
+       * http://lists.freedesktop.org/archives/polkit-devel/2015-June/000425.html
+       * 
+       * Except if the legacy AuthenticationAgentResponse is invoked,
+       * we don't know the uid and hence use -1.  Continue to support
+       * the old behavior for backwards compatibility, although everyone
+       * who is using our own setuid helper will automatically be updated
+       * to the new API.
+       */
+      if (uid != (uid_t)-1)
+        {
+          if (agent->creator_uid != uid)
+            continue;
+        }
+
       for (l = agent->active_sessions; l != NULL; l = l->next)
         {
           AuthenticationSession *session = l->data;
@@ -2544,6 +2571,7 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
   priv->agent_serial++;
   agent = authentication_agent_new (priv->agent_serial,
                                     subject,
+                                    user_of_caller,
                                     polkit_system_bus_name_get_name (POLKIT_SYSTEM_BUS_NAME (caller)),
                                     locale,
                                     object_path,
@@ -2757,6 +2785,7 @@ polkit_backend_interactive_authority_unregister_authentication_agent (PolkitBack
 static gboolean
 polkit_backend_interactive_authority_authentication_agent_response (PolkitBackendAuthority   *authority,
                                                               PolkitSubject            *caller,
+                                                              uid_t                     uid,
                                                               const gchar              *cookie,
                                                               PolkitIdentity           *identity,
                                                               GError                  **error)
@@ -2799,7 +2828,7 @@ polkit_backend_interactive_authority_authentication_agent_response (PolkitBacken
     }
 
   /* find the authentication session */
-  session = get_authentication_session_for_cookie (interactive_authority, cookie);
+  session = get_authentication_session_for_uid_and_cookie (interactive_authority, uid, cookie);
   if (session == NULL)
     {
       g_set_error (error,