CVE-2015-4625: Use unpredictable cookie values, keep them secret

Tavis noted that it'd be possible with a 32 bit counter for someone to
cause the cookie to wrap by creating Authentication requests in a
loop.

Something important to note here is that wrapping of signed integers
is undefined behavior in C, so we definitely want to fix that.  All
counter integers used in this patch are unsigned.

See the comment above `authentication_agent_generate_cookie` for
details, but basically we're now using a cookie of the form:

```
        <agent serial> - <agent random id> - <session serial> - <session
random id>
```

Which has multiple 64 bit counters, plus unpredictable random 128 bit
integer ids (effectively UUIDs, but we're not calling them that
because we don't need to be globally unique.

We further ensure that the cookies are not visible to other processes
by changing the setuid helper to accept them over standard input.  This
means that an attacker would have to guess both ids.

In any case, the security hole here is better fixed with the other
change to bind user id (uid) of the agent with cookie lookups, making
cookie guessing worthless.

Nevertheless, I think it's worth doing this change too, for defense in
depth.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=90832
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>

1 files changed, 80 insertions, 19 deletions

diff --git a/src/polkitbackend/polkitbackendinteractiveauthority.c b/src/polkitbackend/polkitbackendinteractiveauthority.c
index 3f339e9..15adc6a 100644
--- a/src/polkitbackend/polkitbackendinteractiveauthority.c
+++ b/src/polkitbackend/polkitbackendinteractiveauthority.c
@@ -214,6 +214,8 @@ typedef struct
 
   GDBusConnection *system_bus_connection;
   guint name_owner_changed_signal_id;
+
+  guint64 agent_serial;
 } PolkitBackendInteractiveAuthorityPrivate;
 
 /* ---------------------------------------------------------------------------------------------------- */
@@ -439,11 +441,15 @@ struct AuthenticationAgent
   volatile gint ref_count;
 
   PolkitSubject *scope;
+  guint64 serial;
 
   gchar *locale;
   GVariant *registration_options;
   gchar *object_path;
   gchar *unique_system_bus_name;
+  GRand *cookie_pool;
+  gchar *cookie_prefix;
+  guint64  cookie_serial;
 
   GDBusProxy *proxy;
 
@@ -1427,9 +1433,54 @@ authentication_session_cancelled_cb (GCancellable *cancellable,
   authentication_session_cancel (session);
 }
 
+/* We're not calling this a UUID, but it's basically
+ * the same thing, just not formatted that way because:
+ *
+ *  - I'm too lazy to do it
+ *  - If we did, people might think it was actually
+ *    generated from /dev/random, which we're not doing
+ *    because this value doesn't actually need to be
+ *    globally unique.
+ */
+static void
+append_rand_u128_str (GString *buf,
+                      GRand   *pool)
+{
+  g_string_append_printf (buf, "%08x%08x%08x%08x",
+                          g_rand_int (pool),
+                          g_rand_int (pool),
+                          g_rand_int (pool),
+                          g_rand_int (pool));
+}
+
+/* A value that should be unique to the (AuthenticationAgent, AuthenticationSession)
+ * pair, and not guessable by other agents.
+ *
+ * <agent serial> - <agent uuid> - <session serial> - <session uuid>
+ *
+ * See http://lists.freedesktop.org/archives/polkit-devel/2015-June/000425.html
+ *
+ */
+static gchar *
+authentication_agent_generate_cookie (AuthenticationAgent *agent)
+{
+  GString *buf = g_string_new ("");
+
+  g_string_append (buf, agent->cookie_prefix);
+  
+  g_string_append_c (buf, '-');
+  agent->cookie_serial++;
+  g_string_append_printf (buf, "%" G_GUINT64_FORMAT, 
+                          agent->cookie_serial);
+  g_string_append_c (buf, '-');
+  append_rand_u128_str (buf, agent->cookie_pool);
+
+  return g_string_free (buf, FALSE);
+}
+
+
 static AuthenticationSession *
 authentication_session_new (AuthenticationAgent         *agent,
-                            const gchar                 *cookie,
                             PolkitSubject               *subject,
                             PolkitIdentity              *user_of_subject,
                             PolkitSubject               *caller,
@@ -1447,7 +1498,7 @@ authentication_session_new (AuthenticationAgent         *agent,
 
   session = g_new0 (AuthenticationSession, 1);
   session->agent = authentication_agent_ref (agent);
-  session->cookie = g_strdup (cookie);
+  session->cookie = authentication_agent_generate_cookie (agent);
   session->subject = g_object_ref (subject);
   session->user_of_subject = g_object_ref (user_of_subject);
   session->caller = g_object_ref (caller);
@@ -1496,16 +1547,6 @@ authentication_session_free (AuthenticationSession *session)
   g_free (session);
 }
 
-static gchar *
-authentication_agent_new_cookie (AuthenticationAgent *agent)
-{
-  static gint counter = 0;
-
-  /* TODO: use a more random-looking cookie */
-
-  return g_strdup_printf ("cookie%d", counter++);
-}
-
 static PolkitSubject *
 authentication_agent_get_scope (AuthenticationAgent *agent)
 {
@@ -1553,12 +1594,15 @@ authentication_agent_unref (AuthenticationAgent *agent)
       g_free (agent->unique_system_bus_name);
       if (agent->registration_options != NULL)
         g_variant_unref (agent->registration_options);
+      g_rand_free (agent->cookie_pool);
+      g_free (agent->cookie_prefix);
       g_free (agent);
     }
 }
 
 static AuthenticationAgent *
-authentication_agent_new (PolkitSubject *scope,
+authentication_agent_new (guint64      serial,
+                          PolkitSubject *scope,
                           const gchar *unique_system_bus_name,
                           const gchar *locale,
                           const gchar *object_path,
@@ -1592,6 +1636,7 @@ authentication_agent_new (PolkitSubject *scope,
 
   agent = g_new0 (AuthenticationAgent, 1);
   agent->ref_count = 1;
+  agent->serial = serial;
   agent->scope = g_object_ref (scope);
   agent->object_path = g_strdup (object_path);
   agent->unique_system_bus_name = g_strdup (unique_system_bus_name);
@@ -1599,6 +1644,25 @@ authentication_agent_new (PolkitSubject *scope,
   agent->registration_options = registration_options != NULL ? g_variant_ref (registration_options) : NULL;
   agent->proxy = proxy;
 
+  {
+    GString *cookie_prefix = g_string_new ("");
+    GRand *agent_private_rand = g_rand_new ();
+
+    g_string_append_printf (cookie_prefix, "%" G_GUINT64_FORMAT "-", agent->serial);
+
+    /* Use a uniquely seeded PRNG to get a prefix cookie for this agent,
+     * whose sequence will not correlate with the per-authentication session
+     * cookies.
+     */
+    append_rand_u128_str (cookie_prefix, agent_private_rand);
+    g_rand_free (agent_private_rand);
+
+    agent->cookie_prefix = g_string_free (cookie_prefix, FALSE);
+    
+    /* And a newly seeded pool for per-session cookies */
+    agent->cookie_pool = g_rand_new ();
+  }
+
   return agent;
 }
 
@@ -2193,7 +2257,6 @@ authentication_agent_initiate_challenge (AuthenticationAgent         *agent,
 {
   PolkitBackendInteractiveAuthorityPrivate *priv = POLKIT_BACKEND_INTERACTIVE_AUTHORITY_GET_PRIVATE (authority);
   AuthenticationSession *session;
-  gchar *cookie;
   GList *l;
   GList *identities;
   gchar *localized_message;
@@ -2215,8 +2278,6 @@ authentication_agent_initiate_challenge (AuthenticationAgent         *agent,
                                     &localized_icon_name,
                                     &localized_details);
 
-  cookie = authentication_agent_new_cookie (agent);
-
   identities = NULL;
 
   /* select admin user if required by the implicit authorization */
@@ -2279,7 +2340,6 @@ authentication_agent_initiate_challenge (AuthenticationAgent         *agent,
     user_identities = g_list_prepend (NULL, polkit_unix_user_new (0));
 
   session = authentication_session_new (agent,
-                                        cookie,
                                         subject,
                                         user_of_subject,
                                         caller,
@@ -2335,7 +2395,6 @@ authentication_agent_initiate_challenge (AuthenticationAgent         *agent,
   g_list_free_full (user_identities, g_object_unref);
   g_list_foreach (identities, (GFunc) g_object_unref, NULL);
   g_list_free (identities);
-  g_free (cookie);
 
   g_free (localized_message);
   g_free (localized_icon_name);
@@ -2482,7 +2541,9 @@ polkit_backend_interactive_authority_register_authentication_agent (PolkitBacken
       goto out;
     }
 
-  agent = authentication_agent_new (subject,
+  priv->agent_serial++;
+  agent = authentication_agent_new (priv->agent_serial,
+                                    subject,
                                     polkit_system_bus_name_get_name (POLKIT_SYSTEM_BUS_NAME (caller)),
                                     locale,
                                     object_path,