hs_pow: Make proof-of-work support optional in configure

This adds a new "pow" module for the user-visible proof
of work support in ./configure, and this disables
src/feature/hs/hs_pow at compile-time.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>

configure.ac

@@ -373,7 +373,7 @@ dnl Tor modules options. These options are namespaced with --disable-module-XXX
 dnl ---
 
 dnl All our modules.
-m4_define(MODULES, relay dirauth dircache)
+m4_define([MODULES], [relay dirauth dircache pow])
 
 # Some modules are only disabled through another option. For those, we don't
 # want to print the help in the summary at the end of the configure. Any entry
@@ -382,6 +382,9 @@ m4_define(MODULES, relay dirauth dircache)
 m4_set_add_all([MODULES_WITH_NO_OPTIONS], [dircache])
 
 dnl Relay module.
+m4_define([module_option_hints(relay)],
+  [AS_IF([test "x$value" = x1], [HINT_OPT([--disable-module-relay])],
+    [HINT_NONE])])
 AC_ARG_ENABLE([module-relay],
               AS_HELP_STRING([--disable-module-relay],
                              [Build tor without the Relay modules: tor can not run as a relay, bridge, or authority. Implies --disable-module-dirauth]))
@@ -399,6 +402,9 @@ AM_COND_IF(BUILD_MODULE_DIRCACHE,
                      [Compile with directory cache support]))
 
 dnl Directory Authority module.
+m4_define([module_option_hints(dirauth)],
+  [AS_IF([test "x$value" = x1], [HINT_OPT([--disable-module-dirauth])],
+    [HINT_NONE])])
 AC_ARG_ENABLE([module-dirauth],
               AS_HELP_STRING([--disable-module-dirauth],
                              [Build tor without the Directory Authority module: tor can not run as a directory authority or bridge authority]))
@@ -407,6 +413,19 @@ AM_COND_IF(BUILD_MODULE_DIRAUTH,
            AC_DEFINE([HAVE_MODULE_DIRAUTH], [1],
                      [Compile with Directory Authority feature support]))
 
+dnl Hidden Service Proof-of-Work module.
+m4_define([module_option_hints(pow)],
+  [AS_IF([test "x$value" = x1], [HINT_OPT([--disable-module-pow])],
+    [AS_IF([test "x$license_option" != "xGPL"], [HINT_OPT([requires --enable-gpl])],
+      [HINT_NONE])])])
+AC_ARG_ENABLE([module-pow],
+              AS_HELP_STRING([--disable-module-pow],
+                             [Build tor without proof-of-work denial of service mitigation, normally available when building with --enable-gpl]))
+AM_CONDITIONAL(BUILD_MODULE_POW,
+               [test "x$license_option" = "xGPL" && test "x$enable_module_pow" != "xno"])
+AM_COND_IF(BUILD_MODULE_POW,
+           AC_DEFINE([HAVE_MODULE_POW], [1], [Compile with proof-of-work support]))
+
 dnl Helper variables.
 TOR_MODULES_ALL_ENABLED=
 AC_DEFUN([ADD_MODULE], [
@@ -2733,12 +2752,22 @@ PPRINT_PROP_BOOL([Fragile Hardening (--enable-fragile-hardening, dev only)], $va
 AS_ECHO
 PPRINT_SUBTITLE([Modules])
 
+# Modules have documentation hints indicating how they can be enabled
+# or disabled, and those hints can select which version of our message
+# to show based on variables at configure-time.
+#
+# Each "module_option_hints(<name>)" macro, if it exists, must
+# visit exactly one HINT_* macro using shell conditionals.
+
 m4_foreach_w([mname], MODULES,
   [
     AM_COND_IF(m4_join([], [BUILD_MODULE_], m4_toupper([]mname[])), value=1, value=0)
-    m4_set_contains([MODULES_WITH_NO_OPTIONS], mname,
-                    PPRINT_PROP_BOOL([mname], $value),
-                    PPRINT_PROP_BOOL([mname (--disable-module-mname)], $value))
+    m4_pushdef([HINT_OPT], [PPRINT_PROP_BOOL](mname ($1), [[$value]]))
+    m4_pushdef([HINT_NONE], [PPRINT_PROP_BOOL](mname, [[$value]]))
+    m4_ifdef([module_option_hints](mname),
+      [m4_indir([module_option_hints](mname))],
+      [HINT_NONE])
+    m4_popdef([HINT_OPT], [HINT_NONE])
   ]
 )
 

src/app/config/config.c

@@ -88,9 +88,11 @@
 #include "feature/control/control.h"
 #include "feature/control/control_auth.h"
 #include "feature/control/control_events.h"
+#include "feature/dircache/dirserv.h"
 #include "feature/dirclient/dirclient_modes.h"
 #include "feature/hibernate/hibernate.h"
 #include "feature/hs/hs_config.h"
+#include "feature/hs/hs_pow.h"
 #include "feature/metrics/metrics.h"
 #include "feature/nodelist/dirlist.h"
 #include "feature/nodelist/networkstatus.h"
@@ -2731,11 +2733,19 @@ list_deprecated_options(void)
 static void
 list_enabled_modules(void)
 {
-  printf("%s: %s\n", "relay", have_module_relay() ? "yes" : "no");
-  printf("%s: %s\n", "dirauth", have_module_dirauth() ? "yes" : "no");
-  // We don't list dircache, because it cannot be enabled or disabled
-  // independently from relay.  Listing it here would proliferate
-  // test variants in test_parseconf.sh to no useful purpose.
+  static const struct {
+    const char *name;
+    bool have;
+  } list[] = {
+    { "relay", have_module_relay() },
+    { "dirauth", have_module_dirauth() },
+    { "dircache", have_module_dircache() },
+    { "pow", have_module_pow() }
+  };
+
+  for (unsigned i = 0; i < sizeof list / sizeof list[0]; i++) {
+    printf("%s: %s\n", list[i].name, list[i].have ? "yes" : "no");
+  }
 }
 
 /** Prints compile-time and runtime library versions. */

src/core/include.am

@@ -17,6 +17,7 @@ if UNITTESTS_ENABLED
 LIBTOR_APP_TESTING_A_SOURCES += $(MODULE_RELAY_SOURCES)
 LIBTOR_APP_TESTING_A_SOURCES += $(MODULE_DIRCACHE_SOURCES)
 LIBTOR_APP_TESTING_A_SOURCES += $(MODULE_DIRAUTH_SOURCES)
+LIBTOR_APP_TESTING_A_SOURCES += $(MODULE_POW_SOURCES)
 
 src_core_libtor_app_testing_a_SOURCES = $(LIBTOR_APP_TESTING_A_SOURCES)
 else

src/feature/hs/hs_circuit.c

@@ -1369,7 +1369,7 @@ hs_circ_handle_introduce2(const hs_service_t *service,
 
   /* Add the rendezvous request to the priority queue if PoW defenses are
    * enabled, otherwise rendezvous as usual. */
-  if (service->config.has_pow_defenses_enabled) {
+  if (have_module_pow() && service->config.has_pow_defenses_enabled) {
     log_notice(LD_REND,
                "Adding introduction request to pqueue with effort: %u",
                data.rdv_data.pow_effort);

src/feature/hs/hs_client.c

@@ -733,7 +733,8 @@ consider_sending_introduce1(origin_circuit_t *intro_circ,
 
   /* If the descriptor contains PoW parameters then the service is
    * expecting a PoW solution in the INTRODUCE cell, which we solve here. */
-  if (desc->encrypted_data.pow_params &&
+  if (have_module_pow() &&
+      desc->encrypted_data.pow_params &&
       desc->encrypted_data.pow_params->suggested_effort > 0) {
     log_debug(LD_REND, "PoW params present in descriptor.");
 
@@ -752,9 +753,11 @@ consider_sending_introduce1(origin_circuit_t *intro_circ,
 
     /* send it to the client-side pow cpuworker for solving. */
     intro_circ->hs_currently_solving_pow = 1;
-    pow_queue_work(intro_circ->global_identifier,
-                   rend_circ->global_identifier,
-                   desc->encrypted_data.pow_params);
+    if (0 != hs_pow_queue_work(intro_circ->global_identifier,
+                               rend_circ->global_identifier,
+                               desc->encrypted_data.pow_params)) {
+      log_debug(LD_REND, "Failed to enqueue PoW request");
+    }
 
     /* can't proceed with the intro1 cell yet, so yield back to the
      * main loop */

src/feature/hs/hs_config.c

@@ -327,6 +327,12 @@ config_validate_service(const hs_service_config_t *config)
              config->pow_queue_burst, config->pow_queue_rate);
     goto invalid;
   }
+  if (config->has_pow_defenses_enabled && !have_module_pow()) {
+    log_warn(LD_CONFIG, "Hidden service proof-of-work defenses are enabled "
+                        "in our configuration but this build of tor does not "
+                        "include the required 'pow' module.");
+    goto invalid;
+  }
 
   /* Valid. */
   return 0;

src/feature/hs/hs_pow.c

@@ -410,9 +410,9 @@ pow_worker_replyfn(void *work_)
  * Queue the job of solving the pow in a worker thread.
  */
 int
-pow_queue_work(uint32_t intro_circ_identifier,
-               uint32_t rend_circ_identifier,
-               const hs_pow_desc_params_t *pow_params)
+hs_pow_queue_work(uint32_t intro_circ_identifier,
+                  uint32_t rend_circ_identifier,
+                  const hs_pow_desc_params_t *pow_params)
 {
   tor_assert(in_main_thread());
 

src/feature/hs/hs_pow.h

@@ -127,6 +127,9 @@ typedef struct hs_pow_solution_t {
   equix_solution equix_solution;
 } hs_pow_solution_t;
 
+#ifdef HAVE_MODULE_POW
+#define have_module_pow() (1)
+
 /* API */
 int hs_pow_solve(const hs_pow_desc_params_t *pow_params,
                  hs_pow_solution_t *pow_solution_out);
@@ -137,8 +140,54 @@ int hs_pow_verify(const hs_pow_service_state_t *pow_state,
 void hs_pow_remove_seed_from_cache(uint32_t seed);
 void hs_pow_free_service_state(hs_pow_service_state_t *state);
 
-int pow_queue_work(uint32_t intro_circ_identifier,
-                   uint32_t rend_circ_identifier,
-                   const hs_pow_desc_params_t *pow_params);
+int hs_pow_queue_work(uint32_t intro_circ_identifier,
+                      uint32_t rend_circ_identifier,
+                      const hs_pow_desc_params_t *pow_params);
+
+#else /* !defined(HAVE_MODULE_POW) */
+#define have_module_pow() (0)
+
+static inline int
+hs_pow_solve(const hs_pow_desc_params_t *pow_params,
+             hs_pow_solution_t *pow_solution_out)
+{
+  (void)pow_params;
+  (void)pow_solution_out;
+  return -1;
+}
+
+static inline int
+hs_pow_verify(const hs_pow_service_state_t *pow_state,
+              const hs_pow_solution_t *pow_solution)
+{
+  (void)pow_state;
+  (void)pow_solution;
+  return -1;
+}
+
+static inline void
+hs_pow_remove_seed_from_cache(uint32_t seed)
+{
+  (void)seed;
+}
+
+static inline void
+hs_pow_free_service_state(hs_pow_service_state_t *state)
+{
+  (void)state;
+}
+
+static inline int
+hs_pow_queue_work(uint32_t intro_circ_identifier,
+                  uint32_t rend_circ_identifier,
+                  const hs_pow_desc_params_t *pow_params)
+{
+  (void)intro_circ_identifier;
+  (void)rend_circ_identifier;
+  (void)pow_params;
+  return -1;
+}
+
+#endif /* defined(HAVE_MODULE_POW) */
 
 #endif /* !defined(TOR_HS_POW_H) */

src/feature/hs/hs_service.c

@@ -2899,7 +2899,7 @@ run_housekeeping_event(time_t now)
 
     /* Check if we need to initialize or update PoW parameters, if the
      * defenses are enabled. */
-    if (service->config.has_pow_defenses_enabled) {
+    if (have_module_pow() && service->config.has_pow_defenses_enabled) {
       pow_housekeeping(service, now);
     }
 
@@ -2937,8 +2937,10 @@ run_build_descriptor_event(time_t now)
    * is useful for newly built descriptors. */
   update_all_descriptors_intro_points(now);
 
-  /* Update the PoW params if needed. */
-  update_all_descriptors_pow_params(now);
+  if (have_module_pow()) {
+    /* Update the PoW params if needed. */
+    update_all_descriptors_pow_params(now);
+  }
 }
 
 /** For the given service, launch any intro point circuits that could be

src/feature/hs/include.am

@@ -15,12 +15,19 @@ LIBTOR_APP_A_SOURCES += 				\
 	src/feature/hs/hs_intropoint.c		\
 	src/feature/hs/hs_metrics.c			\
 	src/feature/hs/hs_ob.c			\
-	src/feature/hs/hs_pow.c			\
 	src/feature/hs/hs_service.c		\
 	src/feature/hs/hs_stats.c		\
 	src/feature/hs/hs_sys.c		\
 	src/feature/hs/hs_metrics_entry.c
 
+# Proof of Work module
+MODULE_POW_SOURCES =				\
+	src/feature/hs/hs_pow.c
+
+if BUILD_MODULE_POW
+LIBTOR_APP_A_SOURCES +=	$(MODULE_POW_SOURCES)
+endif
+
 # ADD_C_FILE: INSERT HEADERS HERE.
 noinst_HEADERS +=					\
 	src/feature/hs/hs_cache.h			\

src/test/test_parseconf.sh

@@ -98,6 +98,9 @@
 #      want to encode that knowledge in this test script, so we supply a
 #      separate result file for every combination of disabled modules that
 #      has a different result.)
+#
+#      This logic ignores modules that are not listed by --list-modules
+#      (dircache) and some that do not currently affect config parsing (pow).
 
 umask 077
 set -e
@@ -197,6 +200,8 @@ echo "This pattern should not match any log messages" \
                                   "$NON_EMPTY"
 
 STANDARD_LIBS="libevent\\|openssl\\|zlib"
+MODULES_WITHOUT_CONFIG_TESTS="dircache\\|pow"
+
 # Lib names are restricted to [a-z0-9]* at the moment
 # We don't actually want to support foreign accents here
 # shellcheck disable=SC2018,SC2019
@@ -229,6 +234,7 @@ TOR_LIBS_ENABLED_SEARCH="$(echo "$TOR_LIBS_ENABLED_SEARCH" | tr ' ' '\n' \
                            | grep -v '^_*$' | tr '\n' ' ')"
 
 TOR_MODULES_DISABLED="$("$TOR_BINARY" --list-modules | grep ': no' \
+                        | grep -v "$MODULES_WITHOUT_CONFIG_TESTS" \
                         | cut -d ':' -f1 | sort | tr '\n' '_')"
 # Remove the last underscore, if there is one
 TOR_MODULES_DISABLED=${TOR_MODULES_DISABLED%_}