Bug 40897 Bug Bounty: Double the number of max conflux circs

We strongly suspect that bug 40897 was caused by a custom Tor client that
tried to use more than the default number of conflux circuits, for either
performance or traffic analysis defense gains, or both.

This entity hit a safety check on the exit side, which caused a UAF. Our
"belt and suspenders" snapped off, and hit us in the face... again...

Since there are good reasons to try more than 2 conflux legs, and research has
found some traffic analysis benefits with as many as 5, we're going to raise
and parameterize this limit as a form of bug bounty for finding this UAF, so
that this entity can try out a little more confluxing.

This should also make it easier for researchers to try things like gathering
traces with larger amounts of confluxing than normal, to measure real-world
traffic analysis impacts of conflux.

Shine on, you yoloing anonymous diamond. Let us know if you find out anything
interesting!

4 files changed, 21 insertions, 14 deletions

diff --git a/src/core/or/conflux_params.c b/src/core/or/conflux_params.c
index dbf4ae5272..65728032f9 100644
--- a/src/core/or/conflux_params.c
+++ b/src/core/or/conflux_params.c
@@ -56,6 +56,11 @@
 #define NUM_LEGS_SET_MAX (UINT8_MAX)
 #define NUM_LEGS_SET_DEFAULT (2)
 
+/* For "cfx_max_legs_set" */
+#define MAX_LEGS_SET_MIN (3)
+#define MAX_LEGS_SET_MAX (UINT8_MAX)
+#define MAX_LEGS_SET_DEFAULT (8)
+
 /* For "cfx_send_pct". */
 #define CFX_SEND_PCT_MIN (0)
 #define CFX_SEND_PCT_MAX (255)
@@ -81,6 +86,8 @@ static uint8_t max_prebuilt_set = MAX_PREBUILT_SET_DEFAULT;
 STATIC uint32_t max_unlinked_leg_retry = MAX_UNLINKED_LEG_RETRY_DEFAULT;
 /* Number of legs per set. */
 static uint8_t num_legs_set = NUM_LEGS_SET_DEFAULT;
+/* Maximum number of legs per set allowed at exits */
+static uint8_t max_legs_set = MAX_LEGS_SET_DEFAULT;
 /* The low Exit relay threshold, as a ratio between 0 and 1, used as a limit to
  * decide the amount of pre-built set we build depending on how many Exit relay
  * supports conflux in our current consensus. */
@@ -223,6 +230,13 @@ conflux_params_get_num_legs_set(void)
   return num_legs_set;
 }
 
+/** Return the maximum number of legs per set. */
+uint8_t
+conflux_params_get_max_legs_set(void)
+{
+  return max_legs_set;
+}
+
 /** Return the drain percent we must hit before switching */
 uint8_t
 conflux_params_get_drain_pct(void)
@@ -275,6 +289,11 @@ conflux_params_new_consensus(const networkstatus_t *ns)
                             NUM_LEGS_SET_DEFAULT,
                             NUM_LEGS_SET_MIN, NUM_LEGS_SET_MAX);
 
+  max_legs_set =
+    networkstatus_get_param(ns, "cfx_max_legs_set",
+                            MAX_LEGS_SET_DEFAULT,
+                            MAX_LEGS_SET_MIN, MAX_LEGS_SET_MAX);
+
   /* Params used by conflux.c */
   cfx_send_pct = networkstatus_get_param(ns, "cfx_send_pct",
       CFX_SEND_PCT_DFLT,
diff --git a/src/core/or/conflux_params.h b/src/core/or/conflux_params.h
index 22c3e4ad1f..06e902cf03 100644
--- a/src/core/or/conflux_params.h
+++ b/src/core/or/conflux_params.h
@@ -16,6 +16,7 @@ uint8_t conflux_params_get_max_linked_set(void);
 uint8_t conflux_params_get_max_prebuilt(void);
 uint8_t conflux_params_get_max_unlinked_leg_retry(void);
 uint8_t conflux_params_get_num_legs_set(void);
+uint8_t conflux_params_get_max_legs_set(void);
 uint8_t conflux_params_get_drain_pct(void);
 uint8_t conflux_params_get_send_pct(void);
 
diff --git a/src/core/or/conflux_pool.c b/src/core/or/conflux_pool.c
index 5a677fb9aa..74781b307a 100644
--- a/src/core/or/conflux_pool.c
+++ b/src/core/or/conflux_pool.c
@@ -743,7 +743,7 @@ try_finalize_set(unlinked_circuits_t *unlinked)
 
   /* If there are too many legs, we can't link. */
   if (smartlist_len(unlinked->legs) +
-      smartlist_len(unlinked->cfx->legs) > CONFLUX_MAX_CIRCS) {
+      smartlist_len(unlinked->cfx->legs) > conflux_params_get_max_legs_set()) {
     log_fn(LOG_PROTOCOL_WARN, LD_CIRC,
            "Conflux set has too many legs to link. "
            "Rejecting this circuit.");
diff --git a/src/core/or/conflux_st.h b/src/core/or/conflux_st.h
index 8d85ad1fbe..61e38f8268 100644
--- a/src/core/or/conflux_st.h
+++ b/src/core/or/conflux_st.h
@@ -22,19 +22,6 @@ typedef enum {
  CONFLUX_ALG_CWNDRTT = 2,
 } conflux_alg_t;
 
-/**
- * Maximum number of linked circuits.
- *
- * We want to experiment with 3 guards, so we need at least 3 here.
- *
- * However, we need 1 more than this, to support using a test circuit to probe
- * for a faster path, for applications that *require* a specific latency target
- * (like VoIP).
- *
- * We may also want to raise this for traffic analysis defense evaluation.
- */
-#define CONFLUX_MAX_CIRCS 4
-
 /** XXX: Cached consensus params+scheduling alg */
 struct conflux_params_t {
   conflux_alg_t alg;