dos: Add extra safety asserts in cc_stats_refill_bucket()

Never allow the function to set a bucket value above the allowed circuit
burst.

Closes #25202

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket25202

@@ -0,0 +1,4 @@
+  o Minor bugfixes (DoS mitigation):
+    - Add extra safety checks when refilling the circuit creation bucket to
+      ensure we never set a value that is above the allowed burst. Fixes
+      ticket 25202.

src/or/dos.c

@@ -309,6 +309,16 @@ cc_stats_refill_bucket(cc_client_stats_t *stats, const tor_addr_t *addr)
     new_circuit_bucket_count = MIN(stats->circuit_bucket + (uint32_t)num_token,
                                    dos_cc_circuit_burst);
   }
+
+  /* This function is not allowed to make the bucket count larger than the
+   * burst value */
+  tor_assert_nonfatal(new_circuit_bucket_count <= dos_cc_circuit_burst);
+  /* This function is not allowed to make the bucket count smaller, unless it
+   * is decreasing it to a newly configured, lower burst value. We allow the
+   * bucket to stay the same size, in case the circuit rate is zero. */
+  tor_assert_nonfatal(new_circuit_bucket_count >= stats->circuit_bucket ||
+                      new_circuit_bucket_count == dos_cc_circuit_burst);
+
   log_debug(LD_DOS, "DoS address %s has its circuit bucket value: %" PRIu32
                     ". Filling it to %" PRIu32 ". Circuit rate is %" PRIu64
                     ". Elapsed time is %" PRIi64,