simplewallet: plug a timing leak

As reported by Tramèr et al, timing of refresh requests can be used
to see whether a password was requested (and thus at least one output
received) since this will induce a delay in subsequent calls.
To avoid this, we schedule calls at a given time instead of sleeping
for a set time (which would make delays additive).
To further avoid a scheduled call being during the time in which a
password is prompted, the actual scheduled time is now randomized.

src/simplewallet/simplewallet.cpp

@@ -8807,12 +8807,25 @@ void simple_wallet::check_for_messages()
 //----------------------------------------------------------------------------------------------------
 void simple_wallet::wallet_idle_thread()
 {
+  const boost::posix_time::ptime start_time = boost::posix_time::microsec_clock::universal_time();
   while (true)
   {
     boost::unique_lock<boost::mutex> lock(m_idle_mutex);
     if (!m_idle_run.load(std::memory_order_relaxed))
       break;
 
+    // if another thread was busy (ie, a foreground refresh thread), we'll end up here at
+    // some random time that's not what we slept for, so we should not call refresh now
+    // or we'll be leaking that fact through timing
+    const boost::posix_time::ptime now0 = boost::posix_time::microsec_clock::universal_time();
+    const uint64_t dt_actual = (now0 - start_time).total_microseconds() % 1000000;
+#ifdef _WIN32
+    static const uint64_t threshold = 10000;
+#else
+    static const uint64_t threshold = 2000;
+#endif
+    if (dt_actual < threshold) // if less than a threshold... would a very slow machine always miss it ?
+    {
 #ifndef _WIN32
       m_inactivity_checker.do_call(boost::bind(&simple_wallet::check_inactivity, this));
 #endif
@@ -8822,7 +8835,13 @@ void simple_wallet::wallet_idle_thread()
 
       if (!m_idle_run.load(std::memory_order_relaxed))
         break;
-    m_idle_cond.wait_for(lock, boost::chrono::seconds(1));
+    }
+
+    // aim for the next multiple of 1 second
+    const boost::posix_time::ptime now = boost::posix_time::microsec_clock::universal_time();
+    const auto dt = (now - start_time).total_microseconds();
+    const auto wait = 1000000 - dt % 1000000;
+    m_idle_cond.wait_for(lock, boost::chrono::microseconds(wait));
   }
 }
 //----------------------------------------------------------------------------------------------------

src/simplewallet/simplewallet.h

@@ -448,10 +448,12 @@ namespace cryptonote
     std::atomic<bool> m_locked;
     std::atomic<bool> m_in_command;
 
+    template<uint64_t mini, uint64_t maxi> struct get_random_interval { public: uint64_t operator()() const { return crypto::rand_range(mini, maxi); } };
+
     epee::math_helper::once_a_time_seconds<1> m_inactivity_checker;
-    epee::math_helper::once_a_time_seconds<90> m_refresh_checker;
+    epee::math_helper::once_a_time_seconds_range<get_random_interval<80 * 1000000, 100 * 1000000>> m_refresh_checker;
-    epee::math_helper::once_a_time_seconds<90> m_mms_checker;
+    epee::math_helper::once_a_time_seconds_range<get_random_interval<90 * 1000000, 110 * 1000000>> m_mms_checker;
-    epee::math_helper::once_a_time_seconds<90> m_rpc_payment_checker;
+    epee::math_helper::once_a_time_seconds_range<get_random_interval<90 * 1000000, 115 * 1000000>> m_rpc_payment_checker;
     
     std::atomic<bool> m_need_payment;
     boost::posix_time::ptime m_last_rpc_payment_mining_time;