Improve overflow checks in tv_udiff and tv_mdiff

Validate that tv_usec inputs to tv_udiff and tv_mdiff are in range.

Do internal calculations in tv_udiff and tv_mdiff in 64-bit,
which makes the function less prone to integer overflow,
particularly on platforms where long and time_t are 32-bit,
but tv_sec is 64-bit, like some BSD configurations.

Check every addition and subtraction that could overflow.

src/common/util.c

@@ -1386,51 +1386,132 @@ tor_escape_str_for_pt_args(const char *string, const char *chars_to_escape)
  * Time
  * ===== */
 
+#define TOR_USEC_PER_SEC 1000000
+
+/** Return the difference between start->tv_sec and end->tv_sec.
+ * Returns INT64_MAX on overflow and underflow.
+ */
+static int64_t
+tv_secdiff_impl(const struct timeval *start, const struct timeval *end)
+{
+  const int64_t s = (int64_t)start->tv_sec;
+  const int64_t e = (int64_t)end->tv_sec;
+
+  /* This may not be the most efficient way of implemeting this check,
+   * but it's easy to see that it's correct and doesn't overflow */
+
+  if (s > 0 && e < INT64_MIN + s) {
+    /* s is positive: equivalent to e - s < INT64_MIN, but without any
+     * overflow */
+    return INT64_MAX;
+  } else if (s < 0 && e > INT64_MAX + s) {
+    /* s is negative: equivalent to e - s > INT64_MAX, but without any
+     * overflow */
+    return INT64_MAX;
+  }
+
+  return e - s;
+}
+
 /** Return the number of microseconds elapsed between *start and *end.
+ * Returns LONG_MAX on overflow and underflow.
  */
 long
 tv_udiff(const struct timeval *start, const struct timeval *end)
 {
-  long udiff;
-  long secdiff = end->tv_sec - start->tv_sec;
-
-  /* end->tv_usec - start->tv_usec can be up to 1 second */
-  if (labs(secdiff)+1 > LONG_MAX/1000000) {
-    log_warn(LD_GENERAL, "comparing times on microsecond detail too far "
-             "apart: %ld seconds", secdiff);
+  /* Sanity check tv_usec */
+  if (start->tv_usec > TOR_USEC_PER_SEC || start->tv_usec < 0) {
+    log_warn(LD_GENERAL, "comparing times on microsecond detail with bad "
+             "start tv_usec: " I64_FORMAT " microseconds",
+             I64_PRINTF_ARG(start->tv_usec));
     return LONG_MAX;
   }
 
-  udiff = secdiff*1000000L + (end->tv_usec - start->tv_usec);
-  return udiff;
+  if (end->tv_usec > TOR_USEC_PER_SEC || end->tv_usec < 0) {
+    log_warn(LD_GENERAL, "comparing times on microsecond detail with bad "
+             "end tv_usec: " I64_FORMAT " microseconds",
+             I64_PRINTF_ARG(end->tv_usec));
+    return LONG_MAX;
+  }
+
+  /* Some BSDs have struct timeval.tv_sec 64-bit, but time_t (and long) 32-bit
+   */
+  int64_t udiff;
+  const int64_t secdiff = tv_secdiff_impl(start, end);
+
+  /* end->tv_usec - start->tv_usec can be up to 1 second either way */
+  if (secdiff > (int64_t)(LONG_MAX/1000000 - 1) ||
+      secdiff < (int64_t)(LONG_MIN/1000000 + 1)) {
+    log_warn(LD_GENERAL, "comparing times on microsecond detail too far "
+             "apart: " I64_FORMAT " seconds", I64_PRINTF_ARG(secdiff));
+    return LONG_MAX;
+  }
+
+  /* we'll never get an overflow here, because we check that both usecs are
+   * between 0 and TV_USEC_PER_SEC. */
+  udiff = secdiff*1000000 + ((int64_t)end->tv_usec - (int64_t)start->tv_usec);
+
+  if (udiff > (int64_t)LONG_MAX || udiff < (int64_t)LONG_MIN) {
+    return LONG_MAX;
+  } else {
+    return (long)udiff;
+  }
 }
 
 /** Return the number of milliseconds elapsed between *start and *end.
+ * If the tv_usec difference is 500, rounds away from zero.
+ * Returns LONG_MAX on overflow and underflow.
  */
 long
 tv_mdiff(const struct timeval *start, const struct timeval *end)
 {
-  long mdiff;
-  long secdiff = end->tv_sec - start->tv_sec;
+  /* Sanity check tv_usec */
+  if (start->tv_usec > TOR_USEC_PER_SEC || start->tv_usec < 0) {
+    log_warn(LD_GENERAL, "comparing times on millisecond detail with bad "
+             "start tv_usec: " I64_FORMAT " microseconds",
+             I64_PRINTF_ARG(start->tv_usec));
+    return LONG_MAX;
+  }
 
-  /* end->tv_usec - start->tv_usec can be up to 1 second,
-   * but the mdiff calculation adds another temporary second */
-  if (labs(secdiff)+2 > LONG_MAX/1000) {
+  if (end->tv_usec > TOR_USEC_PER_SEC || end->tv_usec < 0) {
+    log_warn(LD_GENERAL, "comparing times on millisecond detail with bad "
+             "end tv_usec: " I64_FORMAT " microseconds",
+             I64_PRINTF_ARG(end->tv_usec));
+    return LONG_MAX;
+  }
+
+  /* Some BSDs have struct timeval.tv_sec 64-bit, but time_t (and long) 32-bit
+   */
+  int64_t mdiff;
+  const int64_t secdiff = tv_secdiff_impl(start, end);
+
+  /* end->tv_usec - start->tv_usec can be up to 1 second either way, but the
+   * mdiff calculation may add another temporary second for rounding.
+   * Whether this actually causes overflow depends on the compiler's constant
+   * folding and order of operations. */
+  if (secdiff > (int64_t)(LONG_MAX/1000 - 2) ||
+      secdiff < (int64_t)(LONG_MIN/1000 + 1)) {
     log_warn(LD_GENERAL, "comparing times on millisecond detail too far "
-             "apart: %ld seconds", secdiff);
+             "apart: " I64_FORMAT " seconds", I64_PRINTF_ARG(secdiff));
     return LONG_MAX;
   }
 
   /* Subtract and round */
-  mdiff = secdiff*1000L +
+  mdiff = secdiff*1000 +
       /* We add a million usec here to ensure that the result is positive,
        * so that the round-towards-zero behavior of the division will give
        * the right result for rounding to the nearest msec. Later we subtract
        * 1000 in order to get the correct result.
-       */
-      ((long)end->tv_usec - (long)start->tv_usec + 500L + 1000000L) / 1000L
+       * We'll never get an overflow here, because we check that both usecs are
+       * between 0 and TV_USEC_PER_SEC. */
+      ((int64_t)end->tv_usec - (int64_t)start->tv_usec + 500 + 1000000) / 1000
       - 1000;
-  return mdiff;
+
+  if (mdiff > (int64_t)LONG_MAX || mdiff < (int64_t)LONG_MIN) {
+    return LONG_MAX;
+  } else {
+    return (long)mdiff;
+  }
 }
 
 /**