Bug 1096741 (part 1) - In mp_exptmod_safe_i(), instantiate |powersArr…

…ay| lazily, r=rrelyea, r=kaie

lib/freebl/mpi/mpmontg.c

@@ -883,8 +883,8 @@ mp_err mp_exptmod_safe_i(const mp_int *   montBase,
   int     expOff;
   mp_int  accum1, accum2, accum[WEAVE_WORD_SIZE];
   mp_int  tmp;
-  unsigned char *powersArray;
-  unsigned char *powers;
+  unsigned char *powersArray = NULL;
+  unsigned char *powers = NULL;
 
   MP_DIGITS(&accum1) = 0;
   MP_DIGITS(&accum2) = 0;
@@ -894,15 +894,6 @@ mp_err mp_exptmod_safe_i(const mp_int *   montBase,
   MP_DIGITS(&accum[3]) = 0;
   MP_DIGITS(&tmp) = 0;
 
-  powersArray = (unsigned char *)malloc(num_powers*(nLen*sizeof(mp_digit)+1));
-  if (powersArray == NULL) {
-    res = MP_MEM;
-    goto CLEANUP;
-  }
-
-  /* powers[i] = base ** (i); */
-  powers = (unsigned char *)MP_ALIGN(powersArray,num_powers);
-
   /* grab the first window value. This allows us to preload accumulator1
    * and save a conversion, some squares and a multiple*/
   MP_CHECKOK( mpl_get_bits(exponent, 
@@ -924,6 +915,13 @@ mp_err mp_exptmod_safe_i(const mp_int *   montBase,
     MP_CHECKOK( mp_copy(montBase, &accum[1]) );
     SQR(montBase, &accum[2]);
     MUL_NOWEAVE(montBase, &accum[2], &accum[3]);
+    powersArray = (unsigned char *)malloc(num_powers*(nLen*sizeof(mp_digit)+1));
+    if (!powersArray) {
+      res = MP_MEM;
+      goto CLEANUP;
+    }
+    /* powers[i] = base ** (i); */ \
+    powers = (unsigned char *)MP_ALIGN(powersArray,num_powers); \
     MP_CHECKOK( mpi_to_weave(accum, powers, nLen, num_powers) );
     if (first_window < 4) {
       MP_CHECKOK( mp_copy(&accum[first_window], &accum1) );
@@ -945,7 +943,10 @@ mp_err mp_exptmod_safe_i(const mp_int *   montBase,
    * odd powers where k is the window size in the two other mp_modexpt
    * implementations in this file. We will get some of that
    * back by not needing the first 'k' squares and one multiply for the 
-   * first window */ 
+   * first window.
+   * Given the value of 4 for WEAVE_WORD_SIZE, this loop will only execute if
+   * num_powers > 2, in which case powers will have been allocated.
+   */
   for (i = WEAVE_WORD_SIZE; i < num_powers; i++) {
     int acc_index = i & (WEAVE_WORD_SIZE-1); /* i % WEAVE_WORD_SIZE */
     if ( i & 1 ) {