Fix tree-hash cnt n^2. Asserts, comment. Squash2

src/crypto/tree-hash.c

@@ -35,7 +35,43 @@
 
 #include "hash-ops.h"
 
+/// Quick check if this is power of two (use on unsigned types; in this case for size_t only)
+bool ispowerof2_size_t(size_t x) {
+	return x && !(x & (x - 1));
+}
+
+/*** 
+* Round to power of two, for count>=3 and for count being not too large (as reasonable for tree hash calculations)
+*/
+size_t tree_hash_cnt(size_t count) {
+	assert( count >= 3); // cases for 0,1,2 are handled elsewhere
+	// Round down the count size: fun(2**n)= 2**(n-1) to round down to power of two
+	size_t tmp = count - 1;
+	size_t jj = 1;
+	for (jj=1 ; tmp != 0 ; ++jj) {
+		tmp /= 2; // dividing by 2 until to get how many powers of 2 fits size_to tmp
+	}
+	size_t cnt = 1 << (jj-2); // cnt is the count, but rounded down to power of two
+	// printf("count=%zu cnt=%zu jj=%zu tmp=%zu \n" , count,cnt,jj,tmp);
+	assert( cnt > 0 );	assert( cnt >= count/2 ); 	assert( cnt <= count );
+	assert( ispowerof2_size_t( cnt ));
+	return cnt;
+}
+
 void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
+// The blockchain block at height 202612 http://monerochain.info/block/bbd604d2ba11ba27935e006ed39c9bfdd99b76bf4a50654bc1e1e61217962698
+// contained 514 transactions, that triggered bad calculation of variable "cnt" in the original version of this function
+// as from CryptoNote code.
+//
+// This bug applies to all CN altcoins.
+//
+// Mathematical bug here was first published on 14:45:34 (GMT+2) 2014-09-04 by Rafal Freeman <rfree>
+// https://github.com/rfree2monero/bitmonero/commit/b417abfb7a297d09f1bbb6de29030f8de9952ac8
+// and soon also applied to CryptoNote (15:10 GMT+2), and BoolBerry used not fully correct work around:
+// the work around of sizeof(size_t)*8 or <<3 as used before in 2 coins and in BBL later was blocking
+// exploitation on normal platforms, how ever we strongly recommend the following fix because it removes
+// mistake in mathematical formula.
+
   assert(count > 0);
   if (count == 1) {
     memcpy(root_hash, hashes, HASH_SIZE);
@@ -43,24 +79,30 @@ void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
     cn_fast_hash(hashes, 2 * HASH_SIZE, root_hash);
   } else {
     size_t i, j;
-    size_t cnt = count - 1;
+
+    size_t cnt = tree_hash_cnt( count );
+    size_t max_size_t = (size_t) -1; // max allowed value of size_t 
+    assert( cnt < max_size_t/2 ); // reasonable size to avoid any overflows. /2 is extra; Anyway should be limited much stronger by logical code 
+    // as we have sane limits on transactions counts in blockchain rules
+
     char (*ints)[HASH_SIZE];
-    for (i = 1; i < sizeof(size_t); i <<= 1) {
-      cnt |= cnt >> i;
-    }
-    cnt &= ~(cnt >> 1);
-    ints = alloca(cnt * HASH_SIZE);
+    size_t ints_size = cnt * HASH_SIZE;
+    ints = alloca(ints_size); 	memset( ints , 0 , ints_size);  // allocate, and zero out as extra protection for using uninitialized mem
+
     memcpy(ints, hashes, (2 * cnt - count) * HASH_SIZE);
+
     for (i = 2 * cnt - count, j = 2 * cnt - count; j < cnt; i += 2, ++j) {
       cn_fast_hash(hashes[i], 64, ints[j]);
     }
     assert(i == count);
+
     while (cnt > 2) {
       cnt >>= 1;
       for (i = 0, j = 0; j < cnt; i += 2, ++j) {
         cn_fast_hash(ints[i], 64, ints[j]);
       }
     }
+
     cn_fast_hash(ints[0], 64, root_hash);
   }
 }