[CVE-2016-9843] Avoid pre-decrement of pointer in big-endian CRC calculation.

There was a small optimization for PowerPCs to pre-increment a
pointer when accessing a word, instead of post-incrementing. This
required prefacing the loop with a decrement of the pointer,
possibly pointing before the object passed. This is not compliant
with the C standard, for which decrementing a pointer before its
allocated memory is undefined. When tested on a modern PowerPC
with a modern compiler, the optimization no longer has any effect.
Due to all that, and per the recommendation of a security audit of
the zlib code by Trail of Bits and TrustInSoft, in support of the
Mozilla Foundation, this "optimization" was removed, in order to
avoid the possibility of undefined behavior.

Change-Id: Ide8a89b834f0b2af170ab8725eccc3fad3a5d1bf
Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>

diff --git a/compat/zlib/crc32.c b/compat/zlib/crc32.c
index 979a719..05733f4 100644 (file)
--- a/compat/zlib/crc32.c
+++ b/compat/zlib/crc32.c
@@ -278,7 +278,7 @@ local unsigned long crc32_little(crc, buf, len)
 }
 
 /* ========================================================================= */
-#define DOBIG4 c ^= *++buf4; \
+#define DOBIG4 c ^= *buf4++; \
         c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
             crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4
@@ -300,7 +300,6 @@ local unsigned long crc32_big(crc, buf, len)
     }
 
     buf4 = (const z_crc_t FAR *)(const void FAR *)buf;
-    buf4--;
     while (len >= 32) {
         DOBIG32;
         len -= 32;
@@ -309,7 +308,6 @@ local unsigned long crc32_big(crc, buf, len)
         DOBIG4;
         len -= 4;
     }
-    buf4++;
     buf = (const unsigned char FAR *)buf4;
 
     if (len) do {