[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: I3af497cf1229a09cdc2f2c20934b29add514084a
Signed-off-by: DongHun Kwak <dh0128.kwak@samsung.com>

diff --git a/zlib/crc32.c b/zlib/crc32.c
index f658a9e..704b748 100644 (file)
--- a/zlib/crc32.c
+++ b/zlib/crc32.c
@@ -293,7 +293,7 @@ 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
@@ -315,7 +315,6 @@ local unsigned long crc32_big(crc, buf, len)
     }
 
     buf4 = (const u4 FAR *)(const void FAR *)buf;
-    buf4--;
     while (len >= 32) {
         DOBIG32;
         len -= 32;
@@ -324,7 +323,6 @@ local unsigned long crc32_big(crc, buf, len)
         DOBIG4;
         len -= 4;
     }
-    buf4++;
     buf = (const unsigned char FAR *)buf4;
 
     if (len) do {