commit 6a043145ca6e9c55184013841a67b2fef87e44c0
Author: Mark Adler <madler@alumni.caltech.edu>
Date:   Wed Sep 21 23:35:50 2016 -0700

    Remove offset pointer optimization in inftrees.c.
    
    inftrees.c was subtracting an offset from a pointer to an array,
    in order to provide a pointer that allowed indexing starting at
    the offset. This is not compliant with the C standard, for which
    the behavior of a pointer decremented before its allocated memory
    is undefined. Per the recommendation of a security audit of the
    zlib code by Trail of Bits and TrustInSoft, in support of the
    Mozilla Foundation, this tiny optimization was removed, in order
    to avoid the possibility of undefined behavior.

Upstream-Status: Backport
http://http.debian.net/debian/pool/main/z/zlib/zlib_1.2.8.dfsg-5.debian.tar.xz
https://github.com/madler/zlib/commit/6a043145ca6e9c55184013841a67b2fef87e44c0

CVE: CVE-2016-9840

Signed-off-by: George McCollister <george.mccollister@gmail.com>

diff --git a/inftrees.c b/inftrees.c
index 22fcd66..0d2670d 100644
--- a/inftrees.c
+++ b/inftrees.c
@@ -54,7 +54,7 @@ unsigned short FAR *work;
     code FAR *next;             /* next available space in table */
     const unsigned short FAR *base;     /* base value table to use */
     const unsigned short FAR *extra;    /* extra bits table to use */
-    int end;                    /* use base and extra for symbol > end */
+    unsigned match;             /* use base and extra for symbol >= match */
     unsigned short count[MAXBITS+1];    /* number of codes of each length */
     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */
     static const unsigned short lbase[31] = { /* Length codes 257..285 base */
@@ -181,19 +181,17 @@ unsigned short FAR *work;
     switch (type) {
     case CODES:
         base = extra = work;    /* dummy value--not used */
-        end = 19;
+        match = 20;
         break;
     case LENS:
         base = lbase;
-        base -= 257;
         extra = lext;
-        extra -= 257;
-        end = 256;
+        match = 257;
         break;
     default:            /* DISTS */
         base = dbase;
         extra = dext;
-        end = -1;
+        match = 0;
     }
 
     /* initialize state for loop */
@@ -216,13 +214,13 @@ unsigned short FAR *work;
     for (;;) {
         /* create table entry */
         here.bits = (unsigned char)(len - drop);
-        if ((int)(work[sym]) < end) {
+        if (work[sym] + 1 < match) {
             here.op = (unsigned char)0;
             here.val = work[sym];
         }
-        else if ((int)(work[sym]) > end) {
-            here.op = (unsigned char)(extra[work[sym]]);
-            here.val = base[work[sym]];
+        else if (work[sym] >= match) {
+            here.op = (unsigned char)(extra[work[sym] - match]);
+            here.val = base[work[sym] - match];
         }
         else {
             here.op = (unsigned char)(32 + 64);         /* end of block */
