[PATCH] lib: zlib: Remove offset pointer optimization in inftrees.c

[Ley Foon Tan]

From: Chin Liang See <chin.liang.see at intel.com>

This fixes the CVE-2016-9840. Commit imported from [1].

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.

[1]: https://github.com/madler/zlib/commit/6a043145ca6e9c55184013841a67b2fef87e44c0

Signed-off-by: Mark Adler <madler at alumni.caltech.edu>
Signed-off-by: Chin Liang See <chin.liang.see at intel.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan at intel.com>
---
 lib/zlib/inftrees.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/lib/zlib/inftrees.c b/lib/zlib/inftrees.c
index b71b9695a849..caee502f885c 100644
--- a/lib/zlib/inftrees.c
+++ b/lib/zlib/inftrees.c
@@ -50,7 +50,7 @@ int inflate_table(codetype type, unsigned short FAR *lens, unsigned codes,
     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 */
@@ -178,19 +178,17 @@ int inflate_table(codetype type, unsigned short FAR *lens, unsigned codes,
     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 */
@@ -212,13 +210,12 @@ int inflate_table(codetype type, unsigned short FAR *lens, unsigned codes,
     for (;;) {
         /* create table entry */
         this.bits = (unsigned char)(len - drop);
-        if ((int)(work[sym]) < end) {
+        if (work[sym] + 1 < match) {
             this.op = (unsigned char)0;
             this.val = work[sym];
-        }
-        else if ((int)(work[sym]) > end) {
-            this.op = (unsigned char)(extra[work[sym]]);
-            this.val = base[work[sym]];
+        } else if (work[sym] >= match) {
+            this.op = (unsigned char)(extra[work[sym] - match]);
+            this.val = base[work[sym] - match];
         }
         else {
             this.op = (unsigned char)(32 + 64);         /* end of block */
-- 
2.19.0