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Re: LZW decompression issues
From: Solar Designer <solar () openwall com>
Date: Wed, 28 Sep 2011 19:42:03 +0400


On Wed, Aug 10, 2011 at 08:22:20PM +0200, Tomas Hoger wrote:
We've recently came across an issue in commonly re-used LZW
decompression implementations - original BSD compress and GIF reader
written by David Koblas.  Due to an insufficient input checking, invalid
LZW stream can create a loop in the decompression table, leading to the
decompression stack buffer overflow.

Following bugzillas list various code bases that were checked for the
issue and if they are affected or not:

FreeBSD has just released an advisory for this:


To my surprise, it lists gzip as affected (and provides a patch for it
too), even though it was believed that neither CVE-2011-2895 nor
CVE-2011-2896 affected current versions of gzip (at least per Tomas'
off-list notification to distro vendors).

The latest security fix in upstream gzip is from January 2010, for


(no idea if that issue was present in FreeBSD's gzip or not).

Trying to match the changes to usr.bin/gzip/zuncompress.c in


against code in gzip 1.4 tarball, it appears that FreeBSD's patch
actually introduces more checks than gzip upstream has - although it is
difficult to tell for sure because of other differences in the code.

For example gzip-1.4/unlzw.c has:

    if (maxbits > BITS) {

FreeBSD now patches it as:

-       if (zs->zs_maxbits > BITS) {
+       if (zs->zs_maxbits > BITS || zs->zs_maxbits < 12) {

Do we possibly want to add the "maxbits < 12" check as well?  And does
it matter for security?

Then there are non-obvious differences related to the "oldcode"
variable.  In one of those places, gzip-1.4/unlzw.c has:

            if (code >= free_ent) { /* Special case for KwKwK string. */
                if (code > free_ent) {

whereas the FreeBSD patch has:

                if (zs->u.r.zs_code >= zs->zs_free_ent) {
+                       if (zs->u.r.zs_code > zs->zs_free_ent ||
+                           zs->u.r.zs_oldcode == -1) {
+                               /* Bad stream. */

which adds an extra "or" condition.

Similarly, gzip-1.4/unlzw.c has:

            if ((code = free_ent) < maxmaxcode) { /* Generate the new entry. */

whereas the FreeBSD patch has:

                /* Generate the new entry. */
-               if ((zs->u.r.zs_code = zs->zs_free_ent) < zs->zs_maxmaxcode) {
+               if ((zs->u.r.zs_code = zs->zs_free_ent) < zs->zs_maxmaxcode &&
+                   zs->u.r.zs_oldcode != -1) {

(an extra "and" condition this time).

Are these differences only a result of other differences in the FreeBSD
revision of gzip?  Or are they generic hardening that could get into
gzip proper and into other distros' revisions of gzip?  Or are they even
security fixes for issues known to FreeBSD (but presumably not to others
yet, in gzip context)?

...looking at similar changes to usr.bin/compress/zopen.c in the same
patch, I guess this may be how those changes were made to the gzip code
as well - due to similarities between the two codebases.  However, even
if so this does not fully address my questions above.

Colin - any comments?



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