[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20110929090657.1136981d@redhat.com>
Date: Thu, 29 Sep 2011 09:06:57 +0200
From: Tomas Hoger <thoger@...hat.com>
To: oss-security@...ts.openwall.com
Cc: Tavis Ormandy <taviso@...xchg8b.com>, joerg@...bsd.org
Subject: Re: LZW decompression issues
On Thu, 29 Sep 2011 04:38:08 +0400 Solar Designer wrote:
> http://cvsweb.openwall.com/cgi/cvsweb.cgi/Owl/packages/gzip/Attic/gzip-1.3.5-google-owl-bound.diff
> http://cvsweb.openwall.com/cgi/cvsweb.cgi/Owl/packages/gzip/Attic/gzip-1.3.5-gentoo-huft_build-return.diff
>
> (these are in Attic because we've since updated to gzip 1.4).
>
> As far as I can see, the sanity checks in
> gzip-1.3.5-google-owl-bound.diff do not overlap with those in FreeBSD's
> latest patch. These are different sets of checks.
Tavis also reported an issue in ncompress - CVE-2006-1168 - with the
following fix added to ncompress:
http://ncompress.git.sourceforge.net/git/gitweb.cgi?p=ncompress/ncompress;a=commitdiff;h=e21aad4a5a3ba0b6c2279b28a80f85b0b226a175
It's rather closely related to CVE-2011-2895, as it was also creating
prefix loop, via bogus first code. At the time that was reported, the
case that I originally started to look at (code > free_ent) was already
fixed in ncompress, afaics.
> As to who originally added the "maxbits < 12" check, when, and why
> exactly (and why this value), I still don't know. In NetBSD, it is
> added with a commit made 6 weeks ago:
>
> http://cvsweb.netbsd.org/bsdweb.cgi/src/usr.bin/gzip/zuncompress.c?only_with_tag=MAIN
>
> The commit message is merely "Do proper input validation without
> penalizing performance", and it makes several other changes as well
> (FreeBSD in fact reused essentially the same patch).
The "without penalizing performance" is reference to my original
libXfont one-liner fix that did not prevent loops, only blocked their
impact by checking for stack buffer overflow. The same kind of fix
Tavis proposed for ncompress to address CVE-2006-1168.
As for < 12, I'm guessing it comes from libXfont too, which had it
before because of this:
if (maxbits > BITS || maxbits < 12)
return 0;
hsize = hsize_table[maxbits - 12];
where:
static int hsize_table[] = {
5003, /* 12 bits - 80% occupancy */
9001, /* 13 bits - 91% occupancy */
18013, /* 14 bits - 91% occupancy */
35023, /* 15 bits - 94% occupancy */
69001 /* 16 bits - 95% occupancy */
};
This seems to be a re-write of the original:
#if BITS == 16
# define HSIZE 69001 /* 95% occupancy */
#endif
#if BITS == 15
# define HSIZE 35023 /* 94% occupancy */
#endif
#if BITS == 14
# define HSIZE 18013 /* 91% occupancy */
#endif
#if BITS == 13
# define HSIZE 9001 /* 91% occupancy */
#endif
#if BITS <= 12
# define HSIZE 5003 /* 80% occupancy */
#endif
The original seems to allow maxbits < 12.
NetBSD / FreeBSD uses following:
#define BITS 16 /* Default bits. */
#define HSIZE 69001 /* 95% occupancy */
hence maxbits < 12 is probably not needed for the same reason it's
needed in libXfont.
Anyway, there seems to be an easy way to test. Can anyone with updated
NetBSD or FreeBSD try this:
echo test | compress -b 10 | uncompress
?
--
Tomas Hoger / Red Hat Security Response Team
Powered by blists - more mailing lists
Please check out the
Open Source Software Security Wiki, which is counterpart to this
mailing list.
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
