Protect against a fax VL(n) codeword commanding a move left.  Without

this, a malicious input file can generate an indefinitely large series

of runs without a0 ever reaching the right margin, thus overrunning

our buffer of run lengths.  Per CVE-2011-0192.  This is a modified

version of a patch proposed by Drew Yao of Apple Product Security.

It adds an unexpected() report, and disallows the equality case except

for the first run of a line, since emitting a run without increasing a0

still allows buffer overrun.  (We have to allow it for the first run to

cover the case of encoding a zero-length run at start of line using VL.)

diff -Naur tiff-3.9.4.orig/libtiff/tif_fax3.h tiff-3.9.4/libtiff/tif_fax3.h

--- tiff-3.9.4.orig/libtiff/tif_fax3.h	2010-06-08 14:50:42.000000000 -0400

+++ tiff-3.9.4/libtiff/tif_fax3.h	2011-03-10 12:11:20.850839162 -0500

@@ -478,6 +478,12 @@

 	    break;							\

 	case S_VL:							\

 	    CHECK_b1;							\

+	    if (b1 <= (int) (a0 + TabEnt->Param)) {			\

+		if (b1 < (int) (a0 + TabEnt->Param) || pa != thisrun) {	\

+		    unexpected("VL", a0);				\

+		    goto eol2d;						\

+		}							\

+	    }								\

 	    SETVALUE(b1 - a0 - TabEnt->Param);				\

 	    b1 -= *--pb;						\

 	    break;							\