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Bug 694949: prevent denial of service with arithmetic decoding

Badly constructed Jbig2 images using arithmetic decoding may result in
a denial of service attack by causing huge images to be created and
initialized from no data at all (at EOS, the arithmetic decoder always
returns the same values instead of failing).

Two cases are prevented with this patch:
* huge generic regions with barely any data to decode it from
* a huge list of height classes with no data at all to decode it from

This can be seen e.g. in:

b534b5caad95dd405735ec9a079fd43b_asan_heap-oob_14bf3ce_6977_5690.pdf

Thanks to Mateusz Jurczyk and Gynvael Coldwind of the Google Security
Team for providing the example files.

--- a/jbig2_arith.c

+++ b/jbig2_arith.c

@@ -379,6 +379,12 @@

     }

 }

+int

+jbig2_arith_get_offset(Jbig2ArithState *as)

+{

+  return as->offset;

+}

+

 #ifdef TEST

 static int

--- a/jbig2_arith.h

+++ b/jbig2_arith.h

@@ -33,4 +33,6 @@

 bool

 jbig2_arith_decode (Jbig2ArithState *as, Jbig2ArithCx *pcx);

-

+/* return the state's offset (for sanity checks) */

+int

+jbig2_arith_get_offset(Jbig2ArithState *as);

--- a/jbig2_generic.c

+++ b/jbig2_generic.c

@@ -760,6 +760,12 @@

 {

   const int8_t *gbat = params->gbat;

+  if (image->stride * image->height > (1 << 24) && segment->data_length < image->stride * image->height / 256) {

+    return jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number,

+                       "region is far larger than data provided (%d << %d), aborting to prevent DOS",

+                       segment->data_length, image->stride * image->height);

+  }

+

   if (!params->MMR && params->TPGDON) 

      return jbig2_decode_generic_region_TPGDON(ctx, segment, params, 

 		as, image, GB_stats);

--- a/jbig2_symbol_dict.c

+++ b/jbig2_symbol_dict.c

@@ -361,6 +361,12 @@

 	  "error or OOB decoding height class delta (%d)\n", code);

       }

+      if (!params->SDHUFF && jbig2_arith_get_offset(as) >= size) {

+          code = jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number,

+              "prevent DOS while decoding height classes");

+          goto cleanup2;

+      }

+

       /* 6.5.5 (4b) */

       HCHEIGHT = HCHEIGHT + HCDH;

       SYMWIDTH = 0;