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jbig2dec: Support up to 16 bit gray-scale image for halftoning.

Without this commit if jbig2dec is fed a fuzzed bitstream where
HBPP ends up larger than 8, jbig2dec will access bits outside of
each 8 bit sample in GSVALS.

HBPP is in Table 22 in 6.6.4 defined to be 32 bits unsigned. The
specification in C.2 limits GSBPP to 6 bits unsigned, a maximum
value of 63, i.e. a gray-scale image with 63 bits per sample.
According to table 23 in 6.6.5 HBPP is assigned to GSBPP, so any
value larger than 63 would be out of spec.

A non-fuzzed bitstream that has HBPP larger than 9 is yet to be
encountered. So for the time being use uint16_t the GSVALS and
limit HBPP to 16. If a file with HBPP larger than 16 is ever
encountered, the type and limit needs to be revisited.

--- a/jbig2_halftone.c

+++ b/jbig2_halftone.c

@@ -258,13 +258,13 @@

  * returns: array of gray-scale values with GSW x GSH width/height

  *          0 on failure

  **/

-static uint8_t **

+static uint16_t **

 jbig2_decode_gray_scale_image(Jbig2Ctx *ctx, Jbig2Segment *segment,

                               const byte *data, const size_t size,

                               bool GSMMR, uint32_t GSW, uint32_t GSH,

                               uint32_t GSBPP, bool GSUSESKIP, Jbig2Image *GSKIP, int GSTEMPLATE, Jbig2ArithCx *GB_stats)

 {

-    uint8_t **GSVALS = NULL;

+    uint16_t **GSVALS = NULL;

     size_t consumed_bytes = 0;

     uint32_t i, j, stride, x, y;

     int code;

@@ -357,13 +357,13 @@

     }

     /* allocate GSVALS */

-    GSVALS = jbig2_new(ctx, uint8_t *, GSW);

+    GSVALS = jbig2_new(ctx, uint16_t *, GSW);

     if (GSVALS == NULL) {

         jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number, "failed to allocate GSVALS: %d bytes", GSW);

         goto cleanup;

     }

     for (i = 0; i < GSW; ++i) {

-        GSVALS[i] = jbig2_new(ctx, uint8_t, GSH);

+        GSVALS[i] = jbig2_new(ctx, uint16_t, GSH);

         if (GSVALS[i] == NULL) {

             jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number, "failed to allocate GSVALS: %d bytes", GSH * GSW);

             /* free already allocated */

@@ -454,13 +454,13 @@

 {

     uint32_t HBPP;

     uint32_t HNUMPATS;

-    uint8_t **GI = NULL;

+    uint16_t **GI = NULL;

     Jbig2Image *HSKIP = NULL;

     Jbig2PatternDict *HPATS;

     uint32_t i;

     int32_t mg, ng;

     int32_t x, y;

-    uint8_t gray_val;

+    uint16_t gray_val;

     int code = 0;

     /* We need the patterns used in this region, get them from the referred pattern dictionary */

@@ -497,6 +497,10 @@

     HNUMPATS = HPATS->n_patterns;

     HBPP = 0;

     while (HNUMPATS > (1U << ++HBPP));

+    if (HBPP > 16) {

+        code = jbig2_error(ctx, JBIG2_SEVERITY_FATAL, segment->number, "HBPP is larger than supported (%u)", HBPP);

+        goto cleanup;

+    }

     /* 6.6.5 point 4. decode gray-scale image as mentioned in annex C */

     GI = jbig2_decode_gray_scale_image(ctx, segment, data, size,