[kerx] Protext against overflows

src/hb-aat-layout-kerx-table.hh

@@ -233,7 +233,7 @@ struct KerxSubTableFormat2
     unsigned int offset = l + r;
     const FWORD *v = &StructAtOffset<FWORD> (&(this+array), offset);
     if (unlikely ((const char *) v < (const char *) &array ||
-		  (const char *) v + v->static_size - (const char *) this > header.length))
+		  (const char *) v - (const char *) this > header.length - v->static_size))
       return 0;
     return *v;
   }
@@ -480,9 +480,11 @@ struct KerxSubTableFormat6
       unsigned int l = (this+t.rowIndexTable).get_value_or_null (left, num_glyphs);
       unsigned int r = (this+t.columnIndexTable).get_value_or_null (right, num_glyphs);
       unsigned int offset = l + r;
+      if (unlikely (offset < l)) return 0; /* Addition overflow. */
+      if (unlikely (hb_unsigned_mul_overflows (offset, sizeof (FWORD32)))) return 0;
       const FWORD32 *v = &StructAtOffset<FWORD32> (&(this+t.array), offset * sizeof (FWORD32));
       if (unlikely ((const char *) v < (const char *) &t.array ||
-		    (const char *) v + v->static_size - (const char *) this > header.length))
+		    (const char *) v - (const char *) this > header.length - v->static_size))
 	return 0;
       return *v;
     }
@@ -494,7 +496,7 @@ struct KerxSubTableFormat6
       unsigned int offset = l + r;
       const FWORD *v = &StructAtOffset<FWORD> (&(this+t.array), offset * sizeof (FWORD));
       if (unlikely ((const char *) v < (const char *) &t.array ||
-		    (const char *) v + v->static_size - (const char *) this > header.length))
+		    (const char *) v - (const char *) this > header.length - v->static_size))
 	return 0;
       return *v;
     }