3027 installgrub can segfault when encountering bogus data on disk

*** 105,123 ****
  /*
   * Given a pointer to the extra information area (a sequence of bb_header_ext_t
   * + payload chunks), find the extended information structure.
   */
  bblk_einfo_t *
! find_einfo(char *extra)
  {
          bb_header_ext_t         *ext_header;
          bblk_einfo_t            *einfo;
          uint32_t                cksum;
  
          assert(extra != NULL);
  
          ext_header = (bb_header_ext_t *)extra;
          cksum = compute_checksum(extra + sizeof (bb_header_ext_t),
              ext_header->size);
          BOOT_DEBUG("Extended information header checksum is %x\n", cksum);
  
          if (cksum != ext_header->checksum) {
--- 105,129 ----
  /*
   * Given a pointer to the extra information area (a sequence of bb_header_ext_t
   * + payload chunks), find the extended information structure.
   */
  bblk_einfo_t *
! find_einfo(char *extra, uint32_t size)
  {
          bb_header_ext_t         *ext_header;
          bblk_einfo_t            *einfo;
          uint32_t                cksum;
  
          assert(extra != NULL);
  
          ext_header = (bb_header_ext_t *)extra;
+         if (ext_header->size > size) {
+                 BOOT_DEBUG("Unable to find extended versioning information, "
+                     "data size too big\n");
+                 return (NULL);
+         }
+ 
          cksum = compute_checksum(extra + sizeof (bb_header_ext_t),
              ext_header->size);
          BOOT_DEBUG("Extended information header checksum is %x\n", cksum);
  
          if (cksum != ext_header->checksum) {