Commit message: rar: file split across multi-part archives must match

Fuzzing uncovered some UAF and memory overrun bugs where a file in a
single file archive reported that it was split across multiple
volumes. This was caused by ppmd7 operations calling
rar_br_fillup. This would invoke rar_read_ahead, which would in some
situations invoke archive_read_format_rar_read_header.  That would
check the new file name against the old file name, and if they didn't
match up it would free the ppmd7 buffer and allocate a new
one. However, because the ppmd7 decoder wasn't actually done with the
buffer, it would continue to used the freed buffer. Both reads and
writes to the freed region can be observed.

This is quite tricky to solve: once the buffer has been freed it is
too late, as the ppmd7 decoder functions almost universally assume
success - there's no way for ppmd_read to signal error, nor are there
good ways for functions like Range_Normalise to propagate them. So we
can't detect after the fact that we're in an invalid state - e.g. by
checking rar->cursor, we have to prevent ourselves from ever ending up
there. So, when we are in the dangerous part or rar_read_ahead that
assumes a valid split, we set a flag force read_header to either go
down the path for split files or bail. This means that the ppmd7
decoder keeps a valid buffer and just runs out of data.

Found with a combination of AFL, afl-rb and qsym.
CVE: CVE-2018-1000878
CWE: CWE-416
Vulnerability description: libarchive version commit 416694915449219d505531b1096384f3237dd6cc onwards (release v3.1.0 onwards) contains a CWE-416: Use After Free vulnerability in RAR decoder - libarchive/archive_read_support_format_rar.c that can result in Crash/DoS - it is unknown if RCE is possible. This attack appear to be exploitable via the victim must open a specially crafted RAR archive.

Project: libarchive
File: archive_read_support_format_rar.c

archive_read_format_rar_read_data(struct archive_read *a, const void **buff,
                                  size_t *size, int64_t *offset)
{
  struct rar *rar = (struct rar *)(a->format->data);
  int ret;

  if (rar->has_encrypted_entries == ARCHIVE_READ_FORMAT_ENCRYPTION_DONT_KNOW) {
	  rar->has_encrypted_entries = 0;
  }

  if (rar->bytes_unconsumed > 0) {
      /* Consume as much as the decompressor actually used. */
      __archive_read_consume(a, rar->bytes_unconsumed);
      rar->bytes_unconsumed = 0;
  }

  *buff = NULL;
  if (rar->entry_eof || rar->offset_seek >= rar->unp_size) {
    *size = 0;
    *offset = rar->offset;
    if (*offset < rar->unp_size)
      *offset = rar->unp_size;
    return (ARCHIVE_EOF);
  }

  switch (rar->compression_method)
  {
  case COMPRESS_METHOD_STORE:
    ret = read_data_stored(a, buff, size, offset);
    break;

  case COMPRESS_METHOD_FASTEST:
  case COMPRESS_METHOD_FAST:
  case COMPRESS_METHOD_NORMAL:
  case COMPRESS_METHOD_GOOD:
  case COMPRESS_METHOD_BEST:
    ret = read_data_compressed(a, buff, size, offset);
    if (ret != ARCHIVE_OK && ret != ARCHIVE_WARN)
      __archive_ppmd7_functions.Ppmd7_Free(&rar->ppmd7_context);
    break;

  default:
    archive_set_error(&a->archive, ARCHIVE_ERRNO_FILE_FORMAT,
                      "Unsupported compression method for RAR file.");
    ret = ARCHIVE_FATAL;
    break;
  }
  return (ret);
}