linux-user: fix netlink memory corruption

Netlink is byte-swapping data in the guest memory (it's bad).

It's ok when the data come from the host as they are generated by the
host.

But it doesn't work when data come from the guest: the guest can
try to reuse these data whereas they have been byte-swapped.

This is what happens in glibc:

glibc generates a sequence number in nlh.nlmsg_seq and calls
sendto() with this nlh. In sendto(), we byte-swap nlmsg.seq.

Later, after the recvmsg(), glibc compares nlh.nlmsg_seq with
sequence number given in return, and of course it fails (hangs),
because nlh.nlmsg_seq is not valid anymore.

The involved code in glibc is:

sysdeps/unix/sysv/linux/check_pf.c:make_request()
...
  req.nlh.nlmsg_seq = time (NULL);
...
  if (TEMP_FAILURE_RETRY (__sendto (fd, (void *) &req, sizeof (req), 0,
                                    (struct sockaddr *) &nladdr,
                                    sizeof (nladdr))) < 0)
<here req.nlh.nlmsg_seq has been byte-swapped>
...
  do
    {
...
      ssize_t read_len = TEMP_FAILURE_RETRY (__recvmsg (fd, &msg, 0));
...
      struct nlmsghdr *nlmh;
      for (nlmh = (struct nlmsghdr *) buf;
           NLMSG_OK (nlmh, (size_t) read_len);
           nlmh = (struct nlmsghdr *) NLMSG_NEXT (nlmh, read_len))
        {
<we compare nlmh->nlmsg_seq with corrupted req.nlh.nlmsg_seq>
          if (nladdr.nl_pid != 0 || (pid_t) nlmh->nlmsg_pid != pid
              || nlmh->nlmsg_seq != req.nlh.nlmsg_seq)
            continue;
...
          else if (nlmh->nlmsg_type == NLMSG_DONE)
            /* We found the end, leave the loop.  */
            done = true;
        }
    }
  while (! done);

As we have a continue on "nlmh->nlmsg_seq != req.nlh.nlmsg_seq",
"done" cannot be set to "true" and we have an infinite loop.

It's why commands like "apt-get update" or "dnf update hangs".

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
