vmxnet3 unable to send IPv6 ESP packets

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1758091
Reported by 'Thomas Jansen' on 2018-03-22 :

My vmxnet3 network driver (in a closed source custom OS) is unable to send
network packets that are structured as follows: Ethernet-
Header(IPv6-Header(ESP(encrypted data))). I can verify that the packet is
sent in the VM but is dropped in qemu. I first encountered this problem on
qemu 2.10.1 but master is affected as well. After some debug printing in
qemu I could identify the following call chain as being problematic:

eth_is_ip6_extension_header_type
eth_parse_ipv6_hdr
net_tx_pkt_parse_headers
net_tx_pkt_parse
vmxnet3_process_tx_queue

The problem seems to be the definition of the ESP header
(https://en.wikipedia.org/wiki/IPsec#Encapsulating_Security_Payload) that
does not follow the standard IPv6 extension header format starting with
next type and length. Thus the parsed ext_hdr in eth_parse_ipv6_hdr does
not contain valid data, in particular the length will contain bogus data
and lead to a info->full_hdr_len that is larger than the packet itself and
the loop would then try to read beyond the end of the packet.

Using the e1000 driver I can send these packets. My guess is that the
net_tx_pkt_parse function is not called in that case.

My guess for a fix would be to remove "case IP6_ESP:" from
eth_is_ip6_extension_header_type and not regard the ESP header as a IPv6
extension header. In a quick test this seems to fix the problem. But that
should be verified by someone who is familiar with the code.