MFH: r531837

security/openvpn: Fix illegal client float (CVE-2020-11810)

There is a time frame between allocating peer-id and initializing data
channel key (which is performed on receiving push request or on async
push-reply) in which the existing peer-id float checks do not work right.

If a "rogue" data channel packet arrives during that time frame from another
address and with same peer-id, this would cause client to float to that new
address.

The net effect of this behaviour is that the VPN session for the "victim
client" is broken. Since the "attacker client" does not have suitable keys,
it can not inject or steal VPN traffic from the other session. The time
window is small and it can not be used to attack a specific client's session,
unless some other way is found to make it disconnect and reconnect first.

This fix is inherited by the openvpn-mbedtls slave port.

Blanket "Backport of security and reliability fixes which only result in
PORTREVISION bumps and no changes to enabled features. for example, adding
a patch fixing a buffer overflow."

Obtained from: Lev Stipakov (OpenVPN)
Security: CVE-2020-11810
Security: 8604121c-7fc2-11ea-bcac-7781e90b0c8f

Approved by: ports-secteam@ (blanket security/reliability fix)