selinux: handle TCP SYN-ACK packets correctly in selinux_ip_postroute()

selinux: handle TCP SYN-ACK packets correctly in selinux_ip_postroute()

In selinux_ip_postroute() we perform access checks based on the
packet's security label.  For locally generated traffic we get the
packet's security label from the associated socket; this works in all
cases except for TCP SYN-ACK packets.  In the case of SYN-ACK packet's
the correct security label is stored in the connection's request_sock,
not the server's socket.  Unfortunately, at the point in time when
selinux_ip_postroute() is called we can't query the request_sock
directly, we need to recreate the label using the same logic that
originally labeled the associated request_sock.

See the inline comments for more explanation.

Reported-by: Janak Desai <[email protected]>
Tested-by: Janak Desai <[email protected]>
Cc: [email protected]
Signed-off-by: Paul Moore <[email protected]>