During xfrm migration copy replay and preplay sequence numbers
from the previous state.

Here is a tcpdump output showing the problem.
10.0.10.46 is running vanilla kernel, is the IKE/IPsec responder.
After the migration it sent wrong sequence number, reset to 1.
The migration is from 10.0.0.52 to 10.0.0.53.

IP 10.0.0.52.4500 > 10.0.10.46.4500: UDP-encap: ESP(spi=0x43ef462d,seq=0x7cf), length 136
IP 10.0.10.46.4500 > 10.0.0.52.4500: UDP-encap: ESP(spi=0xca1c282d,seq=0x7cf), length 136
IP 10.0.0.52.4500 > 10.0.10.46.4500: UDP-encap: ESP(spi=0x43ef462d,seq=0x7d0), length 136
IP 10.0.10.46.4500 > 10.0.0.52.4500: UDP-encap: ESP(spi=0xca1c282d,seq=0x7d0), length 136

IP 10.0.0.53.4500 > 10.0.10.46.4500: NONESP-encap: isakmp: child_sa  inf2[I]
IP 10.0.10.46.4500 > 10.0.0.53.4500: NONESP-encap: isakmp: child_sa  inf2[R]
IP 10.0.0.53.4500 > 10.0.10.46.4500: NONESP-encap: isakmp: child_sa  inf2[I]
IP 10.0.10.46.4500 > 10.0.0.53.4500: NONESP-encap: isakmp: child_sa  inf2[R]

IP 10.0.0.53.4500 > 10.0.10.46.4500: UDP-encap: ESP(spi=0x43ef462d,seq=0x7d1), length 136

NOTE: next sequence is wrong 0x1

IP 10.0.10.46.4500 > 10.0.0.53.4500: UDP-encap: ESP(spi=0xca1c282d,seq=0x1), length 136
IP 10.0.0.53.4500 > 10.0.10.46.4500: UDP-encap: ESP(spi=0x43ef462d,seq=0x7d2), length 136
IP 10.0.10.46.4500 > 10.0.0.53.4500: UDP-encap: ESP(spi=0xca1c282d,seq=0x2), length 136
Signed-off-by: Antony Antony <antony@phenome.org>
Reviewed-by: Richard Guy Briggs <rgb@tricolour.ca>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>