xfrm: iptfs: validate inner IPv4 header length in IPTFS payload

In the Linux kernel, the following vulnerability has been resolved:

xfrm: iptfs: validate inner IPv4 header length in IPTFS payload

Add validation of the inner IPv4 packet tot_len and ihl fields parsed

from decrypted IPTFS payloads in __input_process_payload(). A crafted

ESP packet containing an inner IPv4 header with tot_len=0 causes an

infinite loop: iplen=0 leads to capturelen=min(0, remaining)=0, so the

data offset never advances and the while(data < tail) loop never

terminates, spinning forever in softirq context.

Reject inner IPv4 packets where tot_len < ihl*4 or ihl*4 < sizeof(struct

iphdr), which catches both the tot_len=0 case and malformed ihl values.

The normal IP stack performs this validation in ip_rcv_core(), but IPTFS

extracts and processes inner packets before they reach that layer.