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.

Reported-by: Roshan Kumar <[email protected]>
Fixes: 6c82d24 ("xfrm: iptfs: add basic receive packet (tunnel egress) handling")
Cc: [email protected]
Signed-off-by: Roshan Kumar <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>

Author: youhaveme9

net/xfrm/xfrm_iptfs.c

@@ -991,6 +991,11 @@ static bool __input_process_payload(struct xfrm_state *x, u32 data,
 
 			iplen = be16_to_cpu(iph->tot_len);
 			iphlen = iph->ihl << 2;
+			if (iplen < iphlen || iphlen < sizeof(*iph)) {
+				XFRM_INC_STATS(net,
+					       LINUX_MIB_XFRMINHDRERROR);
+				goto done;
+			}
 			protocol = cpu_to_be16(ETH_P_IP);
 			XFRM_MODE_SKB_CB(skbseq->root_skb)->tos = iph->tos;
 		} else if (iph->version == 0x6) {