[PATCH nf v2] ipvs: ensure inner headers in ICMP errors are in headroom

Sashiko points out that after stripping the outer headers
with pskb_pull() we should ensure the inner IP headers
in ICMP errors from tunnels are present in the skb headroom
for functions like ipv4_update_pmtu(), icmp_send() and
IP_VS_DBG().

Also, add more checks for the length of the inner headers.

Fixes: f2edb9f7706d ("ipvs: implement passive PMTUD for IPIP packets")
Link: https://sashiko.dev/#/patchset/20260702073430.67680-1-zhaoyz24%40mails.tsinghua.edu.cn
Signed-off-by: Julian Anastasov <[email protected]>
---
 net/netfilter/ipvs/ip_vs_core.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index 906f2c361676..5b427349a257 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c
@@ -1767,6 +1767,7 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 	bool tunnel, new_cp = false;
 	union nf_inet_addr *raddr;
 	char *outer_proto = "IPIP";
+	unsigned int hlen_ipip;
 	int ulen = 0;
 
 	*related = 1;
@@ -1804,9 +1805,10 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 	/* Now find the contained IP header */
 	offset += sizeof(_icmph);
 	cih = skb_header_pointer(skb, offset, sizeof(_ciph), &_ciph);
-	if (cih == NULL)
+	if (!(cih && cih->version == 4 && cih->ihl >= 5))
 		return NF_ACCEPT; /* The packet looks wrong, ignore */
 	raddr = (union nf_inet_addr *)&cih->daddr;
+	hlen_ipip = cih->ihl * 4;
 
 	/* Special case for errors for IPIP/UDP/GRE tunnel packets */
 	tunnel = false;
@@ -1822,9 +1824,9 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 		/* Only for known tunnel */
 		if (!dest || dest->tun_type != IP_VS_CONN_F_TUNNEL_TYPE_IPIP)
 			return NF_ACCEPT;
-		offset += cih->ihl * 4;
+		offset += hlen_ipip;
 		cih = skb_header_pointer(skb, offset, sizeof(_ciph), &_ciph);
-		if (cih == NULL)
+		if (!(cih && cih->version == 4 && cih->ihl >= 5))
 			return NF_ACCEPT; /* The packet looks wrong, ignore */
 		tunnel = true;
 	} else if ((cih->protocol == IPPROTO_UDP ||	/* Can be UDP encap */
@@ -1836,7 +1838,7 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 		/* Non-first fragment has no UDP/GRE header */
 		if (unlikely(cih->frag_off & htons(IP_OFFSET)))
 			return NF_ACCEPT;
-		offset2 = offset + cih->ihl * 4;
+		offset2 = offset + hlen_ipip;
 		if (cih->protocol == IPPROTO_UDP) {
 			ulen = ipvs_udp_decap(ipvs, skb, offset2, AF_INET,
 					      raddr, &iproto);
@@ -1905,6 +1907,7 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 	}
 
 	if (tunnel) {
+		unsigned int hlen_orig = cih->ihl * 4;
 		__be32 info = ic->un.gateway;
 		__u8 type = ic->type;
 		__u8 code = ic->code;
@@ -1921,6 +1924,9 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 				goto ignore_tunnel;
 			offset2 -= ihl + sizeof(_icmph);
 			skb_reset_network_header(skb);
+			/* Ensure the IP header is present in headroom */
+			if (!pskb_may_pull(skb, hlen_ipip))
+				goto ignore_tunnel;
 			IP_VS_DBG(12, "ICMP for %s %pI4->%pI4: mtu=%u\n",
 				  outer_proto, &ip_hdr(skb)->saddr,
 				  &ip_hdr(skb)->daddr, mtu);
@@ -1936,8 +1942,8 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 				if (dest_dst)
 					mtu = dst_mtu(dest_dst->dst_cache);
 			}
-			if (mtu > 68 + sizeof(struct iphdr) + ulen)
-				mtu -= sizeof(struct iphdr) + ulen;
+			if (mtu > 68 + hlen_ipip + ulen)
+				mtu -= hlen_ipip + ulen;
 			info = htonl(mtu);
 		}
 		/* Strip outer IP, ICMP and IPIP/UDP/GRE, go to IP header of
@@ -1946,6 +1952,9 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 		if (pskb_pull(skb, offset2) == NULL)
 			goto ignore_tunnel;
 		skb_reset_network_header(skb);
+		/* Ensure the IP header is present in headroom */
+		if (!pskb_may_pull(skb, hlen_orig))
+			goto ignore_tunnel;
 		IP_VS_DBG(12, "Sending ICMP for %pI4->%pI4: t=%u, c=%u, i=%u\n",
 			&ip_hdr(skb)->saddr, &ip_hdr(skb)->daddr,
 			type, code, ntohl(info));
-- 
2.55.0