[PATCH nf] ipvs: revalidate ihl before icmp_send

[Julian Anastasov <ja@ssi.bg>]

While the outer IP header is already pulled into the skb head,
we must be careful and revalidate the embedded headers after
reading them from the skb frags to prevent out-of-bounds
access.

One such place reported by Sashiko is ip_vs_in_icmp() where
local process can change the ihl field and after
pskb_may_pull() we can see larger value which is a problem
for the icmp_send() call.

Add check to drop the packet if the ihl field is changed and
make sure the transport header is updated.

Also, provide correct protocol to ipv4_update_pmtu().

Fixes: 7fcc2fe39fed ("net: icmp: avoid invalid transport header access in icmp_send tracepoint")
Fixes: f2edb9f7706d ("ipvs: implement passive PMTUD for IPIP packets")
Link: https://sashiko.dev/#/patchset/20260806105211.34622-1-ja%40ssi.bg
Signed-off-by: Julian Anastasov <ja@ssi.bg>
---
 net/netfilter/ipvs/ip_vs_core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index 1a0661d19d9f..de6ae759cb29 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c
@@ -1935,7 +1935,8 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 			IP_VS_DBG(12, "ICMP for %s %pI4->%pI4: mtu=%u\n",
 				  outer_proto, &ip_hdr(skb)->saddr,
 				  &ip_hdr(skb)->daddr, mtu);
-			ipv4_update_pmtu(skb, ipvs->net, mtu, 0, 0);
+			ipv4_update_pmtu(skb, ipvs->net, mtu, 0,
+					 ip_hdr(skb)->protocol);
 			/* Client uses PMTUD? */
 			if (!(frag_off & htons(IP_DF)))
 				goto ignore_tunnel;
@@ -1960,6 +1961,12 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
 		/* Ensure the IP header is present in headroom */
 		if (!pskb_may_pull(skb, hlen_orig))
 			goto ignore_tunnel;
+		skb_set_transport_header(skb, hlen_orig);
+		/* Before now we may used ihl from skb frag, revalidate it after
+		 * copying it into skb head to prevent out-of-bounds access
+		 */
+		if (ip_hdr(skb)->ihl * 4 != 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