[PATCH net 6/8] ipvs: revalidate ihl before icmp_send

From: 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 possible 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. Even if icmp_send() has checks to prevent out-of-bounds
access, play safe and add check to drop the packet if the ihl field is
changed.  As the outer headers are pulled, make sure the transport
header is updated too, it was used before commit 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>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/ipvs/ip_vs_core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index ba0957798bad..fd503f0efb57 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c

@@ -1960,6 +1960,12 @@ 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));

-- 
2.47.3