bareudp: Pull inner IP header in bareudp_udp_encap_recv().

JIRA: https://issues.redhat.com/browse/RHEL-59593
Upstream Status: linux.git

commit 45fa29c
Author: Guillaume Nault <gnault@redhat.com>
Date:   Wed Sep 11 11:20:58 2024 +0200

    bareudp: Pull inner IP header in bareudp_udp_encap_recv().

    Bareudp reads the inner IP header to get the ECN value. Therefore, it
    needs to ensure that it's part of the skb's linear data.

    This is similar to the vxlan and geneve fixes for that same problem:
      * commit f778941 ("vxlan: Pull inner IP header in vxlan_rcv().")
      * commit 1ca1ba4 ("geneve: make sure to pull inner header in
        geneve_rx()")

    Fixes: 571912c ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
    Signed-off-by: Guillaume Nault <gnault@redhat.com>
    Reviewed-by: Willem de Bruijn <willemb@google.com>
    Link: https://patch.msgid.link/5205940067c40218a70fbb888080466b2fc288db.1726046181.git.gnault@redhat.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Guillaume Nault <gnault@redhat.com>

Author: Guillaume Nault

drivers/net/bareudp.c

@@ -68,6 +68,7 @@ static int bareudp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 	__be16 proto;
 	void *oiph;
 	int err;
+	int nh;
 
 	bareudp = rcu_dereference_sk_user_data(sk);
 	if (!bareudp)
@@ -148,10 +149,25 @@ static int bareudp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 	}
 	skb_dst_set(skb, &tun_dst->dst);
 	skb->dev = bareudp->dev;
-	oiph = skb_network_header(skb);
-	skb_reset_network_header(skb);
 	skb_reset_mac_header(skb);
 
+	/* Save offset of outer header relative to skb->head,
+	 * because we are going to reset the network header to the inner header
+	 * and might change skb->head.
+	 */
+	nh = skb_network_header(skb) - skb->head;
+
+	skb_reset_network_header(skb);
+
+	if (!pskb_inet_may_pull(skb)) {
+		DEV_STATS_INC(bareudp->dev, rx_length_errors);
+		DEV_STATS_INC(bareudp->dev, rx_errors);
+		goto drop;
+	}
+
+	/* Get the outer header. */
+	oiph = skb->head + nh;
+
 	if (!ipv6_mod_enabled() || family == AF_INET)
 		err = IP_ECN_decapsulate(oiph, skb);
 	else