[PATCH net-next v2 4/8] ip_gre: add drop reasons to the RX path

[Anton Danilov <littlesmilingcloud@gmail.com>]

A packet that reaches gre_rcv() and does not belong to any tunnel is
dropped, after an ICMP port unreachable is sent back, with a plain
kfree_skb().  This is the GRE counterpart of a UDP packet hitting no
socket, and by far the most common way a GRE packet is dropped on
receive, yet nothing tells it apart from a malformed one.

Add SKB_DROP_REASON_GRE_TUNNEL_NOT_FOUND for it, in the spirit of the
existing SKB_DROP_REASON_VXLAN_VNI_NOT_FOUND, and report it from
erspan_rcv() and __ipgre_rcv() through a new output parameter, so that a
failed lookup is not reported the same way as a header that could not be
pulled or as a metadata allocation failure.

The header pull failures reuse SKB_DROP_REASON_HDR_TRUNC and the
metadata allocation failures reuse SKB_DROP_REASON_NOMEM.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
---
 include/net/dropreason-core.h |  6 ++++++
 net/ipv4/ip_gre.c             | 37 +++++++++++++++++++++++------------
 2 files changed, 30 insertions(+), 13 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 6ae7a604722d..fa8bd552122f 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -133,6 +133,7 @@
 	FN(TNL_OLD_SEQ)			\
 	FN(GRE_INVALID_HDR)		\
 	FN(GRE_CSUM)			\
+	FN(GRE_TUNNEL_NOT_FOUND)	\
 	FNe(MAX)
 
 /**
@@ -637,6 +638,11 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_GRE_INVALID_HDR,
 	/** @SKB_DROP_REASON_GRE_CSUM: GRE checksum error */
 	SKB_DROP_REASON_GRE_CSUM,
+	/**
+	 * @SKB_DROP_REASON_GRE_TUNNEL_NOT_FOUND: no GRE tunnel found for the
+	 * endpoints and the key the packet carries.
+	 */
+	SKB_DROP_REASON_GRE_TUNNEL_NOT_FOUND,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
 	 * shouldn't be used as a real 'reason' - only for tracing code gen
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 1894c5746a73..4d9bb6d186ae 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -265,7 +265,7 @@ static bool is_erspan_type1(int gre_hdr_len)
 }
 
 static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
-		      int gre_hdr_len)
+		      int gre_hdr_len, enum skb_drop_reason *reason)
 {
 	struct net *net = dev_net(skb->dev);
 	struct metadata_dst *tun_dst = NULL;
@@ -289,8 +289,10 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 					  iph->saddr, iph->daddr, 0);
 	} else {
 		if (unlikely(!pskb_may_pull(skb,
-					    gre_hdr_len + sizeof(*ershdr))))
+					    gre_hdr_len + sizeof(*ershdr)))) {
+			*reason = SKB_DROP_REASON_HDR_TRUNC;
 			return PACKET_REJECT;
+		}
 
 		ershdr = (struct erspan_base_hdr *)(skb->data + gre_hdr_len);
 		ver = ershdr->ver;
@@ -306,8 +308,10 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		else
 			len = gre_hdr_len + erspan_hdr_len(ver);
 
-		if (unlikely(!pskb_may_pull(skb, len)))
+		if (unlikely(!pskb_may_pull(skb, len))) {
+			*reason = SKB_DROP_REASON_HDR_TRUNC;
 			return PACKET_REJECT;
+		}
 
 		if (__iptunnel_pull_header(skb,
 					   len,
@@ -327,8 +331,10 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 
 			tun_dst = ip_tun_rx_dst(skb, flags,
 						tun_id, sizeof(*md));
-			if (!tun_dst)
+			if (!tun_dst) {
+				*reason = SKB_DROP_REASON_NOMEM;
 				return PACKET_REJECT;
+			}
 
 			/* MUST set options_len before referencing options */
 			info = &tun_dst->u.tun_info;
@@ -356,15 +362,17 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		ip_tunnel_rcv(tunnel, skb, tpi, tun_dst, log_ecn_error);
 		return PACKET_RCVD;
 	}
+	*reason = SKB_DROP_REASON_GRE_TUNNEL_NOT_FOUND;
 	return PACKET_REJECT;
 
 drop:
-	kfree_skb(skb);
+	kfree_skb_reason(skb, SKB_DROP_REASON_HDR_TRUNC);
 	return PACKET_RCVD;
 }
 
 static int __ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
-		       struct ip_tunnel_net *itn, int hdr_len, bool raw_proto)
+		       struct ip_tunnel_net *itn, int hdr_len, bool raw_proto,
+		       enum skb_drop_reason *reason)
 {
 	struct metadata_dst *tun_dst = NULL;
 	const struct iphdr *iph;
@@ -400,22 +408,25 @@ static int __ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 
 			tun_id = key32_to_tunnel_id(tpi->key);
 			tun_dst = ip_tun_rx_dst(skb, flags, tun_id, 0);
-			if (!tun_dst)
+			if (!tun_dst) {
+				*reason = SKB_DROP_REASON_NOMEM;
 				return PACKET_REJECT;
+			}
 		}
 
 		ip_tunnel_rcv(tunnel, skb, tpi, tun_dst, log_ecn_error);
 		return PACKET_RCVD;
 	}
+	*reason = SKB_DROP_REASON_GRE_TUNNEL_NOT_FOUND;
 	return PACKET_NEXT;
 
 drop:
-	kfree_skb(skb);
+	kfree_skb_reason(skb, SKB_DROP_REASON_HDR_TRUNC);
 	return PACKET_RCVD;
 }
 
 static int ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
-		     int hdr_len)
+		     int hdr_len, enum skb_drop_reason *reason)
 {
 	struct net *net = dev_net(skb->dev);
 	struct ip_tunnel_net *itn;
@@ -426,13 +437,13 @@ static int ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 	else
 		itn = net_generic(net, ipgre_net_id);
 
-	res = __ipgre_rcv(skb, tpi, itn, hdr_len, false);
+	res = __ipgre_rcv(skb, tpi, itn, hdr_len, false, reason);
 	if (res == PACKET_NEXT && tpi->proto == htons(ETH_P_TEB)) {
 		/* ipgre tunnels in collect metadata mode should receive
 		 * also ETH_P_TEB traffic.
 		 */
 		itn = net_generic(net, ipgre_net_id);
-		res = __ipgre_rcv(skb, tpi, itn, hdr_len, true);
+		res = __ipgre_rcv(skb, tpi, itn, hdr_len, true, reason);
 	}
 	return res;
 }
@@ -457,12 +468,12 @@ static int gre_rcv(struct sk_buff *skb)
 
 	if (unlikely(tpi.proto == htons(ETH_P_ERSPAN) ||
 		     tpi.proto == htons(ETH_P_ERSPAN2))) {
-		if (erspan_rcv(skb, &tpi, hdr_len) == PACKET_RCVD)
+		if (erspan_rcv(skb, &tpi, hdr_len, &reason) == PACKET_RCVD)
 			return 0;
 		goto out;
 	}
 
-	if (ipgre_rcv(skb, &tpi, hdr_len) == PACKET_RCVD)
+	if (ipgre_rcv(skb, &tpi, hdr_len, &reason) == PACKET_RCVD)
 		return 0;
 
 out:
-- 
2.47.3