ip_tunnel: make __iptunnel_pull_header() return a drop reason

__iptunnel_pull_header() returns -ENOMEM whenever it fails. It can
fail in two pskb_may_pull() calls, one for the tunnel header and one
for the inner Ethernet header of ETH_P_TEB, and in the skb_unclone()
done for GSO packets. pskb_may_pull() fails when the packet is shorter
than the requested length as well as when pulling from the frags cannot
allocate, so a truncated packet and an allocation failure look the same
to the callers. The ones that report a drop reason can only pick
SKB_DROP_REASON_NOMEM, as vxlan_rcv() does, and so would the GRE
receive paths once they report drop reasons. In ip6_gre, gre_rcv()
pulls the header before the tunnel lookup, so a packet from any sender
whose ETH_P_TEB inner Ethernet header or WCCPv2 extra word is cut short
would be reported as an out of memory condition.

Make __iptunnel_pull_header() and iptunnel_pull_header() return the
reason pskb_may_pull_reason() already computes, SKB_DROP_REASON_NOMEM
when skb_unclone() fails, and SKB_NOT_DROPPED_YET on success. A failure
is still non-zero, so the callers that only test the result keep
working. Three callers, in ip_gre and ip6_gre, test it with "< 0"
instead; the enum has no negative values, so that test would always be
false, and the compiler does not warn about it. Make them test for a
non-zero value. vxlan_rcv() keeps reporting NOMEM for any failure until
the next patch has it report the returned reason.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20261005201417.906344-3-littlesmilingcloud@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/ip_tunnels.h

@@ -614,11 +614,13 @@ static inline u8 ip_tunnel_ecn_encap(u8 tos, const struct iphdr *iph,
 	return INET_ECN_encapsulate(tos, inner);
 }
 
-int __iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
-			   __be16 inner_proto, bool raw_proto, bool xnet);
+enum skb_drop_reason
+__iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
+		       __be16 inner_proto, bool raw_proto, bool xnet);
 
-static inline int iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
-				       __be16 inner_proto, bool xnet)
+static inline enum skb_drop_reason
+iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
+		     __be16 inner_proto, bool xnet)
 {
 	return __iptunnel_pull_header(skb, hdr_len, inner_proto, false, xnet);
 }

net/ipv4/ip_gre.c

@@ -312,7 +312,7 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		if (__iptunnel_pull_header(skb,
 					   len,
 					   htons(ETH_P_TEB),
-					   false, false) < 0)
+					   false, false))
 			goto drop;
 
 		if (tunnel->collect_md) {
@@ -378,7 +378,7 @@ static int __ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 		const struct iphdr *tnl_params;
 
 		if (__iptunnel_pull_header(skb, hdr_len, tpi->proto,
-					   raw_proto, false) < 0)
+					   raw_proto, false))
 			goto drop;
 
 		/* Special case for ipgre_header_parse(), which expects the

net/ipv4/ip_tunnel_core.c

@@ -106,19 +106,24 @@ void iptunnel_xmit(struct sock *sk, struct rtable *rt, struct sk_buff *skb,
 }
 EXPORT_SYMBOL_GPL(iptunnel_xmit);
 
-int __iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
-			   __be16 inner_proto, bool raw_proto, bool xnet)
+enum skb_drop_reason
+__iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
+		       __be16 inner_proto, bool raw_proto, bool xnet)
 {
-	if (unlikely(!pskb_may_pull(skb, hdr_len)))
-		return -ENOMEM;
+	enum skb_drop_reason reason;
+
+	reason = pskb_may_pull_reason(skb, hdr_len);
+	if (unlikely(reason))
+		return reason;
 
 	skb_pull_rcsum(skb, hdr_len);
 
 	if (!raw_proto && inner_proto == htons(ETH_P_TEB)) {
 		struct ethhdr *eh;
 
-		if (unlikely(!pskb_may_pull(skb, ETH_HLEN)))
-			return -ENOMEM;
+		reason = pskb_may_pull_reason(skb, ETH_HLEN);
+		if (unlikely(reason))
+			return reason;
 
 		eh = (struct ethhdr *)skb->data;
 		if (likely(eth_proto_is_802_3(eh->h_proto)))
@@ -135,7 +140,10 @@ int __iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
 	skb_set_queue_mapping(skb, 0);
 	skb_scrub_packet(skb, xnet);
 
-	return iptunnel_pull_offloads(skb);
+	if (unlikely(iptunnel_pull_offloads(skb)))
+		return SKB_DROP_REASON_NOMEM;
+
+	return SKB_NOT_DROPPED_YET;
 }
 EXPORT_SYMBOL_GPL(__iptunnel_pull_header);
 

net/ipv6/ip6_gre.c

@@ -512,7 +512,7 @@ static int ip6erspan_rcv(struct sk_buff *skb,
 
 		if (__iptunnel_pull_header(skb, len,
 					   htons(ETH_P_TEB),
-					   false, false) < 0)
+					   false, false))
 			return PACKET_REJECT;
 
 		if (tunnel->parms.collect_md) {