From 3a39c214fd2c3dd8266649e7f9f85ca1439eb738 Mon Sep 17 00:00:00 2001
From: Nick Hudson <nhudson@akamai.com>
Date: Wed, 12 Aug 2026 09:31:13 +0100
Subject: [PATCH] bpf: Allow new DECAP flags and add guard rails

Add checks to require shrink-only decap, reject conflicting decap flag
combinations, and verify removed length is sufficient for claimed header
decapsulation.

Co-developed-by: Max Tottenham <mtottenh@akamai.com>
Co-developed-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Max Tottenham <mtottenh@akamai.com>
Signed-off-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Nick Hudson <nhudson@akamai.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/20260812083115.73100-5-nhudson@akamai.com
---
 net/core/filter.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 00b7ebe4a02b..3ba653ab3239 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -56,6 +56,7 @@
 #include <net/sock_reuseport.h>
 #include <net/busy_poll.h>
 #include <net/tcp.h>
+#include <net/gre.h>
 #include <net/xfrm.h>
 #include <net/udp.h>
 #include <linux/bpf_trace.h>
@@ -3572,6 +3573,12 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
 #define BPF_F_ADJ_ROOM_DECAP_L3_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_IPV4 | \
 					 BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
 
+#define BPF_F_ADJ_ROOM_DECAP_L4_MASK	(BPF_F_ADJ_ROOM_DECAP_L4_UDP | \
+					 BPF_F_ADJ_ROOM_DECAP_L4_GRE)
+
+#define BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK	(BPF_F_ADJ_ROOM_DECAP_IPXIP4 | \
+					 BPF_F_ADJ_ROOM_DECAP_IPXIP6)
+
 #define BPF_F_ADJ_ROOM_ENCAP_MASK	(BPF_F_ADJ_ROOM_ENCAP_L3_MASK | \
 					 BPF_F_ADJ_ROOM_ENCAP_L4_GRE | \
 					 BPF_F_ADJ_ROOM_ENCAP_L4_UDP | \
@@ -3579,7 +3586,9 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
 					 BPF_F_ADJ_ROOM_ENCAP_L2( \
 					  BPF_ADJ_ROOM_ENCAP_L2_MASK))
 
-#define BPF_F_ADJ_ROOM_DECAP_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_MASK)
+#define BPF_F_ADJ_ROOM_DECAP_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_MASK | \
+					 BPF_F_ADJ_ROOM_DECAP_L4_MASK | \
+					 BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
 
 #define BPF_F_ADJ_ROOM_MASK		(BPF_F_ADJ_ROOM_FIXED_GSO | \
 					 BPF_F_ADJ_ROOM_ENCAP_MASK | \
@@ -3827,6 +3836,8 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 	}
 
 	if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
+		u32 len_decap_min = 0;
+
 		if (!shrink)
 			return -EINVAL;
 
@@ -3835,6 +3846,37 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 		    BPF_F_ADJ_ROOM_DECAP_L3_MASK)
 			return -EINVAL;
 
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) ==
+		    BPF_F_ADJ_ROOM_DECAP_L4_MASK)
+			return -EINVAL;
+
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) ==
+		    BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
+			return -EINVAL;
+
+		/* Reject mutually exclusive decap tunnel type flags. */
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
+		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
+			return -EINVAL;
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK)
+			len_decap_min += bpf_skb_net_base_len(skb);
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP)
+			len_decap_min += sizeof(struct udphdr);
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE)
+			len_decap_min += sizeof(struct gre_base_hdr);
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP4)
+			len_decap_min += sizeof(struct iphdr);
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP6)
+			len_decap_min += sizeof(struct ipv6hdr);
+
+		if (len_diff_abs < len_decap_min)
+			return -EINVAL;
+
 		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV4)
 			len_min = sizeof(struct iphdr);