[PATCH v1 4/5] bpf: add guard rails for new DECAP flags

[Nick Hudson]

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@xxxxxxxxxx>
Signed-off-by: Max Tottenham <mtottenh@xxxxxxxxxx>
Co-developed-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
Signed-off-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
Signed-off-by: Nick Hudson <nhudson@xxxxxxxxxx>
---
 net/core/filter.c | 45 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index ac7e1068fe4c..437e0da34f84 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>
@@ -3745,20 +3746,46 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 		return -ENOTSUPP;
 	}
 
-	if (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
+	if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
+		u32 len_decap_min = 0;
+
 		if (!shrink)
 			return -EINVAL;
 
-		switch (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
-		case BPF_F_ADJ_ROOM_DECAP_L3_IPV4:
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
+		    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;
+
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
+		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
+			return -EINVAL;
+
+		if (mode == BPF_ADJ_ROOM_MAC)
+			len_decap_min += proto == htons(ETH_P_IP) ?
+					 sizeof(struct iphdr) : sizeof(struct ipv6hdr);
+
+		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 (len_diff_abs < len_decap_min)
+			return -EINVAL;
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV4)
 			len_min = sizeof(struct iphdr);
-			break;
-		case BPF_F_ADJ_ROOM_DECAP_L3_IPV6:
+
+		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
 			len_min = sizeof(struct ipv6hdr);
-			break;
-		default:
-			return -EINVAL;
-		}
 	}
 
 	len_cur = skb->len - skb_network_offset(skb);
-- 
2.34.1