bpf: add End.DT6 action to bpf_lwt_seg6_action helper

The seg6local LWT provides the End.DT6 action, which allows to
decapsulate an outer IPv6 header containing a Segment Routing Header
(SRH), full specification is available here:

https://tools.ietf.org/html/draft-filsfils-spring-srv6-network-programming-05

This patch adds this action now to the seg6local BPF
interface. Since it is not mandatory that the inner IPv6 header also
contains a SRH, seg6_bpf_srh_state has been extended with a pointer to
a possible SRH of the outermost IPv6 header. This helps assessing if the
validation must be triggered or not, and avoids some calls to
ipv6_find_hdr.

v3: s/1/true, s/0/false for boolean values
v2: - changed true/false -> 1/0
    - preempt_enable no longer called in first conditional block

Signed-off-by: Mathieu Xhonneux <m.xhonneux@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/net/seg6_local.h

@@ -21,10 +21,12 @@
 
 extern int seg6_lookup_nexthop(struct sk_buff *skb, struct in6_addr *nhaddr,
 			       u32 tbl_id);
+extern bool seg6_bpf_has_valid_srh(struct sk_buff *skb);
 
 struct seg6_bpf_srh_state {
-	bool valid;
+	struct ipv6_sr_hdr *srh;
 	u16 hdrlen;
+	bool valid;
 };
 
 DECLARE_PER_CPU(struct seg6_bpf_srh_state, seg6_bpf_srh_states);

net/core/filter.c

@@ -4542,26 +4542,28 @@ BPF_CALL_4(bpf_lwt_seg6_store_bytes, struct sk_buff *, skb, u32, offset,
 {
 	struct seg6_bpf_srh_state *srh_state =
 		this_cpu_ptr(&seg6_bpf_srh_states);
+	struct ipv6_sr_hdr *srh = srh_state->srh;
 	void *srh_tlvs, *srh_end, *ptr;
-	struct ipv6_sr_hdr *srh;
 	int srhoff = 0;
 
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+	if (srh == NULL)
 		return -EINVAL;
 
-	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
 	srh_tlvs = (void *)((char *)srh + ((srh->first_segment + 1) << 4));
 	srh_end = (void *)((char *)srh + sizeof(*srh) + srh_state->hdrlen);
 
 	ptr = skb->data + offset;
 	if (ptr >= srh_tlvs && ptr + len <= srh_end)
-		srh_state->valid = 0;
+		srh_state->valid = false;
 	else if (ptr < (void *)&srh->flags ||
 		 ptr + len > (void *)&srh->segments)
 		return -EFAULT;
 
 	if (unlikely(bpf_try_make_writable(skb, offset + len)))
 		return -EFAULT;
+	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+		return -EINVAL;
+	srh_state->srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
 
 	memcpy(skb->data + offset, from, len);
 	return 0;
@@ -4577,52 +4579,78 @@ static const struct bpf_func_proto bpf_lwt_seg6_store_bytes_proto = {
 	.arg4_type	= ARG_CONST_SIZE
 };
 
+static void bpf_update_srh_state(struct sk_buff *skb)
+{
+	struct seg6_bpf_srh_state *srh_state =
+		this_cpu_ptr(&seg6_bpf_srh_states);
+	int srhoff = 0;
+
+	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0) {
+		srh_state->srh = NULL;
+	} else {
+		srh_state->srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
+		srh_state->hdrlen = srh_state->srh->hdrlen << 3;
+		srh_state->valid = true;
+	}
+}
+
 BPF_CALL_4(bpf_lwt_seg6_action, struct sk_buff *, skb,
 	   u32, action, void *, param, u32, param_len)
 {
 	struct seg6_bpf_srh_state *srh_state =
 		this_cpu_ptr(&seg6_bpf_srh_states);
-	struct ipv6_sr_hdr *srh;
+	int hdroff = 0;
-	int srhoff = 0;
 	int err;
 
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
-		return -EINVAL;
-	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
-
-	if (!srh_state->valid) {
-		if (unlikely((srh_state->hdrlen & 7) != 0))
-			return -EBADMSG;
-
-		srh->hdrlen = (u8)(srh_state->hdrlen >> 3);
-		if (unlikely(!seg6_validate_srh(srh, (srh->hdrlen + 1) << 3)))
-			return -EBADMSG;
-
-		srh_state->valid = 1;
-	}
-
 	switch (action) {
 	case SEG6_LOCAL_ACTION_END_X:
+		if (!seg6_bpf_has_valid_srh(skb))
+			return -EBADMSG;
 		if (param_len != sizeof(struct in6_addr))
 			return -EINVAL;
 		return seg6_lookup_nexthop(skb, (struct in6_addr *)param, 0);
 	case SEG6_LOCAL_ACTION_END_T:
+		if (!seg6_bpf_has_valid_srh(skb))
+			return -EBADMSG;
 		if (param_len != sizeof(int))
 			return -EINVAL;
 		return seg6_lookup_nexthop(skb, NULL, *(int *)param);
+	case SEG6_LOCAL_ACTION_END_DT6:
+		if (!seg6_bpf_has_valid_srh(skb))
+			return -EBADMSG;
+		if (param_len != sizeof(int))
+			return -EINVAL;
+
+		if (ipv6_find_hdr(skb, &hdroff, IPPROTO_IPV6, NULL, NULL) < 0)
+			return -EBADMSG;
+		if (!pskb_pull(skb, hdroff))
+			return -EBADMSG;
+
+		skb_postpull_rcsum(skb, skb_network_header(skb), hdroff);
+		skb_reset_network_header(skb);
+		skb_reset_transport_header(skb);
+		skb->encapsulation = 0;
+
+		bpf_compute_data_pointers(skb);
+		bpf_update_srh_state(skb);
+		return seg6_lookup_nexthop(skb, NULL, *(int *)param);
 	case SEG6_LOCAL_ACTION_END_B6:
+		if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
+			return -EBADMSG;
 		err = bpf_push_seg6_encap(skb, BPF_LWT_ENCAP_SEG6_INLINE,
 					  param, param_len);
 		if (!err)
-			srh_state->hdrlen =
+			bpf_update_srh_state(skb);
-				((struct ipv6_sr_hdr *)param)->hdrlen << 3;
+
 		return err;
 	case SEG6_LOCAL_ACTION_END_B6_ENCAP:
+		if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
+			return -EBADMSG;
 		err = bpf_push_seg6_encap(skb, BPF_LWT_ENCAP_SEG6,
 					  param, param_len);
 		if (!err)
-			srh_state->hdrlen =
+			bpf_update_srh_state(skb);
-				((struct ipv6_sr_hdr *)param)->hdrlen << 3;
+
 		return err;
 	default:
 		return -EINVAL;
@@ -4644,15 +4672,14 @@ BPF_CALL_3(bpf_lwt_seg6_adjust_srh, struct sk_buff *, skb, u32, offset,
 {
 	struct seg6_bpf_srh_state *srh_state =
 		this_cpu_ptr(&seg6_bpf_srh_states);
+	struct ipv6_sr_hdr *srh = srh_state->srh;
 	void *srh_end, *srh_tlvs, *ptr;
-	struct ipv6_sr_hdr *srh;
 	struct ipv6hdr *hdr;
 	int srhoff = 0;
 	int ret;
 
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+	if (unlikely(srh == NULL))
 		return -EINVAL;
-	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
 
 	srh_tlvs = (void *)((unsigned char *)srh + sizeof(*srh) +
 			((srh->first_segment + 1) << 4));
@@ -4682,8 +4709,11 @@ BPF_CALL_3(bpf_lwt_seg6_adjust_srh, struct sk_buff *, skb, u32, offset,
 	hdr = (struct ipv6hdr *)skb->data;
 	hdr->payload_len = htons(skb->len - sizeof(struct ipv6hdr));
 
+	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+		return -EINVAL;
+	srh_state->srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
 	srh_state->hdrlen += len;
-	srh_state->valid = 0;
+	srh_state->valid = false;
 	return 0;
 }
 

net/ipv6/seg6_local.c

@@ -459,36 +459,57 @@ drop:
 
 DEFINE_PER_CPU(struct seg6_bpf_srh_state, seg6_bpf_srh_states);
 
+bool seg6_bpf_has_valid_srh(struct sk_buff *skb)
+{
+	struct seg6_bpf_srh_state *srh_state =
+		this_cpu_ptr(&seg6_bpf_srh_states);
+	struct ipv6_sr_hdr *srh = srh_state->srh;
+
+	if (unlikely(srh == NULL))
+		return false;
+
+	if (unlikely(!srh_state->valid)) {
+		if ((srh_state->hdrlen & 7) != 0)
+			return false;
+
+		srh->hdrlen = (u8)(srh_state->hdrlen >> 3);
+		if (!seg6_validate_srh(srh, (srh->hdrlen + 1) << 3))
+			return false;
+
+		srh_state->valid = true;
+	}
+
+	return true;
+}
+
 static int input_action_end_bpf(struct sk_buff *skb,
 				struct seg6_local_lwt *slwt)
 {
 	struct seg6_bpf_srh_state *srh_state =
 		this_cpu_ptr(&seg6_bpf_srh_states);
-	struct seg6_bpf_srh_state local_srh_state;
 	struct ipv6_sr_hdr *srh;
-	int srhoff = 0;
 	int ret;
 
 	srh = get_and_validate_srh(skb);
-	if (!srh)
+	if (!srh) {
-		goto drop;
+		kfree_skb(skb);
+		return -EINVAL;
+	}
 	advance_nextseg(srh, &ipv6_hdr(skb)->daddr);
 
 	/* preempt_disable is needed to protect the per-CPU buffer srh_state,
 	 * which is also accessed by the bpf_lwt_seg6_* helpers
 	 */
 	preempt_disable();
+	srh_state->srh = srh;
 	srh_state->hdrlen = srh->hdrlen << 3;
-	srh_state->valid = 1;
+	srh_state->valid = true;
 
 	rcu_read_lock();
 	bpf_compute_data_pointers(skb);
 	ret = bpf_prog_run_save_cb(slwt->bpf.prog, skb);
 	rcu_read_unlock();
 
-	local_srh_state = *srh_state;
-	preempt_enable();
-
 	switch (ret) {
 	case BPF_OK:
 	case BPF_REDIRECT:
@@ -500,24 +521,17 @@ static int input_action_end_bpf(struct sk_buff *skb,
 		goto drop;
 	}
 
-	if (unlikely((local_srh_state.hdrlen & 7) != 0))
+	if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
-		goto drop;
-
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
-		goto drop;
-	srh = (struct ipv6_sr_hdr *)(skb->data + srhoff);
-	srh->hdrlen = (u8)(local_srh_state.hdrlen >> 3);
-
-	if (!local_srh_state.valid &&
-	    unlikely(!seg6_validate_srh(srh, (srh->hdrlen + 1) << 3)))
 		goto drop;
 
+	preempt_enable();
 	if (ret != BPF_REDIRECT)
 		seg6_lookup_nexthop(skb, NULL, 0);
 
 	return dst_input(skb);
 
 drop:
+	preempt_enable();
 	kfree_skb(skb);
 	return -EINVAL;
 }