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>

1 files changed, 32 insertions, 18 deletions

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index e1025b493a18..60325dbfe88b 100644
--- a/net/ipv6/seg6_local.c
+++ b/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)
-		goto drop;
+	if (!srh) {
+		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))
-		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)))
+	if (srh_state->srh && !seg6_bpf_has_valid_srh(skb))
 		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;
 }