linux-kernel - [PATCH net 1/2] net: ioam6: use "new" dst entry with skb_cow

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Message-Id: <20240702174451.22735-2-justin.iurman@uliege.be>
Date: Tue,  2 Jul 2024 19:44:50 +0200
From: Justin Iurman <justin.iurman@...ege.be>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	linux-kernel@...r.kernel.org,
	justin.iurman@...ege.be
Subject: [PATCH net 1/2] net: ioam6: use "new" dst entry with skb_cow_head

In ioam6_output(), we call skb_cow_head() with LL_RESERVED_SPACE(). The
latter uses "dst", which is potentially not the good one anymore. As a
consequence, there might not be enough headroom, depending on the "new"
dev (e.g., needed_headroom > 0). Therefore, we first need to get the
"new" dst entry (from the cache or by calling ip6_route_output()), just
like seg6 and rpl both do, and only then call skb_cow_head() with
LL_RESERVED_SPACE() on the "new" dst entry.

Fixes: 8cb3bf8bff3c ("ipv6: ioam: Add support for the ip6ip6 encapsulation")
Signed-off-by: Justin Iurman <justin.iurman@...ege.be>
---
 net/ipv6/ioam6_iptunnel.c | 65 +++++++++++++++++++--------------------
 1 file changed, 32 insertions(+), 33 deletions(-)

diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
index bf7120ecea1e..b08c13550144 100644
--- a/net/ipv6/ioam6_iptunnel.c
+++ b/net/ipv6/ioam6_iptunnel.c
@@ -295,7 +295,7 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
 
 static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
-	struct dst_entry *dst = skb_dst(skb);
+	struct dst_entry *dst, *orig_dst = skb_dst(skb);
 	struct in6_addr orig_daddr;
 	struct ioam6_lwt *ilwt;
 	int err = -EINVAL;
@@ -304,7 +304,7 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	if (skb->protocol != htons(ETH_P_IPV6))
 		goto drop;
 
-	ilwt = ioam6_lwt_state(dst->lwtstate);
+	ilwt = ioam6_lwt_state(orig_dst->lwtstate);
 
 	/* Check for insertion frequency (i.e., "k over n" insertions) */
 	pkt_cnt = atomic_fetch_inc(&ilwt->pkt_cnt);
@@ -346,45 +346,44 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	}
 
-	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
-	if (unlikely(err))
-		goto drop;
+	local_bh_disable();
+	dst = dst_cache_get(&ilwt->cache);
+	local_bh_enable();
+
+	if (unlikely(!dst)) {
+		struct ipv6hdr *hdr = ipv6_hdr(skb);
+		struct flowi6 fl6;
+
+		memset(&fl6, 0, sizeof(fl6));
+		fl6.daddr = hdr->daddr;
+		fl6.saddr = hdr->saddr;
+		fl6.flowlabel = ip6_flowinfo(hdr);
+		fl6.flowi6_mark = skb->mark;
+		fl6.flowi6_proto = hdr->nexthdr;
+
+		dst = ip6_route_output(net, NULL, &fl6);
+		if (dst->error) {
+			err = dst->error;
+			dst_release(dst);
+			goto drop;
+		}
 
-	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
 		local_bh_disable();
-		dst = dst_cache_get(&ilwt->cache);
+		dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
 		local_bh_enable();
+	}
 
-		if (unlikely(!dst)) {
-			struct ipv6hdr *hdr = ipv6_hdr(skb);
-			struct flowi6 fl6;
-
-			memset(&fl6, 0, sizeof(fl6));
-			fl6.daddr = hdr->daddr;
-			fl6.saddr = hdr->saddr;
-			fl6.flowlabel = ip6_flowinfo(hdr);
-			fl6.flowi6_mark = skb->mark;
-			fl6.flowi6_proto = hdr->nexthdr;
-
-			dst = ip6_route_output(net, NULL, &fl6);
-			if (dst->error) {
-				err = dst->error;
-				dst_release(dst);
-				goto drop;
-			}
-
-			local_bh_disable();
-			dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
-			local_bh_enable();
-		}
+	skb_dst_drop(skb);
+	skb_dst_set(skb, dst);
 
-		skb_dst_drop(skb);
-		skb_dst_set(skb, dst);
+	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+	if (unlikely(err))
+		goto drop;
 
+	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr))
 		return dst_output(net, sk, skb);
-	}
 out:
-	return dst->lwtstate->orig_output(net, sk, skb);
+	return orig_dst->lwtstate->orig_output(net, sk, skb);
 drop:
 	kfree_skb(skb);
 	return err;
-- 
2.34.1