net: ipv6: ioam6: fix double reallocation

If the dst_entry is the same post transformation (which is a valid use
case for IOAM), we don't add it to the cache to avoid a reference loop.
Instead, we use a "fake" dst_entry and add it to the cache as a signal.
When we read the cache, we compare it with our "fake" dst_entry and
therefore detect if we're in the special case.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20250415112554.23823-3-justin.iurman@uliege.be
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: IurmanJ

net/ipv6/ioam6_iptunnel.c

@@ -38,6 +38,7 @@ struct ioam6_lwt_freq {
 };
 
 struct ioam6_lwt {
+	struct dst_entry null_dst;
 	struct dst_cache cache;
 	struct ioam6_lwt_freq freq;
 	atomic_t pkt_cnt;
@@ -177,6 +178,14 @@ static int ioam6_build_state(struct net *net, struct nlattr *nla,
 	if (err)
 		goto free_lwt;
 
+	/* This "fake" dst_entry will be stored in a dst_cache, which will call
+	 * dst_hold() and dst_release() on it. We must ensure that dst_destroy()
+	 * will never be called. For that, its initial refcount is 1 and +1 when
+	 * it is stored in the cache. Then, +1/-1 each time we read the cache
+	 * and release it. Long story short, we're fine.
+	 */
+	dst_init(&ilwt->null_dst, NULL, NULL, DST_OBSOLETE_NONE, DST_NOCOUNT);
+
 	atomic_set(&ilwt->pkt_cnt, 0);
 	ilwt->freq.k = freq_k;
 	ilwt->freq.n = freq_n;
@@ -356,6 +365,17 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 	dst = dst_cache_get(&ilwt->cache);
 	local_bh_enable();
 
+	/* This is how we notify that the destination does not change after
+	 * transformation and that we need to use orig_dst instead of the cache
+	 */
+	if (dst == &ilwt->null_dst) {
+		dst_release(dst);
+
+		dst = orig_dst;
+		/* keep refcount balance: dst_release() is called at the end */
+		dst_hold(dst);
+	}
+
 	switch (ilwt->mode) {
 	case IOAM6_IPTUNNEL_MODE_INLINE:
 do_inline:
@@ -408,12 +428,19 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 			goto drop;
 		}
 
-		/* cache only if we don't create a dst reference loop */
-		if (orig_dst->lwtstate != dst->lwtstate) {
-			local_bh_disable();
+		/* If the destination is the same after transformation (which is
+		 * a valid use case for IOAM), then we don't want to add it to
+		 * the cache in order to avoid a reference loop. Instead, we add
+		 * our fake dst_entry to the cache as a way to detect this case.
+		 * Otherwise, we add the resolved destination to the cache.
+		 */
+		local_bh_disable();
+		if (orig_dst->lwtstate == dst->lwtstate)
+			dst_cache_set_ip6(&ilwt->cache,
+					  &ilwt->null_dst, &fl6.saddr);
+		else
 			dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
-			local_bh_enable();
-		}
+		local_bh_enable();
 
 		err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
 		if (unlikely(err))
@@ -439,6 +466,10 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 static void ioam6_destroy_state(struct lwtunnel_state *lwt)
 {
+	/* Since the refcount of per-cpu dst_entry caches will never be 0 (see
+	 * why above) when our "fake" dst_entry is used, it is not necessary to
+	 * remove them before calling dst_cache_destroy()
+	 */
 	dst_cache_destroy(&ioam6_lwt_state(lwt)->cache);
 }