netdev - Re: [PATCH net 1/3] net: ipv6: fix dst ref loops on input in lwtunnels

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Message-ID: <af6c02ca-2f40-4e1a-88d2-89fc8c110756@uliege.be>
Date: Sun, 9 Feb 2025 20:52:17 +0100
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, horms@...nel.org,
 Alexander Aring <aahringo@...hat.com>, David Lebrun <dlebrun@...gle.com>
Subject: Re: [PATCH net 1/3] net: ipv6: fix dst ref loops on input in
 lwtunnels

On 2/9/25 20:38, Justin Iurman wrote:
> As a follow up to 92191dd10730 ("net: ipv6: fix dst ref loops in rpl,
> seg6 and ioam6 lwtunnels"), we also need a conditional dst cache on
> input for seg6_iptunnel and rpl_iptunnel to prevent dst ref loops (i.e.,
> if the packet destination did not change, we may end up recording a
> reference to the lwtunnel in its own cache, and the lwtunnel state will
> never be freed).
> 
> Fixes: a7a29f9c361f ("net: ipv6: add rpl sr tunnel")
> Fixes: af4a2209b134 ("ipv6: sr: use dst_cache in seg6_input")
> Signed-off-by: Justin Iurman <justin.iurman@...ege.be>
> Cc: Alexander Aring <aahringo@...hat.com>
> Cc: David Lebrun <dlebrun@...gle.com>
> ---
>   net/ipv6/rpl_iptunnel.c  | 14 ++++++++++++--
>   net/ipv6/seg6_iptunnel.c | 14 ++++++++++++--
>   2 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv6/rpl_iptunnel.c b/net/ipv6/rpl_iptunnel.c
> index 0ac4283acdf2..c26bf284459f 100644
> --- a/net/ipv6/rpl_iptunnel.c
> +++ b/net/ipv6/rpl_iptunnel.c
> @@ -262,10 +262,18 @@ static int rpl_input(struct sk_buff *skb)
>   {
>   	struct dst_entry *orig_dst = skb_dst(skb);
>   	struct dst_entry *dst = NULL;
> +	struct lwtunnel_state *lwtst;
>   	struct rpl_lwt *rlwt;
>   	int err;
>   
> -	rlwt = rpl_lwt_lwtunnel(orig_dst->lwtstate);
> +	/* Get the address of lwtstate now, because "orig_dst" may not be there
> +	 * anymore after a call to skb_dst_drop(). Note that ip6_route_input()
> +	 * also calls skb_dst_drop(). Below, we compare the address of lwtstate
> +	 * to detect loops.
> +	 */
> +	lwtst = orig_dst->lwtstate;
> +
> +	rlwt = rpl_lwt_lwtunnel(lwtst);
>   
>   	local_bh_disable();
>   	dst = dst_cache_get(&rlwt->cache);
> @@ -280,7 +288,9 @@ static int rpl_input(struct sk_buff *skb)
>   	if (!dst) {
>   		ip6_route_input(skb);
>   		dst = skb_dst(skb);
> -		if (!dst->error) {
> +
> +		/* cache only if we don't create a dst reference loop */
> +		if (!dst->error && lwtst != dst->lwtstate) {
>   			local_bh_disable();
>   			dst_cache_set_ip6(&rlwt->cache, dst,
>   					  &ipv6_hdr(skb)->saddr);
> diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
> index 33833b2064c0..6045e850b4bf 100644
> --- a/net/ipv6/seg6_iptunnel.c
> +++ b/net/ipv6/seg6_iptunnel.c
> @@ -472,10 +472,18 @@ static int seg6_input_core(struct net *net, struct sock *sk,
>   {
>   	struct dst_entry *orig_dst = skb_dst(skb);
>   	struct dst_entry *dst = NULL;
> +	struct lwtunnel_state *lwtst;
>   	struct seg6_lwt *slwt;
>   	int err;
>   
> -	slwt = seg6_lwt_lwtunnel(orig_dst->lwtstate);
> +	/* Get the address of lwtstate now, because "orig_dst" may not be there
> +	 * anymore after a call to skb_dst_drop(). Note that ip6_route_input()
> +	 * also calls skb_dst_drop(). Below, we compare the address of lwtstate
> +	 * to detect loops.
> +	 */
> +	lwtst = orig_dst->lwtstate;
> +
> +	slwt = seg6_lwt_lwtunnel(lwtst);
>   
>   	local_bh_disable();
>   	dst = dst_cache_get(&slwt->cache);
> @@ -490,7 +498,9 @@ static int seg6_input_core(struct net *net, struct sock *sk,
>   	if (!dst) {
>   		ip6_route_input(skb);
>   		dst = skb_dst(skb);
> -		if (!dst->error) {
> +
> +		/* cache only if we don't create a dst reference loop */
> +		if (!dst->error && lwtst != dst->lwtstate) {
>   			local_bh_disable();
>   			dst_cache_set_ip6(&slwt->cache, dst,
>   					  &ipv6_hdr(skb)->saddr);

... which makes me think while we're at it: should we directly drop the 
packet when dst->error (just like it's already done in output() 
handlers)? Not sure it's useful to send a DestUnreach in this case, 
especially considering that it may leak data. At least, we probably need 
to have consistency between input and output behavior. Thoughts?