netdev - Re: [PATCH net 2/3] udp: check encap socket in __udp_lib

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Date:   Mon, 12 Jul 2021 11:07:16 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Vadim Fedorenko <vfedorenko@...ek.ru>,
        David Ahern <dsahern@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Xin Long <lucien.xin@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net 2/3] udp: check encap socket in __udp_lib_err

Hello,

On Mon, 2021-07-12 at 03:55 +0300, Vadim Fedorenko wrote:
> Commit d26796ae5894 ("udp: check udp sock encap_type in __udp_lib_err")
> added checks for encapsulated sockets but it broke cases when there is
> no implementation of encap_err_lookup for encapsulation, i.e. ESP in
> UDP encapsulation. Fix it by calling encap_err_lookup only if socket
> implements this method otherwise treat it as legal socket.
> 
> Fixes: d26796ae5894 ("udp: check udp sock encap_type in __udp_lib_err")
> Signed-off-by: Vadim Fedorenko <vfedorenko@...ek.ru>
> ---
>  net/ipv4/udp.c | 24 +++++++++++++++++++++++-
>  net/ipv6/udp.c | 22 ++++++++++++++++++++++
>  2 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index e5cb7fedfbcd..4980e0f19990 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -707,7 +707,29 @@ int __udp4_lib_err(struct sk_buff *skb, u32 info, struct udp_table *udptable)
>  	sk = __udp4_lib_lookup(net, iph->daddr, uh->dest,
>  			       iph->saddr, uh->source, skb->dev->ifindex,
>  			       inet_sdif(skb), udptable, NULL);
> -	if (!sk || udp_sk(sk)->encap_enabled) {
> +	if (sk && udp_sk(sk)->encap_enabled) {
> +		int (*lookup)(struct sock *sk, struct sk_buff *skb);
> +
> +		lookup = READ_ONCE(udp_sk(sk)->encap_err_lookup);
> +		if (lookup) {
> +			int network_offset, transport_offset;
> +
> +			network_offset = skb_network_offset(skb);
> +			transport_offset = skb_transport_offset(skb);
> +
> +			/* Network header needs to point to the outer IPv4 header inside ICMP */
> +			skb_reset_network_header(skb);
> +
> +			/* Transport header needs to point to the UDP header */
> +			skb_set_transport_header(skb, iph->ihl << 2);
> +			if (lookup(sk, skb))
> +				sk = NULL;
> +			skb_set_transport_header(skb, transport_offset);
> +			skb_set_network_header(skb, network_offset);
> +		}
> +	}
> +
> +	if (!sk) {
>  		/* No socket for error: try tunnels before discarding */
>  		sk = ERR_PTR(-ENOENT);
>  		if (static_branch_unlikely(&udp_encap_needed_key)) {
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 798916d2e722..ed49a8589d9f 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -558,6 +558,28 @@ int __udp6_lib_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>  
>  	sk = __udp6_lib_lookup(net, daddr, uh->dest, saddr, uh->source,
>  			       inet6_iif(skb), inet6_sdif(skb), udptable, NULL);
> +	if (sk && udp_sk(sk)->encap_enabled) {
> +		int (*lookup)(struct sock *sk, struct sk_buff *skb);
> +
> +		lookup = READ_ONCE(udp_sk(sk)->encap_err_lookup);
> +		if (lookup) {
> +			int network_offset, transport_offset;
> +
> +			network_offset = skb_network_offset(skb);
> +			transport_offset = skb_transport_offset(skb);
> +
> +			/* Network header needs to point to the outer IPv6 header inside ICMP */
> +			skb_reset_network_header(skb);
> +
> +			/* Transport header needs to point to the UDP header */
> +			skb_set_transport_header(skb, offset);
> +			if (lookup(sk, skb))
> +				sk = NULL;
> +			skb_set_transport_header(skb, transport_offset);
> +			skb_set_network_header(skb, network_offset);
> +		}
> +	}

I can't follow this code. I guess that before d26796ae5894,
__udp6_lib_err() used to invoke ICMP processing on the ESP in UDP
socket, and after d26796ae5894 'sk' was cleared
by __udp4_lib_err_encap(), is that correct?

After this patch, the above chunk will not clear 'sk' for packets
targeting ESP in UDP sockets, but AFAICS we will still enter the
following conditional, preserving the current behavior - no ICMP
processing. 

Can you please clarify?

Why can't you use something alike the following instead?

---
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index c0f9f3260051..96a3b640e4da 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -707,7 +707,7 @@ int __udp4_lib_err(struct sk_buff *skb, u32 info, struct udp_table *udptable)
        sk = __udp4_lib_lookup(net, iph->daddr, uh->dest,
                               iph->saddr, uh->source, skb->dev->ifindex,
                               inet_sdif(skb), udptable, NULL);
-       if (!sk || udp_sk(sk)->encap_type) {
+       if (!sk || READ_ONCE(udp_sk(sk)->encap_err_lookup)) {
                /* No socket for error: try tunnels before discarding */
                sk = ERR_PTR(-ENOENT);
                if (static_branch_unlikely(&udp_encap_needed_key)) {

---

Thanks!

/P