Date:   Tue, 5 Jul 2022 11:04:16 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Sewook Seo <ssewook@...il.com>
Cc:     Sewook Seo <sewookseo@...gle.com>,
        Linux Network Development Mailing List 
        <netdev@...r.kernel.org>, "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Maciej Żenczykowski <maze@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Sehee Lee <seheele@...gle.com>
Subject: Re: [PATCH v2] net-tcp: Find dst with sk's xfrm policy not ctl_sk

On Fri, Jul 1, 2022 at 5:45 PM Sewook Seo <ssewook@...il.com> wrote:
>
> From: sewookseo <sewookseo@...gle.com>
>
> If we set XFRM security policy by calling setsockopt with option
> IPV6_XFRM_POLICY, the policy will be stored in 'sock_policy' in 'sock'
> struct. However tcp_v6_send_response doesn't look up dst_entry with the
> actual socket but looks up with tcp control socket. This may cause a
> problem that a RST packet is sent without ESP encryption & peer's TCP
> socket can't receive it.
> This patch will make the function look up dest_entry with actual socket,
> if the socket has XFRM policy(sock_policy), so that the TCP response
> packet via this function can be encrypted, & aligned on the encrypted
> TCP socket.
>
> Tested: We encountered this problem when a TCP socket which is encrypted
> in ESP transport mode encryption, receives challenge ACK at SYN_SENT
> state. After receiving challenge ACK, TCP needs to send RST to
> establish the socket at next SYN try. But the RST was not encrypted &
> peer TCP socket still remains on ESTABLISHED state.
> So we verified this with test step as below.
> [Test step]
> 1. Making a TCP state mismatch between client(IDLE) & server(ESTABLISHED).
> 2. Client tries a new connection on the same TCP ports(src & dst).
> 3. Server will return challenge ACK instead of SYN,ACK.
> 4. Client will send RST to server to clear the SOCKET.
> 5. Client will retransmit SYN to server on the same TCP ports.
> [Expected result]
> The TCP connection should be established.
>
> Effort: net-tcp
> Cc: Maciej Żenczykowski <maze@...gle.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Steffen Klassert <steffen.klassert@...unet.com>
> Cc: Sehee Lee <seheele@...gle.com>
> Signed-off-by: Sewook Seo <sewookseo@...gle.com>
> ---
> Changelog since v1:
> - Remove unnecessary null check of sk at ip_output.c
>   Narrow down patch scope: sending RST at SYN_SENT state
>   Remove unnecessay condition to call xfrm_sk_free_policy()
>   Verified at KASAN build
>
>  net/ipv4/ip_output.c | 7 ++++++-
>  net/ipv4/tcp_ipv4.c  | 5 +++++
>  net/ipv6/tcp_ipv6.c  | 7 ++++++-
>  3 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 00b4bf26fd93..1da430c8fee2 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1704,7 +1704,12 @@ void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
>                            tcp_hdr(skb)->source, tcp_hdr(skb)->dest,
>                            arg->uid);
>         security_skb_classify_flow(skb, flowi4_to_flowi_common(&fl4));
> -       rt = ip_route_output_key(net, &fl4);

Please avoid these #ifdef ?

You probably can write something like

     if (IS_ENABLED(CONFIG_XFRM) && sk->sk_policy[XFRM_POLICY_OUT])
         rt = ip_route_output_flow(net, &fl4, sk);
    else
          rt = ip_route_output_key(net, &fl4);

> +#ifdef CONFIG_XFRM
> +       if (sk->sk_policy[XFRM_POLICY_OUT])
> +               rt = ip_route_output_flow(net, &fl4, sk);
> +       else
> +#endif
> +               rt = ip_route_output_key(net, &fl4);
>         if (IS_ERR(rt))
>                 return;
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index fda811a5251f..459669f9e13f 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -819,6 +819,10 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb)
>                 ctl_sk->sk_priority = (sk->sk_state == TCP_TIME_WAIT) ?
>                                    inet_twsk(sk)->tw_priority : sk->sk_priority;
>                 transmit_time = tcp_transmit_time(sk);
> +#ifdef CONFIG_XFRM
> +               if (sk->sk_policy[XFRM_POLICY_OUT] && sk->sk_state == TCP_SYN_SENT)
> +                       xfrm_sk_clone_policy(ctl_sk, sk);
> +#endif
>         }
>         ip_send_unicast_reply(ctl_sk,
>                               skb, &TCP_SKB_CB(skb)->header.h4.opt,
> @@ -827,6 +831,7 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb)
>                               transmit_time);
>
>         ctl_sk->sk_mark = 0;
> +       xfrm_sk_free_policy(ctl_sk);
>         sock_net_set(ctl_sk, &init_net);
>         __TCP_INC_STATS(net, TCP_MIB_OUTSEGS);
>         __TCP_INC_STATS(net, TCP_MIB_OUTRSTS);
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index c72448ba6dc9..453452f87a7c 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -952,7 +952,12 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
>          * Underlying function will use this to retrieve the network
>          * namespace
>          */
> -       dst = ip6_dst_lookup_flow(sock_net(ctl_sk), ctl_sk, &fl6, NULL);
> +#ifdef CONFIG_XFRM
> +       if (sk && sk->sk_policy[XFRM_POLICY_OUT] && sk->sk_state == TCP_SYN_SENT && rst)


Why not using sk_fullsock(sk)  instead of 'sk->sk_state == TCP_SYN_SENT' ?

sk_fullsock() is really telling us if we can use sk as a full socket,
and this is all we need to know when reviewing this code.

> +               dst = ip6_dst_lookup_flow(net, sk, &fl6, NULL);  /* Get dst with sk's XFRM policy */
> +       else
> +#endif
> +               dst = ip6_dst_lookup_flow(sock_net(ctl_sk), ctl_sk, &fl6, NULL);
>         if (!IS_ERR(dst)) {
>                 skb_dst_set(buff, dst);
>                 ip6_xmit(ctl_sk, buff, &fl6, fl6.flowi6_mark, NULL,
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>

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