Re: [PATCH v2] net-tcp: Find dst with sk's xfrm policy not ctl_sk

[서세욱 <ssewook@gmail.com>]

Hi, Eric.

Thanks for your review.

 if (IS_ENABLED(CONFIG_XFRM) && sk->sk_policy[XFRM_POLICY_OUT])
 --> This causes a compile error when CONFIG_XFRM is disabled.
ldd: /usr/bin/ld: No such file or directory
net/ipv4/ip_output.c:1739:37: error: no member named 'sk_policy' in
'struct sock'
        if (IS_ENABLED(CONFIG_XFRM) && sk->sk_policy[XFRM_POLICY_OUT])
I think we need to use preprocessor directives at here.
Is there any reason to use #if than #ifdef?  Then I will modify it to use #if.
    #if IS_ENABLED(CONFIG_XFRM) or #if defined(CONFIG_XFRM)

The reason I added the condition only for the state 'TCP_SYN_SENT' is
that I just intended to limit
the scope of the patch to the issue scenario(RST packet following
challenge ACK is not ESP encapsulated)
so that we can have at least a difference as before.
I also agree with you about using sk_fullsock() instead of SYN_SENT
check. will update the patch soon.

Thanks.
Sewook.


2022년 7월 5일 (화) 오전 9:04, Eric Dumazet <edumazet@google.com>님이 작성:
>
> On Fri, Jul 1, 2022 at 5:45 PM Sewook Seo <ssewook@gmail.com> wrote:
> >
> > From: sewookseo <sewookseo@google.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@google.com>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Steffen Klassert <steffen.klassert@secunet.com>
> > Cc: Sehee Lee <seheele@google.com>
> > Signed-off-by: Sewook Seo <sewookseo@google.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
> >