[net,v3] net: Update window_clamp if SOCK_RCVBUF is set

When net.ipv4.tcp_syncookies=1 and syn flood is happened,
cookie_v4_check or cookie_v6_check tries to redo what
tcp_v4_send_synack or tcp_v6_send_synack did,
rsk_window_clamp will be changed if SOCK_RCVBUF is set,
which will make rcv_wscale is different, the client
still operates with initial window scale and can overshot
granted window, the client use the initial scale but local
server use new scale to advertise window value, and session
work abnormally.

Fixes: e88c64f0a425 ("tcp: allow effective reduction of TCP's
rcv-buffer via setsockopt")

Signed-off-by: Mao Wenan <wenan.mao@linux.alibaba.com>
---
 v3: add local variable full_space, add fixes tag.
 v2: fix for ipv6.
 net/ipv4/syncookies.c | 7 ++++++-
 net/ipv6/syncookies.c | 8 +++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

On Mon, Nov 9, 2020 at 5:54 PM Mao Wenan <wenan.mao@linux.alibaba.com> wrote:
>
> When net.ipv4.tcp_syncookies=1 and syn flood is happened,
> cookie_v4_check or cookie_v6_check tries to redo what
> tcp_v4_send_synack or tcp_v6_send_synack did,
> rsk_window_clamp will be changed if SOCK_RCVBUF is set,
> which will make rcv_wscale is different, the client
> still operates with initial window scale and can overshot
> granted window, the client use the initial scale but local
> server use new scale to advertise window value, and session
> work abnormally.
>
> Fixes: e88c64f0a425 ("tcp: allow effective reduction of TCP's
> rcv-buffer via setsockopt")

Please put this tag in a single line (no line wrap)
And do not add an empty line after it.
The Fixes: tag is part of the official tags, all grouped together.
>
> Signed-off-by: Mao Wenan <wenan.mao@linux.alibaba.com>
> ---
>  v3: add local variable full_space, add fixes tag.
>  v2: fix for ipv6.
>  net/ipv4/syncookies.c | 7 ++++++-
>  net/ipv6/syncookies.c | 8 +++++++-
>  2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
> index 6ac473b..eea4698 100644
> --- a/net/ipv4/syncookies.c
> +++ b/net/ipv4/syncookies.c
> @@ -327,6 +327,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
>         struct inet_request_sock *ireq;
>         struct tcp_request_sock *treq;
>         struct tcp_sock *tp = tcp_sk(sk);
> +       int full_space = tcp_full_space(sk);

Please delay the actual call to tcp_full_space() until we need it.
If a packet does not validate the cookie, no need to init @full_space.

>         const struct tcphdr *th = tcp_hdr(skb);
>         __u32 cookie = ntohl(th->ack_seq) - 1;
>         struct sock *ret = sk;
> @@ -427,8 +428,12 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
>
>         /* Try to redo what tcp_v4_send_synack did. */
>         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
> +       /* limit the window selection if the user enforce a smaller rx buffer */

eg:
      full_space = tcp_full_space(sk);

> +       if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
> +           (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
> +               req->rsk_window_clamp = full_space;
>
> -       tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
> +       tcp_select_initial_window(sk, full_space, req->mss,
>                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
>                                   ireq->wscale_ok, &rcv_wscale,
>                                   dst_metric(&rt->dst, RTAX_INITRWND));
> diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
> index e796a64..5b09bb6 100644
> --- a/net/ipv6/syncookies.c
> +++ b/net/ipv6/syncookies.c
> @@ -132,6 +132,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
>         struct tcp_request_sock *treq;
>         struct ipv6_pinfo *np = inet6_sk(sk);
>         struct tcp_sock *tp = tcp_sk(sk);
> +       int full_space = tcp_full_space(sk);

Same remark here.

>         const struct tcphdr *th = tcp_hdr(skb);
>         __u32 cookie = ntohl(th->ack_seq) - 1;
>         struct sock *ret = sk;
> @@ -241,7 +242,12 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
>         }
>
>         req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
> -       tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
> +       /* limit the window selection if the user enforce a smaller rx buffer */
> +       if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
> +           (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
> +               req->rsk_window_clamp = full_space;
> +
> +       tcp_select_initial_window(sk, full_space, req->mss,
>                                   &req->rsk_rcv_wnd, &req->rsk_window_clamp,
>                                   ireq->wscale_ok, &rcv_wscale,
>                                   dst_metric(dst, RTAX_INITRWND));
> --
> 1.8.3.1
>

Thanks !