Messages in this thread | | | Subject | Re: [PATCH net v2] net: Update window_clamp if SOCK_RCVBUF is set | From | Mao Wenan <> | Date | Mon, 9 Nov 2020 18:19:31 +0800 |
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在 2020/11/9 下午6:12, Mao Wenan 写道: > > > 在 2020/11/9 下午5:56, Eric Dumazet 写道: >> On Mon, Nov 9, 2020 at 10:33 AM 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. >> >> What is not working exactly ? >> >> Sending a 'big wscale' should not really matter, unless perhaps there >> is a buggy stack at the remote end ? > 1)in tcp_v4_send_synack, if SO_RCVBUF is set and > tcp_full_space(sk)=65535, pass req->rsk_window_clamp=65535 to > tcp_select_initial_window, rcv_wscale will be zero, and send to client, > the client consider wscale is 0; > 2)when ack is back from client, if there is no this patch, > req->rsk_window_clamp is 0, and pass to tcp_select_initial_window, > wscale will be 7, this new rcv_wscale is no way to advertise to client. > 3)if server send rcv_wind to client with window=63, it consider the real > window is 63*2^7=8064, but client consider the server window is only > 63*2^0=63, it can't send big packet to server, and the send-q of client > is full. > > >> >>> >>> Signed-off-by: Mao Wenan <wenan.mao@linux.alibaba.com> >>> --- >>> v2: fix for ipv6. >>> net/ipv4/syncookies.c | 4 ++++ >>> net/ipv6/syncookies.c | 5 +++++ >>> 2 files changed, 9 insertions(+) >>> >>> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c >>> index 6ac473b..57ce317 100644 >>> --- a/net/ipv4/syncookies.c >>> +++ b/net/ipv4/syncookies.c >>> @@ -427,6 +427,10 @@ 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 */ >>> + if (sk->sk_userlocks & SOCK_RCVBUF_LOCK && >>> + (req->rsk_window_clamp > tcp_full_space(sk) || >>> req->rsk_window_clamp == 0)) >>> + req->rsk_window_clamp = tcp_full_space(sk); >> >> This seems not needed to me. >> >> We call tcp_select_initial_window() with tcp_full_space(sk) passed as >> the 2nd parameter. >> >> tcp_full_space(sk) will then apply : >> >> space = min(*window_clamp, space); > > if cookie_v4_check pass window_clamp=0 to tcp_select_initial_window, it > will set window_clamp to max value. > (*window_clamp) = (U16_MAX << TCP_MAX_WSCALE);
window_clamp=0 is from req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW);
and if SO_RCVBUF is set and equal to 65535,req->rsk_window_clamp will be 65535. req->rsk_window_clamp = tcp_full_space(sk);
> > but space will fetch from sysctl_rmem_max and sysctl_tcp_rmem[2] which > is also big value. > space = max_t(u32, space, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]); > space = max_t(u32, space, sysctl_rmem_max); > > Then,space = min(*window_clamp, space) is a big value, lead wscale to 7, > is different from tcp_v4_send_synack. > > >> >> Please cook a packetdrill test to demonstrate what you are seeing ? >> > I have real environment and reproduce this case, this patch can fix > that, i will try to use packetdrill with syn cookies and syn flood happen.
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