Re: [PATCH net v2] net: Update window_clamp if SOCK_RCVBUF is set

[Mao Wenan <wenan.mao@linux.alibaba.com>]

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.