Re: [PATCH net] net: mptcp: fix unreleased socket in accept queue

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Mon, 2022-09-05 at 13:04 +0800, menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.com>
> 
> The mptcp socket and its subflow sockets in accept queue can't be
> released after the process exit.
> 
> While the release of a mptcp socket in listening state, the
> corresponding tcp socket will be released too. Meanwhile, the tcp
> socket in the unaccept queue will be released too. However, only init
> subflow is in the unaccept queue, and the joined subflow is not in the
> unaccept queue, which makes the joined subflow won't be released, and
> therefore the corresponding unaccepted mptcp socket will not be released
> to.
> 
> This can be reproduced easily with following steps:
> 
> 1. create 2 namespace and veth:
>    $ ip netns add mptcp-client
>    $ ip netns add mptcp-server
>    $ sysctl -w net.ipv4.conf.all.rp_filter=0
>    $ ip netns exec mptcp-client sysctl -w net.mptcp.enabled=1
>    $ ip netns exec mptcp-server sysctl -w net.mptcp.enabled=1
>    $ ip link add red-client netns mptcp-client type veth peer red-server \
>      netns mptcp-server
>    $ ip -n mptcp-server address add 10.0.0.1/24 dev red-server
>    $ ip -n mptcp-server address add 192.168.0.1/24 dev red-server
>    $ ip -n mptcp-client address add 10.0.0.2/24 dev red-client
>    $ ip -n mptcp-client address add 192.168.0.2/24 dev red-client
>    $ ip -n mptcp-server link set red-server up
>    $ ip -n mptcp-client link set red-client up
> 
> 2. configure the endpoint and limit for client and server:
>    $ ip -n mptcp-server mptcp endpoint flush
>    $ ip -n mptcp-server mptcp limits set subflow 2 add_addr_accepted 2
>    $ ip -n mptcp-client mptcp endpoint flush
>    $ ip -n mptcp-client mptcp limits set subflow 2 add_addr_accepted 2
>    $ ip -n mptcp-client mptcp endpoint add 192.168.0.2 dev red-client id \
>      1 subflow
> 
> 3. listen and accept on a port, such as 9999. The nc command we used
>    here is modified, which makes it uses mptcp protocol by default.
>    And the default backlog is 1:
>    ip netns exec mptcp-server nc -l -k -p 9999
> 
> 4. open another *two* terminal and connect to the server with the
>    following command:
>    $ ip netns exec mptcp-client nc 10.0.0.1 9999
>    input something after connect, to triger the connection of the second
>    subflow
> 
> 5. exit all the nc command, and check the tcp socket in server namespace.
>    And you will find that there is one tcp socket in CLOSE_WAIT state
>    and can't release forever.

Thank you for the report! 

I have a doubt WRT the above scenario: AFAICS 'nc' will accept the
incoming sockets ASAP, so the unaccepted queue should be empty at
shutdown, but that does not fit with your description?!?

> There are some solutions that I thought:
> 
> 1. release all unaccepted mptcp socket with mptcp_close() while the
>    listening tcp socket release in mptcp_subflow_queue_clean(). This is
>    what we do in this commit.
> 2. release the mptcp socket with mptcp_close() in subflow_ulp_release().
> 3. etc
> 

Can you please point to a commit introducing the issue?

> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
>  net/mptcp/subflow.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index c7d49fb6e7bd..e39dff5d5d84 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -1770,6 +1770,10 @@ void mptcp_subflow_queue_clean(struct sock *listener_ssk)
>  		msk->first = NULL;
>  		msk->dl_next = NULL;
>  		unlock_sock_fast(sk, slow);
> +
> +		/*  */
> +		sock_hold(sk);
> +		sk->sk_prot->close(sk);

You can call mptcp_close() directly here.

Perhaps we could as well drop the mptcp_sock_destruct() hack?

Perhpas even providing a __mptcp_close() variant not acquiring the
socket lock and move such close call inside the existing sk socket lock
above?

Thanks,

Paolo