Re: [PATCH 1/1] IPVS: Fix use-after-free issue in ip_vs_unbind_app()

[Julian Anastasov <ja@xxxxxx>]

        Hello,

On Mon, 8 Sep 2025, Slavin Liu wrote:

> When exiting a network namespace, in cleanup_net()->ops_undo_list(),
> ip_vs_ftp_ops->exit() is called before ip_vs_core_ops->exit_batch().
> The ip_vs_app ip_vs_ftp and its incarnations will be freed by 
> unregister_ip_vs_app().
> However, there could still be connections bound to ip_vs_ftp's incarnation.
> cp->app points to the free'd incarnation, which will be accessed later by
> __ip_vs_cleanup_batch()->ip_vs_conn_net_cleanup()->ip_vs_conn_flush()->ip_vs_conn_del()->
> ip_vs_conn_expire()->ip_vs_unbind_app(), causing a uaf. This vulnarability can
> lead to a local privilege escalation.
> 
> Reproduction steps:
> 1. create a ipvs service on (127.0.0.1:21)
> 2. create a ipvs destination on the service, to (127.0.0.1:<any>)
> 3. send a tcp packet to (127.0.0.1:21)
> 4. exit the network namespace
> 
> I think the fix should flush all connection to ftp before unregistration.
> The simpler fix is to delete ip_vs_ftp_ops->exit, and defer the unregistration
> of ip_vs_ftp to ip_vs_app_net_cleanup(), which will unregister all ip_vs_app.
> It's after ip_vs_conn_net_cleanup() so there is no uaf issue. This patch
> seems to solve the issue but has't been fully tested yet, and is also not 
> graceful.
> 
> Signed-off-by: Slavin Liu <slavin452@xxxxxxxxx>
> ---
>  net/netfilter/ipvs/ip_vs_ftp.c | 13 -------------
>  1 file changed, 13 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_ftp.c b/net/netfilter/ipvs/ip_vs_ftp.c
> index d8a284999544..68def1106681 100644
> --- a/net/netfilter/ipvs/ip_vs_ftp.c
> +++ b/net/netfilter/ipvs/ip_vs_ftp.c
> @@ -598,22 +598,9 @@ static int __net_init __ip_vs_ftp_init(struct net *net)
>       unregister_ip_vs_app(ipvs, &ip_vs_ftp);
>       return ret;
>  }
> -/*
> - *   netns exit
> - */
> -static void __ip_vs_ftp_exit(struct net *net)
> -{
> -     struct netns_ipvs *ipvs = net_ipvs(net);
> -
> -     if (!ipvs)

        What if we change this 'if' check to:

        if (!ipvs || !ipvs->enable)

        If netns exits, the cleanup order is:

1. exit handlers for pernet device (&ipvs_core_dev_ops) where
        ipvs->enable is set to 0

2. exit handlers for ip_vs_ftp_ops (as last pernet subsys)
        By checking for ipvs->enable, we should not call
        unregister_ip_vs_app() in __ip_vs_ftp_exit() because
        there can be existing conns with valid cp->app

3. exit handlers for pernet subsys (&ipvs_core_ops) where
        ip_vs_app_net_cleanup() unregisters all apps for netns.
        Here the apps will be freed after all conns are gone

        Why we should keep the ftp pernet subsys: because when
there are no conns using the app, the module can be removed and
it must unregister its app from all netns where ipvs->enable will
be 1.

        But note that we have a pending patch that changes
the access to ipvs->enable to use READ_ONCE/WRITE_ONCE:

https://archive.linuxvirtualserver.org/html/lvs-devel/2025-09/msg00000.html

> -             return;
> -
> -     unregister_ip_vs_app(ipvs, &ip_vs_ftp);
> -}
>  
>  static struct pernet_operations ip_vs_ftp_ops = {
>       .init = __ip_vs_ftp_init,
> -     .exit = __ip_vs_ftp_exit,
>  };
>  
>  static int __init ip_vs_ftp_init(void)
> -- 
> 2.34.1

Regards

--
Julian Anastasov <ja@xxxxxx>