Re: [PATCH net] ipvs: Defer application parent freeing until after RCU readers

[Julian Anastasov]

	Hello,

On Sun, 27 Sep 2026, Chengfeng Ye wrote:

> unregister_ip_vs_app() removes each incarnation from its protocol list and
> uses call_rcu() to defer freeing it, but frees the parent application
> immediately. An RCU reader that already found an incarnation can still
> access the parent through inc->app in ip_vs_app_inc_get().
> 
> During FTP helper module unload, the following interleaving is possible:
> 
>   CPU 0 (RCU reader)                 CPU 1 (module unload)
>   tcp_app_conn_bind()
>     find inc in protocol list
>                                      unregister_ip_vs_app()
>                                        ip_vs_app_inc_release(inc)
>                                          list_del_rcu(&inc->p_list)
>                                          call_rcu(&inc->rcu_head, ...)
>                                        kfree(a)
>     ip_vs_app_inc_get(inc)
>       try_module_get(inc->app->module)
> 
> The helper module is already going away, so try_module_get() would fail,
> but evaluating its argument first reads the freed parent. The subsequent
> rcu_barrier() in pernet unregistration cannot protect this earlier free.
> KASAN reported:
> 
>   BUG: KASAN: slab-use-after-free in ip_vs_app_inc_get+0x7c/0x90
>   Call Trace:
>    ip_vs_app_inc_get+0x7c/0x90
>    tcp_app_conn_bind+0x1bc/0x290
>    ip_vs_conn_new+0x1915/0x20d0
>    ip_vs_schedule+0x697/0xea0
>    tcp_conn_schedule+0x489/0x820
>    ip_vs_in_hook+0x7bf/0x1f40
>   Allocated by task 88:
>    kmemdup_noprof+0x20/0x50
>    register_ip_vs_app+0x12d/0x2c0
>    __ip_vs_ftp_init+0x56/0x160 [ip_vs_ftp]
>   Freed by task 104:
>    kfree+0x131/0x3c0
>    unregister_ip_vs_app+0x2f4/0x5b0
>    unregister_pernet_operations+0x232/0x490
>    unregister_pernet_subsys+0x1c/0x30
>    __do_sys_delete_module+0x346/0x510
> 
> Use kfree_rcu() with the existing rcu_head to keep the parent alive until
> these readers finish. Successful helper references already prevent normal
> module unload, and incarnation RCU callbacks do not access the parent.
> 
> Fixes: 363c97d7435e ("ipvs: convert app locks")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>

	Looks good to me for the nf tree, thanks!

Acked-by: Julian Anastasov <ja@xxxxxx>

> ---
>  net/netfilter/ipvs/ip_vs_app.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_app.c b/net/netfilter/ipvs/ip_vs_app.c
> index 11cbdbaf561d..2c1b47702da2 100644
> --- a/net/netfilter/ipvs/ip_vs_app.c
> +++ b/net/netfilter/ipvs/ip_vs_app.c
> @@ -242,7 +242,7 @@ void unregister_ip_vs_app(struct netns_ipvs *ipvs, struct ip_vs_app *app)
>  		}
>  
>  		list_del(&a->a_list);
> -		kfree(a);
> +		kfree_rcu(a, rcu_head);
>  
>  		/* decrease the module use count */
>  		ip_vs_use_count_dec();
> -- 
> 2.43.0

Regards

--
Julian Anastasov <ja@xxxxxx>