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

[Chengfeng Ye <nicoyip.dev@gmail.com>]

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@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 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