[RFC PATCH v3] ipvs: Defer ip_vs_ftp unregister during netns cleanup

[Slavin Liu <slavin452@xxxxxxxxx>]

On the netns cleanup path, __ip_vs_ftp_exit() may unregister ip_vs_ftp
before connections with valid cp->app pointers are flushed, leading to a
use-after-free.

Fix this by introducing a global `module_removing` flag, set to true in
ip_vs_ftp_exit() before unregistering the pernet subsystem. In
__ip_vs_ftp_exit(), skip ip_vs_ftp unregister if called during netns
cleanup (when module_removing is false) and defer it to
__ip_vs_cleanup_batch(), which unregisters all apps after all connections
are flushed. If called during module exit, unregister ip_vs_ftp
immediately.

Fixes: 61b1ab4583e2 ("IPVS: netns, add basic init per netns.")
Suggested-by: Julian Anastasov <ja@xxxxxx>
Signed-off-by: Slavin Liu <slavin452@xxxxxxxxx>
---
 net/netfilter/ipvs/ip_vs_ftp.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/ipvs/ip_vs_ftp.c b/net/netfilter/ipvs/ip_vs_ftp.c
index d8a284999544..4db58c42ff9a 100644
--- a/net/netfilter/ipvs/ip_vs_ftp.c
+++ b/net/netfilter/ipvs/ip_vs_ftp.c
@@ -53,6 +53,7 @@ enum {
        IP_VS_FTP_EPSV,
 };
 
+static bool module_exiting;
 /*
  * List of ports (up to IP_VS_APP_MAX_PORTS) to be handled by helper
  * First port is set to the default port.
@@ -605,7 +606,7 @@ static void __ip_vs_ftp_exit(struct net *net)
 {
        struct netns_ipvs *ipvs = net_ipvs(net);
 
-       if (!ipvs)
+       if (!ipvs || !module_exiting)
                return;
 
        unregister_ip_vs_app(ipvs, &ip_vs_ftp);
@@ -627,7 +628,9 @@ static int __init ip_vs_ftp_init(void)
  */
 static void __exit ip_vs_ftp_exit(void)
 {
+       module_exiting = true;
        unregister_pernet_subsys(&ip_vs_ftp_ops);
+       module_exiting = false;
        /* rcu_barrier() is called by netns */
 }
 
-- 
2.34.1