[PATCH net v4] psp: fix NULL genl_sock deref race with concurrent netns teardown

The race occurs between network namespace removal and PSP device
unregistration.  When a netns is deleted while a PSP device associated
with that netns is concurrently being removed, psp_dev_unregister()
triggers psp_nl_notify_dev() to send a device change notification.
Concurrently, cleanup_net() running in the netns workqueue calls
genl_pernet_exit(), which sets net->genl_sock to NULL. If
genl_pernet_exit() wins the race, two sites in psp_nl_multicast_per_ns()
then dereference the NULL socket and crash:

CPU 0 (netns teardown)       CPU 1 (PSP device unregister)
======================       =============================
cleanup_net [workqueue]
  genl_pernet_exit()         psp_dev_unregister()
    net->genl_sock = NULL      psp_nl_notify_dev()
                                 psp_nl_multicast_per_ns()
                                   build_ntf()
                                     -> netlink_has_listeners(NULL)
                                     /* crash */
                                   genlmsg_multicast_netns()
                                     -> nlmsg_multicast_filtered(NULL)
                                     /* crash */

Fix by replacing the bare dev_net() calls with maybe_get_net().
maybe_get_net() returns NULL if the namespace is already dying.
Holding the reference ensures genl_sock remains valid across both the
build_ntf() and genlmsg_multicast_netns() calls.

Fixes: 00c94ca2b99e ("psp: base PSP device support")
Fixes: 06c2dce2d0f6 ("psp: add new netlink cmd for dev-assoc and dev-disassoc")
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Bhargava Marreddy <redacted>
Reviewed-by: Akhilesh Samineni <redacted>
Signed-off-by: Kiran Kella <redacted>
---
v4:
  Fixes done based on feedback from Jakub Kicinski
   - maybe_get_net() logic removed for assoc_dev_list
   - Added RCU protection on main netns before invoking maybe_get_net()

v3:
  Fixed typo in the commit log
  https://lore.kernel.org/all/20260715110701.3775026-1-kiran.kella@broadcom.com/ (local)

v2:
  get rid of the extra struct net *net, by doing
  (!maybe_get_net(assoc_net)) directly (as suggested by Daniel Zahka)
  https://lore.kernel.org/all/20260707185937.3177211-1-kiran.kella@broadcom.com/ (local)

v1: https://lore.kernel.org/all/20260703112431.2860506-1-kiran.kella@broadcom.com/ (local)

 net/psp/psp_nl.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/psp/psp_nl.c b/net/psp/psp_nl.c
index 9610d8c456ff..43b066353c65 100644
--- a/net/psp/psp_nl.c
+++ b/net/psp/psp_nl.c

@@ -62,7 +62,14 @@ psp_nl_multicast_per_ns(struct psp_dev *psd, unsigned int group,
 	struct net *main_net;
 	struct sk_buff *ntf;
 
-	main_net = dev_net(psd->main_netdev);
+	/* device may be changing netns in parallel */
+	rcu_read_lock();
+	main_net = maybe_get_net(dev_net_rcu(psd->main_netdev));
+	rcu_read_unlock();
+
+	if (!main_net)
+		return;
+
 	xa_init(&sent_nets);
 
 	list_for_each_entry(entry, &psd->assoc_dev_list, dev_list) {

@@ -88,10 +95,10 @@ psp_nl_multicast_per_ns(struct psp_dev *psd, unsigned int group,
 
 	/* Send to main device netns */
 	ntf = build_ntf(psd, main_net, ctx);
-	if (!ntf)
-		return;
-	genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
-				GFP_KERNEL);
+	if (ntf)
+		genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
+					GFP_KERNEL);
+	put_net(main_net);
 }
 
 static struct sk_buff *psp_nl_clone_ntf(struct psp_dev *psd, struct net *net,

-- 
2.54.0