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

[Kiran Kella <kiran.kella@broadcom.com>]

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 */

Both the main_net path (derived from psd->main_netdev) and each
assoc_net entry in psd->assoc_dev_list are affected.

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")
Signed-off-by: Kiran Kella <kiran.kella@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
---
 net/psp/psp_nl.c | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/net/psp/psp_nl.c b/net/psp/psp_nl.c
index 9610d8c456ff..24ab626a9e8a 100644
--- a/net/psp/psp_nl.c
+++ b/net/psp/psp_nl.c
@@ -62,36 +62,45 @@ 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);
+	main_net = maybe_get_net(dev_net(psd->main_netdev));
+	if (!main_net)
+		return;
+
 	xa_init(&sent_nets);
 
 	list_for_each_entry(entry, &psd->assoc_dev_list, dev_list) {
 		struct net *assoc_net = dev_net(entry->assoc_dev);
+		struct net *net;
 		int ret;
 
 		if (net_eq(assoc_net, main_net))
 			continue;
 
-		ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
-				GFP_KERNEL);
-		if (ret == -EBUSY)
+		net = maybe_get_net(assoc_net);
+		if (!net)
 			continue;
 
-		ntf = build_ntf(psd, assoc_net, ctx);
-		if (!ntf)
+		ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
+				GFP_KERNEL);
+		if (ret == -EBUSY) {
+			put_net(net);
 			continue;
+		}
 
-		genlmsg_multicast_netns(&psp_nl_family, assoc_net, ntf, 0,
-					group, GFP_KERNEL);
+		ntf = build_ntf(psd, net, ctx);
+		if (ntf)
+			genlmsg_multicast_netns(&psp_nl_family, net, ntf, 0,
+						group, GFP_KERNEL);
+		put_net(net);
 	}
 	xa_destroy(&sent_nets);
 
 	/* 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