[PATCH] kobject: fix uevent socket use-after-free on net namespace delete

[Adarsh Das <adarshdas950@gmail.com>]

When a network namespace is deleted, we free its uevent socket. But
sometimes a device in that namespace is still being removed and tries to
send a uevent through that socket. The socket is already freed, so KASAN
reports a use-after-free.

This patch attempts the fix of taking a lock around both the send path
and the free path, and clear the pointer before freeing so nobody uses
it after free.

Reported-by: syzbot <syzbot+4393dfdddf166f2de2b0@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=4393dfdddf166f2de2b0
Signed-off-by: Adarsh Das <adarshdas950@gmail.com>
---
 lib/kobject_uevent.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index ddbc4d7482d2..ecba33abb6b1 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -411,9 +411,15 @@ static int kobject_uevent_net_broadcast(struct kobject *kobj,
 						    devpath);
 	else {
 		const struct net *net = container_of(ns, struct net, ns);
+		struct uevent_sock *ue_sk;
 
-		ret = uevent_net_broadcast_tagged(net->uevent_sock->sk, env,
-						  action_string, devpath);
+		mutex_lock(&uevent_sock_mutex);
+		ue_sk = net->uevent_sock;
+		if (ue_sk && ue_sk->sk)
+			ret = uevent_net_broadcast_tagged(ue_sk->sk, env,
+							  action_string,
+							  devpath);
+		mutex_unlock(&uevent_sock_mutex);
 	}
 #endif
 
@@ -804,13 +810,17 @@ static int uevent_net_init(struct net *net)
 
 static void uevent_net_exit(struct net *net)
 {
-	struct uevent_sock *ue_sk = net->uevent_sock;
+	struct uevent_sock *ue_sk;
 
-	if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) {
-		mutex_lock(&uevent_sock_mutex);
+	mutex_lock(&uevent_sock_mutex);
+	ue_sk = net->uevent_sock;
+	net->uevent_sock = NULL;
+	if (ue_sk && sock_net(ue_sk->sk)->user_ns == &init_user_ns)
 		list_del(&ue_sk->list);
-		mutex_unlock(&uevent_sock_mutex);
-	}
+	mutex_unlock(&uevent_sock_mutex);
+
+	if (!ue_sk)
+		return;
 
 	netlink_kernel_release(ue_sk->sk);
 	kfree(ue_sk);
-- 
2.55.0