net: initialize net->notrefcnt_tracker earlier

syzbot was able to trigger a warning [1] from net_free()
calling ref_tracker_dir_exit(&net->notrefcnt_tracker)
while the corresponding ref_tracker_dir_init() has not been
done yet.

copy_net_ns() can indeed bypass the call to setup_net()
in some error conditions.

Note:

We might factorize/move more code in preinit_net() in the future.

[1]
INFO: trying to register non-static key.
The code is fine but needs lockdep annotation, or maybe
you didn't initialize this object before use?
turning off the locking correctness validator.
CPU: 0 PID: 5817 Comm: syz-executor.3 Not tainted 6.2.0-rc7-next-20230208-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/12/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
assign_lock_key kernel/locking/lockdep.c:982 [inline]
register_lock_class+0xdb6/0x1120 kernel/locking/lockdep.c:1295
__lock_acquire+0x10a/0x5df0 kernel/locking/lockdep.c:4951
lock_acquire.part.0+0x11c/0x370 kernel/locking/lockdep.c:5691
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x3d/0x60 kernel/locking/spinlock.c:162
ref_tracker_dir_exit+0x52/0x600 lib/ref_tracker.c:24
net_free net/core/net_namespace.c:442 [inline]
net_free+0x98/0xd0 net/core/net_namespace.c:436
copy_net_ns+0x4f3/0x6b0 net/core/net_namespace.c:493
create_new_namespaces+0x3f6/0xb20 kernel/nsproxy.c:110
unshare_nsproxy_namespaces+0xc1/0x1f0 kernel/nsproxy.c:228
ksys_unshare+0x449/0x920 kernel/fork.c:3205
__do_sys_unshare kernel/fork.c:3276 [inline]
__se_sys_unshare kernel/fork.c:3274 [inline]
__x64_sys_unshare+0x31/0x40 kernel/fork.c:3274
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80

Fixes: 0cafd77d ("net: add a refcount tracker for kernel sockets")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230208182123.3821604-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net_namespace.c

@@ -304,6 +304,12 @@ struct net *get_net_ns_by_id(const struct net *net, int id)
 }
 EXPORT_SYMBOL_GPL(get_net_ns_by_id);
 
+/* init code that must occur even if setup_net() is not called. */
+static __net_init void preinit_net(struct net *net)
+{
+	ref_tracker_dir_init(&net->notrefcnt_tracker, 128);
+}
+
 /*
  * setup_net runs the initializers for the network namespace object.
  */
@@ -316,7 +322,6 @@ static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
 
 	refcount_set(&net->ns.count, 1);
 	ref_tracker_dir_init(&net->refcnt_tracker, 128);
-	ref_tracker_dir_init(&net->notrefcnt_tracker, 128);
 
 	refcount_set(&net->passive, 1);
 	get_random_bytes(&net->hash_mix, sizeof(u32));
@@ -472,6 +477,8 @@ struct net *copy_net_ns(unsigned long flags,
 		rv = -ENOMEM;
 		goto dec_ucounts;
 	}
+
+	preinit_net(net);
 	refcount_set(&net->passive, 1);
 	net->ucounts = ucounts;
 	get_user_ns(user_ns);
@@ -1118,6 +1125,7 @@ void __init net_ns_init(void)
 	init_net.key_domain = &init_net_key_domain;
 #endif
 	down_write(&pernet_ops_rwsem);
+	preinit_net(&init_net);
 	if (setup_net(&init_net, &init_user_ns))
 		panic("Could not setup the initial network namespace");