linux-kernel - Re: unregister_netdevice: waiting for DEV to become free (2)

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Date:   Thu, 05 Dec 2019 12:00:04 +0200
From:   jouni.hogander@...kie.com (Jouni Högander)
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     syzbot <syzbot+30209ea299c09d8785c9@...kaller.appspotmail.com>,
        YueHaibing <yuehaibing@...wei.com>, Julian Anastasov <ja@....bg>,
        ddstreet@...e.org, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, Hulk Robot <hulkci@...wei.com>,
        "David S. Miller" <davem@...emloft.net>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>
Subject: Re: unregister_netdevice: waiting for DEV to become free (2)

Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> writes:

> [   61.584734] Code: bd b1 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 8b b1 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> [   61.590407] RSP: 002b:00007f25d540ec88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [   61.592488] RAX: ffffffffffffffda RBX: 000000000071bf00 RCX: 000000000045a729
> [   61.594552] RDX: 0000000020000040 RSI: 00000000400454d9 RDI: 0000000000000003
> [   61.596829] RBP: 00007f25d540eca0 R08: 0000000000000000 R09: 0000000000000000
> [   61.598540] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f25d540f6d4
> [   61.600278] R13: 00000000004ac5a5 R14: 00000000006ee8a0 R15: 0000000000000005
> [   61.655323] kobject_add_internal failed for tx-1 (error: -12 parent: queues)
> [   71.760970] unregister_netdevice: waiting for vet to become free. Usage count = -1
> [   82.028434] unregister_netdevice: waiting for vet to become free. Usage count = -1
> [   92.140031] unregister_netdevice: waiting for vet to become free. Usage count = -1
> ----------
>
> Worrisome part is that tun_attach() calls tun_set_real_num_queues() at the end of tun_attach()
> but tun_set_real_num_queues() is not handling netif_set_real_num_tx_queues() failure.
> That is, tun_attach() is returning success even if netdev_queue_update_kobjects() from
> netif_set_real_num_tx_queues() failed.
>
>   static void tun_set_real_num_queues(struct tun_struct *tun)
>   {
>           netif_set_real_num_tx_queues(tun->dev, tun->numqueues);
>           netif_set_real_num_rx_queues(tun->dev, tun->numqueues);
>   }
>
> And I guess that ignoring that failure causes clean-up function to drop a refcount
> which was not held by initialization function. Applying below diff seems to avoid
> this problem. Please check.
>
> ----------
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index ae3bcb1540ec..562d06c274aa 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -1459,14 +1459,14 @@ static int netdev_queue_add_kobject(struct net_device *dev, int index)
>  	struct kobject *kobj = &queue->kobj;
>  	int error = 0;
>  
> +	dev_hold(queue->dev);
> +
>  	kobj->kset = dev->queues_kset;
>  	error = kobject_init_and_add(kobj, &netdev_queue_ktype, NULL,
>  				     "tx-%u", index);
>  	if (error)
>  		goto err;
>  
> -	dev_hold(queue->dev);
> -
>  #ifdef CONFIG_BQL
>  	error = sysfs_create_group(kobj, &dql_group);
>  	if (error)

Now after reproducing the issue I think this is actually proper fix for
the issue.  It's not related to missing error handling in in
tun_set_real_num_queues as I commented earlier. Can you prepare patch
for this?

BR,

Jouni Högander