Subject: Re: [PATCH net-next v2 6/8] netdev: depend on netdev->lock for xdp features

From: Jakub Kicinski <[email-masked]>
Date: Thu, 10 Apr 2025 19:23:26 -0700

On Thu, 10 Apr 2025 19:10:28 -0700 Jakub Kicinski wrote:
> On Thu, 10 Apr 2025 10:10:01 -0700 Kuniyuki Iwashima wrote:
> > syzkaller reported splats in register_netdevice() and
> > unregister_netdevice_many_notify().
> > 
> > In register_netdevice(), some devices cannot use
> > netdev_assert_locked().
> > 
> > In unregister_netdevice_many_notify(), maybe we need to
> > hold ops lock in UNREGISTER as you initially suggested.
> > Now do_setlink() deadlock does not happen.  
> 
> Ah...  Thank you.
> 
> Do you have a reference to use as Reported-by, or its from a
> non-public instance ?
> 
> I'll test this shortly:
> 
> diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
> index b64c614a00c4..891e2f60922f 100644
> --- a/net/core/netdev-genl.c
> +++ b/net/core/netdev-genl.c
> @@ -38,7 +38,8 @@ netdev_nl_dev_fill(struct net_device *netdev, struct sk_buff *rsp,
>         u64 xdp_rx_meta = 0;
>         void *hdr;
>  
> -       netdev_assert_locked(netdev); /* note: rtnl_lock may not be held! */
> +       /* note: rtnl_lock may or may not be held! */
> +       netdev_assert_locked_or_invisible(netdev);
>  
>         hdr = genlmsg_iput(rsp, info);
>         if (!hdr)
> @@ -966,7 +967,9 @@ static int netdev_genl_netdevice_event(struct notifier_block *nb,
>                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_ADD_NTF);
>                 break;
>         case NETDEV_UNREGISTER:
> +               netdev_lock(netdev);
>                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_DEL_NTF);
> +               netdev_unlock(netdev);
>                 break;
>         case NETDEV_XDP_FEAT_CHANGE:
>                 netdev_genl_dev_notify(netdev, NETDEV_CMD_DEV_CHANGE_NTF);

Ugh, REGISTER is ops locked we'd need conditional locking here.

Stanislav, I can make the REGISTERED notifier fully locked, right?
I suspect any new object we add that's protected by the instance
lock will want to lock the dev.