linux-kernel - Re: [syzbot] [net?] general protection fault in rtnl_create

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Message-ID: <20250414182341.31191-1-kuniyu@amazon.com>
Date: Mon, 14 Apr 2025 11:23:27 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kuba@...nel.org>
CC: <cratiu@...dia.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<horms@...nel.org>, <kuniyu@...zon.com>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>, <sdf@...ichev.me>,
	<syzbot+de1c7d68a10e3f123bdd@...kaller.appspotmail.com>,
	<syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] [net?] general protection fault in rtnl_create_link

From: Jakub Kicinski <kuba@...nel.org>
Date: Mon, 14 Apr 2025 11:15:36 -0700
> On Mon, 14 Apr 2025 11:01:59 -0700 Kuniyuki Iwashima wrote:
> > From: Jakub Kicinski <kuba@...nel.org>
> > Date: Mon, 14 Apr 2025 10:37:27 -0700
> > > On Sun, 13 Apr 2025 19:30:46 -0700 Kuniyuki Iwashima wrote:  
> > > > diff --git a/include/net/netdev_lock.h b/include/net/netdev_lock.h
> > > > index 5706835a660c..270e157a4a79 100644
> > > > --- a/include/net/netdev_lock.h
> > > > +++ b/include/net/netdev_lock.h
> > > > @@ -30,7 +30,8 @@ static inline bool netdev_need_ops_lock(const struct net_device *dev)
> > > >  	bool ret = dev->request_ops_lock || !!dev->queue_mgmt_ops;
> > > >  
> > > >  #if IS_ENABLED(CONFIG_NET_SHAPER)
> > > > -	ret |= !!dev->netdev_ops->net_shaper_ops;
> > > > +	if (dev->netdev_ops)
> > > > +		ret |= !!dev->netdev_ops->net_shaper_ops;
> > > >  #endif  
> > > 
> > > This is a bit surprising, we pretty much never validate if dev has ops.
> > > 
> > > I think we're guaranteed that IFF_UP will not be set if we just
> > > allocated the device, so we can remove the locks in rtnl_create_link()
> > > and to double confirm add a netdev_ops_assert_locked_or_invisible() 
> > > in netif_state_change() ?  
> > 
> > Removing the lock from NEWLINK makes sense, but my concern
> > was NETDEV_CHANGE, which will requires more caution ?
> > 
> > commit 04efcee6ef8d0f01eef495db047e7216d6e6e38f
> > Author: Stanislav Fomichev <sdf@...ichev.me>
> > Date:   Fri Apr 4 09:11:22 2025 -0700
> > 
> >     net: hold instance lock during NETDEV_CHANGE
> 
> How could we fire a notifier for a device that hasn't been initialized,
> let alone registered?

Ah, now got it.  Exactly, we won't trigger NETDEV_CHANGE in the path,
rtnl_configure_link() is the place where IFF_UP is flagged later also
under the ops lock.


> 
> I'm hoping that the _or_invisible assert in my suggestion will flag to
> future developers trying to change netif_state_change() that the device
> here may not be fully constructed.

Totally agree now.