linux-kernel - Re: [syzbot] [bpf?] [net?] possible deadlock in xsk

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Message-ID: <20250625140357.6203d0af@kernel.org>
Date: Wed, 25 Jun 2025 14:03:57 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Jason Xing <kerneljasonxing@...il.com>, syzbot
 <syzbot+e67ea9c235b13b4f0020@...kaller.appspotmail.com>, andrii@...nel.org,
 ast@...nel.org, bjorn@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, davem@...emloft.net, edumazet@...gle.com,
 horms@...nel.org, jonathan.lemon@...il.com, linux-kernel@...r.kernel.org,
 maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
 netdev@...r.kernel.org, pabeni@...hat.com, sdf@...ichev.me,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bpf?] [net?] possible deadlock in xsk_notifier (3)

On Wed, 25 Jun 2025 13:48:03 -0700 Stanislav Fomichev wrote:
> > > I'm still learning the af_xdp. Sure, I'm interested in it, just a bit
> > > worried if I'm capable of completing it. I will try then.  
> > 
> > SG, thanks! If you need more details lmk, but basically we need to reorder
> > netdev_lock_ops() and mutex_lock(lock: &xs->mutex)+XSK_READY check.
> > And similarly for cleanup (out_unlock/out_release) path.  
> 
> Jakub just told me that I'm wrong and it looks similar to commit
> f0433eea4688 ("net: don't mix device locking in dev_close_many()
> calls"). So this is not as easy as flipping the lock ordering :-(

I don't think registering a netdev from NETDEV_UP even of another
netdev is going to play way with instance locks and lockdep.
This is likely a false positive but if syzbot keeps complaining
we could:

diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index 995a7207bdf8..f357a7ac70ac 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -81,7 +81,7 @@ static struct lapbethdev *lapbeth_get_x25_dev(struct net_device *dev)
 
 static __inline__ int dev_is_ethdev(struct net_device *dev)
 {
-       return dev->type == ARPHRD_ETHER && strncmp(dev->name, "dummy", 5);
+       return dev->type == ARPHRD_ETHER && !netdev_need_ops_lock(dev);
 }
 
IDK what the dummy hack is there for, it's been like that since 
git begun..