netdev - Re: [PATCH] net: procfs: Fix RCU stall and soft lockup in ptype_seq

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Message-ID: <CANn89i+pv6ECtn5zrPYgUfcKkoEMF1BABfxQb13yTP41QhcGvg@mail.gmail.com>
Date: Sat, 31 Jan 2026 18:50:10 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, fengwei_yin@...ux.alibaba.com, davem@...emloft.net, 
	pabeni@...hat.com, horms@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: procfs: Fix RCU stall and soft lockup in ptype_seq_next()

On Sat, Jan 31, 2026 at 6:41 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jakub Kicinski wrote:
> > On Wed, 28 Jan 2026 15:03:59 +0800 fengwei_yin@...ux.alibaba.com wrote:
> > > The root cause is in ptype_seq_next(): when iterating over packet
> > > types, it's possible that a packet type entry (pt) has been removed,
> > > its dev set to NULL, and pt->af_packet_net is not initialized.
> > > In that case, the function may return the same 'nxt' pointer indefinitely.
> > > This results in an infinite loop under RCU read-side critical section,
> > > causing an RCU stall and eventually a soft lockup.
> > >
> > > Fix the issue by properly handling the case where 'nxt' points to
> > > an empty list, ensuring forward progress in the iterator.
> >
> > > @@ -247,7 +247,7 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > >
> > >     if (pt->af_packet_net) {
> > >  net_ptype_all:
> > > -           if (nxt != &net->ptype_all && nxt != &net->ptype_specific)
> > > +           if (!list_empty(nxt) && nxt != &net->ptype_all && nxt != &net->ptype_specific)
> > >                     goto found;
> > >
> > >             if (nxt == &net->ptype_all) {
> > > @@ -267,6 +267,9 @@ static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > >                     return NULL;
> > >             nxt = ptype_base[hash].next;
> > >     }
> > > +
> > > +   if (list_empty(nxt))
> > > +           return NULL;
> > >  found:
> > >     return list_entry(nxt, struct packet_type, list);
> > >  }
> >
> > I'm not sure this fix works, TBH, we're dealing with an RCU list here.
> > The elements are not deleted with list_del_init(), so they won't
> > look "empty".
> >
> > If the pt entries are under RCU protection I think the issue is that
> > af_packet is clearing pt->dev before waiting for the grace period to
> > expire.
> >
> > Willem, is there a reason for that or just convenience?
>
> That would be wrong. Do we see it doing that somewhere?
>
> These handlers should get removed with dev_remove_pack. Or
> __dev_remove_pack and observe the RCU grace period some other way.
> I can review these, but was not aware of any abuses.
>

packet_notifier()

case NETDEV_DOWN:
if (dev->ifindex == po->ifindex) {
spin_lock(&po->bind_lock);
if (packet_sock_flag(po, PACKET_SOCK_RUNNING)) {
__unregister_prot_hook(sk, false);
/* removed without a synchronize_rcu() */
sk->sk_err = ENETDOWN;
if (!sock_flag(sk, SOCK_DEAD))
sk_error_report(sk);
}
if (msg == NETDEV_UNREGISTER) {
packet_cached_dev_reset(po);
WRITE_ONCE(po->ifindex, -1);
netdev_put(po->prot_hook.dev,
   &po->prot_hook.dev_tracker);
po->prot_hook.dev = NULL;                       // pointer set to NULL
}
spin_unlock(&po->bind_lock);
}
break;