net: add proper RCU protection to /proc/net/ptype

net: add proper RCU protection to /proc/net/ptype

Yin Fengwei reported an RCU stall in ptype_seq_show() and provided
a patch.

Real issue is that ptype_seq_next() and ptype_seq_show() violate
RCU rules.

ptype_seq_show() runs under rcu_read_lock(), and reads pt->dev
to get device name without any barrier.

At the same time, concurrent writers can remove a packet_type structure
(which is correctly freed after an RCU grace period) and clear pt->dev
without an RCU grace period.

Define ptype_iter_state to carry a dev pointer along seq_net_private:

struct ptype_iter_state {
	struct seq_net_private	p;
	struct net_device	*dev; // added in this patch
};

We need to record the device pointer in ptype_get_idx() and
ptype_seq_next() so that ptype_seq_show() is safe against
concurrent pt->dev changes.

We also need to add full RCU protection in ptype_seq_next().
(Missing READ_ONCE() when reading list.next values)

Many thanks to Dong Chenchen for providing a repro.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 1d10f8a1f40b ("net-procfs: show net devices bound packet types")
Fixes: c353e8983e0d ("net: introduce per netns packet chains")
Reported-by: Yin Fengwei <fengwei_yin@linux.alibaba.com>
Reported-by: Dong Chenchen <dongchenchen2@huawei.com>
Closes: https://lore.kernel.org/netdev/CANn89iKRRKPnWjJmb-_3a=sq+9h6DvTQM4DBZHT5ZRGPMzQaiA@mail.gmail.com/T/#m7b80b9fc9b9267f90e0b7aad557595f686f9c50d
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Tested-by: Yin Fengwei <fengwei_yin@linux.alibaba.com>
Link: https://patch.msgid.link/20260202205217.2881198-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>