Re: [PATCH v3 net] net: add proper RCU protection to /proc/net/ptype

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Eric Dumazet wrote:
> 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")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> 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>

Thanks for the fix Eric! This one is super tricky.