Subject: Re: [PATCH] neighbour: purge nf_bridged skb from foreign device neigh

From: Florian Westphal <[email-masked]>
Date: Mon, 8 Jan 2024 12:15:04 +0100

Pavel Tikhomirov <[email-masked]> wrote:
> An skb can be added to a neigh->arp_queue while waiting for an arp
> reply. Where original skb's skb->dev can be different to neigh's
> neigh->dev. For instance in case of bridging dnated skb from one veth to
> another, the skb would be added to a neigh->arp_queue of the bridge.
> 
> There is no explicit mechanism that prevents the original skb->dev link
> of such skb from being freed under us. For instance neigh_flush_dev does
> not cleanup skbs from different device's neigh queue. But that original
> link can be used and lead to crash on e.g. this stack:
> 
> arp_process
>   neigh_update
>     skb = __skb_dequeue(&neigh->arp_queue)
>       neigh_resolve_output(..., skb)
>         ...
>           br_nf_dev_xmit
>             br_nf_pre_routing_finish_bridge_slow
>               skb->dev = nf_bridge->physindev
>               br_handle_frame_finish
> 
> So let's improve neigh_flush_dev to also purge skbs when device
> equal to their skb->nf_bridge->physindev gets destroyed.

Can we fix this by replacing physindev pointer with plain
ifindex instead?  There are not too many places that need to
peek into the original net_device struct, so I don't think
the additional dev_get_by_index_rcu() would be an issue.