CVE-2024-35839

In the Linux kernel, the following vulnerability has been resolved:

netfilter: bridge: replace physindev with physinif in nf_bridge_info

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.

As skb->dev can be reset back to nf_bridge->physindev and used, and as there is no explicit mechanism that prevents this physindev from been freed under us (for instance neigh_flush_dev doesn't cleanup skbs from different device's neigh queue) we can 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

Let's use plain ifindex instead of net_device link. To peek into the original net_device we will use dev_get_by_index_rcu(). Thus either we get device and are safe to use it or we don't get it and drop skb.