CVE-2024-35839: netfilter: bridge: replace physindev with physinif in nf_bridge_info

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

netfilter: bridge: replace physindev with physinif in nfbridgeinfo

An skb can be added to a neigh->arpqueue 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->arpqueue of the bridge.

As skb->dev can be reset back to nfbridge->physindev and used, and as there is no explicit mechanism that prevents this physindev from been freed under us (for instance neighflushdev doesn't cleanup skbs from different device's neigh queue) we can crash on e.g. this stack:

arpprocess neighupdate skb = skbdequeue(&neigh->arpqueue) neighresolveoutput(..., skb) ... brnfdevxmit brnfpreroutingfinishbridgeslow skb->dev = nfbridge->physindev brhandleframefinish

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