lwt: Fix return values of BPF xmit ops

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: lwt: Fix return values of BPF xmit ops BPF encap ops can return different types of positive values, such like NET_RX_DROP, NET_XMIT_CN, NETDEV_TX_BUSY, and so on, from function skb_do_redirect and bpf_lwt_xmit_reroute. At the xmit hook, such return values would be treated implicitly as LWTUNNEL_XMIT_CONTINUE in ip(6)_finish_output2. When this happens, skbs that have been freed would continue to the neighbor subsystem, causing use-after-free bug and kernel crashes. To fix the incorrect behavior, skb_do_redirect return values can be simply discarded, the same as tc-egress behavior. On the other hand, bpf_lwt_xmit_reroute returns useful errors to local senders, e.g. PMTU information. Thus convert its return values to avoid the conflict with LWTUNNEL_XMIT_CONTINUE.

References

• https://git.kernel.org/stable/c/67f8f2bae8e7ac72e09def2b667e44704c4d1ee1
• https://git.kernel.org/stable/c/a97f221651fcdc891166e9bc270e3d9bfa5a0080
• https://git.kernel.org/stable/c/e3f647e4b642f9f6d32795a16f92c116c138d2af
• https://git.kernel.org/stable/c/065d5f17096ec9161180e2c890afdff4dc6125f2
• https://git.kernel.org/stable/c/d68c17402442f5f494a2c3ebde5cb82f6aa9160a
• https://git.kernel.org/stable/c/65583f9e070db7bece20710cfa2e3daeb0b831d9
• https://git.kernel.org/stable/c/29b22badb7a84b783e3a4fffca16f7768fb31205