bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap
In the Linux kernel, the following vulnerability has been resolved: bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap The bpf_lwt_xmit_push_encap helper needs to access skb_dst(skb)->dev to calculate the needed headroom: err = skb_cow_head(skb, len + LL_RESERVED_SPACE(skb_dst(skb)->dev)); But skb->_skb_refdst may not be initialized when the skb is set up by bpf_prog_test_run_skb function. Executing bpf_lwt_push_ip_encap function in this scenario will trigger null pointer dereference, causing a kernel crash as Yinhao reported: [ 105.186365] BUG: kernel NULL pointer dereference, address: 0000000000000