bpf: Don't redirect packets with invalid pkt_len

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: bpf: Don't redirect packets with invalid pkt_len Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any skbs, that is, the flow->head is null. The root cause, as the [2] says, is because that bpf_prog_test_run_skb() run a bpf prog which redirects empty skbs. So we should determine whether the length of the packet modified by bpf prog or others like bpf_prog_test is valid before forwarding it directly.

References

• https://git.kernel.org/stable/c/8b68e53d56697a59b5c53893b53f508bbdf272a0
• https://git.kernel.org/stable/c/6204bf78b2a903b96ba43afff6abc0b04d6e0462
• https://git.kernel.org/stable/c/a75987714bd2d8e59840667a28e15c1fa5c47554
• https://git.kernel.org/stable/c/72f2dc8993f10262092745a88cb2dd0fef094f23
• https://git.kernel.org/stable/c/fd1894224407c484f652ad456e1ce423e89bb3eb