Messages in this thread | | | Date | Fri, 14 Oct 2022 09:55:40 -0700 | Subject | Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len | From | sdf@google ... |
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On 10/14, Lorenz Bauer wrote: > On Thu, 13 Oct 2022, at 11:44, shaozhengchao wrote: > > Sorry, I haven't fully understood your intentions yet. > > Can you explain it more detail?
> I'll try! Roughly, we do the following:
> 1. Create a BPF_PROG_TYPE_SOCKET_FILTER program that just returns 0 > 2. Load the program into the kernel > 3. Call BPF_PROG_RUN with data_size_in == 14
> After your bugfix, it seems like step 3 is rejected due to data_size_in > == 14. We had to increase data_size_in to 15 to > avoid this, see [0].
> This breaks user space, so it would be great if you could fix this in a > way that doesn't refuse BPF_PROG_RUN with
[..]
> data_size_in == 14. Since I don't understand the original problem very > well I can't tell you what the best fix is however.
The problem was that we were able to generate skb with len=0 via BPF_PROG_RUN. Prohibiting those cases breaks backwards compatibility, so we either have to:
a) (preferred?) accept inputs with <14, but maybe internally pad to 14 bytes to make the core stack happy b) revert the patch and instead have length checks at runtime; doesn't seem to be worth the penalty in the forwarding path because of some corner cases like these ?
> 0: > https://github.com/cilium/ebpf/commit/a38fb6b5a46ab3b5639ea4d421232a10013596c0
> Thanks > Lorenz
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