On 10/31/16 at 09:07am, Tom Herbert wrote: > I guess this leads to a more general question I have about the effects > of allowing userspace to insert code in the kernel that modifies > packets. If we allow BPF programs to arbitrarily modify packets in > LWT, how do we ensure that there are no insidious effects later in the > path? For instance, what someone uses BPF to convert an IPv6 packet > to IPv4, or maybe convert packet to something that isn't even IP, or > what if someone just decides to overwrite every byte in a packet with > 0xff?
This is why modifying packets is not allowed on input at all as it would invalidate the IP parsing that has already been done. Writing is allowed for dst_output() on the basis that it is the equivalent of a raw socket with header inclusion. If you look at rawv6_send_hdrinc(), it does not perform any validation and calls into dst_output() directly. I agree though that this must be made water proof. Pushing additional headers is only allowed at xmit, this is the equivalent LWT MPLS. > Are these thing allowed, and if so what is the effect? I would > assume a policy that these can't cause any insidious effects to > unrelated traffic or the rest of the system, in particular such things > should not cause the kernel to crash (based on the principle that > user space code should never cause kernel to crash). I think XDP might Agreed. Although it's already possible to hook a kernel module at LWT or Netfilter to do arbitrary packet modifications, BPF must be held at a higher standard even in privileged mode.
