On Tue, Feb 28, 2017 at 4:58 PM, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote: > On Tue, Feb 28, 2017 at 7:28 PM, Tom Herbert <tom@xxxxxxxxxxxxxxx> wrote: >> On Tue, Feb 28, 2017 at 3:22 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: >>> On Tue, 2017-02-28 at 14:52 -0800, Andy Lutomirski wrote: >>> >>>> The user pages are a gift to the kernel. The application may not >>>> modify this memory ever, otherwise the page cache and on-disk data may >>>> differ. >>>> >>>> This is just not okay IMO. >>> >>> TCP works just fine in this case. >>> >>> TX checksum will be computed by the NIC after/while data is copied. >>> >>> If really the application changes the data, that will not cause any >>> problems, other than user side consistency. >>> >>> This is why we require a copy (for all buffers that came from zero-copy) >>> if network stack hits a device that can not offload TX checksum. >>> >>> Even pwrite() does not guarantee consistency if multiple threads are >>> using it on overlapping regions. >>> >> The Mellanox team working on TLS offload pointed out to us that if >> data is changed for a retransmit then it becomes trivial for someone >> snooping to break the encryption. Sounds pretty scary and it would be >> a shame if we couldn't use zero-copy in that use case :-( Hopefully we >> can find a solution... >> > > This requires collusion by the process initiating the zerocopy send > to help the entity snooping the link. That could be an attack on admin > configured tunnels, but user-directed encryption offload like AF_TLS > can still use zerocopy. Yes, but we can't trust the user to always understand or correctly implement the semantic nuances when security is involved. If we can't provide a robust API then the only recourse is to not allow zero copy in that case. We could suggest COW to solve all problems, but I think we know where the conversation will go ;-) Tom -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
