netdev - Re: [PATCH RFC v2 00/12] socket sendmsg MSG

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Date:   Tue, 28 Feb 2017 17:50:07 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, Feb 28, 2017 at 4:58 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Tue, Feb 28, 2017 at 7:28 PM, Tom Herbert <tom@...bertland.com> wrote:
>> On Tue, Feb 28, 2017 at 3:22 PM, Eric Dumazet <eric.dumazet@...il.com> 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