netdev - Re: [PATCH] tls: add zerocopy device sendpage

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Date:   Mon, 13 Jul 2020 15:59:06 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Boris Pismenny <borisp@...lanox.com>
Cc:     David Miller <davem@...emloft.net>, john.fastabend@...il.com,
        daniel@...earbox.net, tariqt@...lanox.com, netdev@...r.kernel.org
Subject: Re: [PATCH] tls: add zerocopy device sendpage

On Tue, 14 Jul 2020 01:15:26 +0300 Boris Pismenny wrote:
> On 13/07/2020 22:05, David Miller wrote:
> > The TLS signatures are supposed to be even stronger than the protocol
> > checksum, and therefore we should send out valid ones rather than
> > incorrect ones.  
> 
> Right, but one is on packet payload, while the other is part of the payload.
> 
> > Why can't the device generate the correct TLS signature when
> > offloading?  Just like for the protocol checksum, the device should
> > load the payload into the device over DMA and make it's calculations
> > on that copy.  
> 
> Right. The problematic case is when some part of the record is already
> received by the other party, and then some (modified) data including
> the TLS authentication tag is re-transmitted.
> The modified tag is calculated over the new data, while the other party
> will use the already received old data, resulting in authentication error.
> 
> > For SW kTLS, we must copy.  Potentially sending out garbage signatures
> > in a packet cannot be an "option".  
> 
> Obviously, SW kTLS must encrypt the data into a different kernel buffer,
> which is the same as copying for that matter. TLS_DEVICE doesn't require this.

This proposal is one big attrition of requirements, which I personally
dislike quite a bit. Nothing material has changed since the first
version of the code was upstreamed, let's ask ourselves - why was the
knob not part of the initial submission?