Date:   Thu, 19 Jan 2023 16:40:39 +0100
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Frantisek Krenzelok <fkrenzel@...hat.com>,
        Gal Pressman <gal@...dia.com>,
        Apoorv Kothari <apoorvko@...zon.com>
Subject: Re: [PATCH net-next 0/5] tls: implement key updates for TLS1.3

2023-01-18, 18:55:22 -0800, Jakub Kicinski wrote:
> On Wed, 18 Jan 2023 11:06:25 +0100 Sabrina Dubroca wrote:
> > 2023-01-17, 18:03:51 -0800, Jakub Kicinski wrote:
> > > On Tue, 17 Jan 2023 14:45:26 +0100 Sabrina Dubroca wrote:  
> > > > This adds support for receiving KeyUpdate messages (RFC 8446, 4.6.3
> > > > [1]). A sender transmits a KeyUpdate message and then changes its TX
> > > > key. The receiver should react by updating its RX key before
> > > > processing the next message.
> > > > 
> > > > This patchset implements key updates by:
> > > >  1. pausing decryption when a KeyUpdate message is received, to avoid
> > > >     attempting to use the old key to decrypt a record encrypted with
> > > >     the new key
> > > >  2. returning -EKEYEXPIRED to syscalls that cannot receive the
> > > >     KeyUpdate message, until the rekey has been performed by userspace  
> > > 
> > > Why? We return to user space after hitting a cmsg, don't we?
> > > If the user space wants to keep reading with the old key - 🤷️  
> > 
> > But they won't be able to read anything. Either we don't pause
> > decryption, and the socket is just broken when we look at the next
> > record, or we pause, and there's nothing to read until the rekey is
> > done. I think that -EKEYEXPIRED is better than breaking the socket
> > just because a read snuck in between getting the cmsg and setting the
> > new key.
> 
> IDK, we don't interpret any other content types/cmsgs, and for well
> behaved user space there should be no problem (right?).
> I'm weakly against, if nobody agrees with me you can keep as is.

I was concerned (I don't know if it's realistic) about a userspace
application with two threads:


  Thread A            Thread B
  --------            --------

  read cmsg

                      read some data (still on the old key)

  sets the new key


I guess one could claim that's a userspace bug.

František's implementation in gnutls doesn't seem to rely on this.

Apoorv, since you were also looking into key updates, do you have an
opinion on pausing decryption/reads until userspace has provides the
new key?

> > > >  3. passing the KeyUpdate message to userspace as a control message
> > > >  4. allowing updates of the crypto_info via the TLS_TX/TLS_RX
> > > >     setsockopts
> > > > 
> > > > This API has been tested with gnutls to make sure that it allows
> > > > userspace libraries to implement key updates [2]. Thanks to Frantisek
> > > > Krenzelok <fkrenzel@...hat.com> for providing the implementation in
> > > > gnutls and testing the kernel patches.  
> > > 
> > > Please explain why - the kernel TLS is not faster than user space, 
> > > the point of it is primarily to enable offload. And you don't add
> > > offload support here.  
> > 
> > Well, TLS1.3 support was added 4 years ago, and yet the offload still
> > doesn't support 1.3 at all.
> 
> I'm pretty sure some devices support it. None of the vendors could 
> be bothered to plumb in the kernel support, yet, tho.
> I don't know of anyone supporting rekeying.
>
> > IIRC support for KeyUpdates is mandatory in TLS1.3, so currently the
> > kernel can't claim to support 1.3, independent of offloading.
> 
> The problem is that we will not be able to rekey offloaded connections.
> For Tx it's a non-trivial problem given the current architecture.
> The offload is supposed to be transparent, we can't fail the rekey just
> because the TLS gotten offloaded.

What's their plan when the peer sends a KeyUpdate request then? Let
the connection break?

-- 
Sabrina

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