netdev - Re: [PATCH net-next 0/5] tls: implement key updates for TLS1.3

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Message-ID: <45c5fe42-29a8-5322-636b-e30da0f2ee18@novek.ru>
Date:   Fri, 20 Jan 2023 01:37:25 +0000
From:   Vadim Fedorenko <vfedorenko@...ek.ru>
To:     Jakub Kicinski <kuba@...nel.org>,
        Sabrina Dubroca <sd@...asysnail.net>
Cc:     netdev@...r.kernel.org, Frantisek Krenzelok <fkrenzel@...hat.com>,
        Gal Pressman <gal@...dia.com>
Subject: Re: [PATCH net-next 0/5] tls: implement key updates for TLS1.3

On 19.01.2023 02:55, 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.
> 
>>>>   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.

But we really don't have any HW offload implementation for TLSv1.3. But it would 
be great to have SW implementation better align to RFC.

>> Some folks did tests with and without kTLS using nbdcopy and found a
>> small but noticeable performance improvement (around 8-10%).