Re: [PATCH RFC net-next v2 0/5] tls: perform TLS 1.3 KeyUpdate in the kernel

[Martin Belanger <martin.nvme.dev@gmail.com>]

On 10/28/26 10:42, Hannes Reinecke wrote:
> On 10/28/26 3:21 PM, Chuck Lever wrote:
>>
>>
>> On Wed, Oct 28, 2026, at 9:52 AM, Martin Belanger via B4 Relay wrote:
>>> kTLS does not currently handle TLS 1.3 KeyUpdate. It never sends a
>>> KeyUpdate. And when it receives one, it blocks the socket until the
>>> application installs the next key. kTLS also does not enforce the key
>>> usage limit on transmit.
>>
>> No argument here.
>>
>>
>>> This applies to TLS libraries in user space as well as kernel users such
>>> as NVMe/TCP or sunrpc:
>>>
>>> - GnuTLS sends a KeyUpdate after 2^24 records when it runs its own
>>>    record layer. With kTLS, the record layer is handled in the kernel 
>>> and
>>>    GnuTLS therefore never runs the check. OpenSSL never sends a 
>>> KeyUpdate
>>>    on its own.
>>> - NVMe/TCP and sunrpc don't have a mechanism to install a new key. A
>>>    conforming peer that sends a KeyUpdate stalls their connection.
>>
>> The design intention of these consumers is that they are supposed to 
>> break
>> the connection and create a new session, rather than stall. If that is 
>> not
>> happening, that's a bug. KeyUpdate is complicated enough that backporting
>> to LTS is not really an option, so that bug needs to be addressed first,
>> IMHO.
>>
> Yes. But Martin send already a patchset for that (well, for NVMe, at 
> least).

Thanks, Hannes. Two points to add.

A reconnect is also a valid answer to the record limit: RFC 9846
section 5.5 allows closing the connection. The cost is the problem.
Depending on the load and link speed, the 2^24 limit can be reached in
less than 10 minutes in many cases. Each reconnect stops the I/O of the
queue and needs a new handshake upcall, for every queue of every
controller. A peer that sends KeyUpdate on its own causes the same
reconnects. With this series, the connection stays up.

> 
>> The argument that was made that put tlshd in user space is that we do
>> not want the kernel to handle something that the libraries are already
>> good at (IOW, mechanisms that have been soaked in commodity usage by many
>> consumers).
>>
>> Is that rationale different for handling TLS KeyUpdate? I see that the
>> series adds back the HKDF functionality to the kernel. That seems like
>> an indication that KeyUpdate should be handled via an upcall.
>>

The series does not add back the HKDF library. It computes the HMACs
inline with the HMAC library, as the NVMe PSK derivation in
drivers/nvme/common/auth.c does. That is why commit 6d888db2cfd9
removed the library. The QUIC series also derives its key updates in
the kernel.

GnuTLS does handle a received KeyUpdate with kTLS since kernel 6.14,
but only when user space keeps the GnuTLS session. tlshd does not: it
hands the socket to the kernel after the handshake. So the library
cannot help nvme-tcp or sunrpc here.

Martin.

> Yes. TLS KeyUpdate requires that all records _after_ the KeyUpdate 
> message are encrypted with the next key; idea is here that one can
> (and should) do this in-line without stopping/resetting the connection.
> _And_ TLS KeyUpdate is done independently on the RX and the TX side.
> 
> This makes it extremely hard to implement via tlshd; we would need
> to pass the KeyUpdate message to tlshd (which is easy), but then
> we would need to stop processing incoming packets until we get
> a response from tlshd indicating that it has installed the new
> secret.
> 
> Which makes the whole idea of KeyUpdate (namely that you can
> do it _without_ stopping packet processing) a bit pointless.
> 
> And I haven't even mentioned that you would need to be able to
> stop RX and TX _independently_, which I'm not sure the various
> ULPs can do.
> 
> Plus there is the question why ULPs need to be burdened with
> protocol specific knowledge; why would I need to code hooks/
> handling for an extremely protocol-specific know?
> It's far better be handled by the protocol itself.
> 
>> Are there policy decisions that need to be made during KeyUpdate (like,
>> has the peer's certificate expired, or has it been put on a revocation
>> list in the meantime) ?
>>
> No. This is not about the certificate, it's about traffic secrets.
> If a certificate is expired you need to reset the connection
> to install a new certificate; TLS 1.3 does not allow you to do
> that while the connection is established.
> And handling of expired certificates is an admin choice, not
> something one should do automatically.
> 
> (The last thing you want is your root fs becoming inaccessible
> just because a lazy admin forgot to renew the certificate ...)
> 
> Cheers,
> 
> Hannes