linux-kernel - Re: [PATCH] net/tls: allow limiting maximum record size

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Message-ID: <76d986e3f304dc199826228e90ad3f68160ba8f3.camel@gmail.com>
Date: Tue, 02 Sep 2025 13:30:49 +1000
From: Wilfred Mallawa <wilfred.opensource@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: chuck.lever@...cle.com, kernel-tls-handshake@...ts.linux.dev, 
	donald.hunter@...il.com, edumazet@...gle.com, horms@...nel.org,
 hare@...nel.org, 	john.fastabend@...il.com, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, 	pabeni@...hat.com, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH] net/tls: allow limiting maximum record size

On Mon, 2025-09-01 at 11:39 -0700, Jakub Kicinski wrote:
> On Mon,  1 Sep 2025 15:36:19 +1000 Wilfred Mallawa wrote:
> > During a handshake, an endpoint may specify a maximum record size
> > limit.
> > Currently, the kernel defaults to TLS_MAX_PAYLOAD_SIZE (16KB) for
> > the
> > maximum record size. Meaning that, the outgoing records from the
> > kernel
> > can exceed a lower size negotiated during the handshake. In such a
> > case,
> > the TLS endpoint must send a fatal "record_overflow" alert [1], and
> > thus the record is discarded.
> > 
> > Upcoming Western Digital NVMe-TCP hardware controllers implement
> > TLS
> > support. For these devices, supporting TLS record size negotiation
> > is
> > necessary because the maximum TLS record size supported by the
> > controller
> > is less than the default 16KB currently used by the kernel.
> > 
> > This patch adds support for retrieving the negotiated record size
> > limit
> > during a handshake, and enforcing it at the TLS layer such that
> > outgoing
> > records are no larger than the size negotiated. This patch depends
> > on
> > the respective userspace support in tlshd [2] and GnuTLS [3].
> 
> I don't get why you are putting this in the handshake handling code.
> Add a TLS setsockopt, why any TLS socket can use, whether the socket 
> is opened by the kernel or user. GnuTLS can call it directly before 
> it returns the socket to kernel ownership.
> 
> I feel like I already commented to this effect. If you don't
> understand
> comments from the maintainers - ask for clarifications.

Hey Jakub,

I don't think I saw your email, apologies! But this makes sense, I have
drafted a V2 using setsockopt(). I will send it out soon. Thanks for
the feedback!

Regards,
Wilfred