Re: [PATCH v3] net/tls: support maximum record size limit

[Sabrina Dubroca <sd@queasysnail.net>]

2025-09-03, 11:47:57 +1000, Wilfred Mallawa wrote:
> +static int do_tls_setsockopt_tx_record_size(struct sock *sk, sockptr_t optval,
> +					    unsigned int optlen)
> +{
> +	struct tls_context *ctx = tls_get_ctx(sk);
> +	u16 value;
> +
> +	if (sockptr_is_null(optval) || optlen != sizeof(value))
> +		return -EINVAL;
> +
> +	if (copy_from_sockptr(&value, optval, sizeof(value)))
> +		return -EFAULT;
> +
> +	if (ctx->prot_info.version == TLS_1_2_VERSION &&
> +	    value > TLS_MAX_PAYLOAD_SIZE)
> +		return -EINVAL;
> +
> +	if (ctx->prot_info.version == TLS_1_3_VERSION &&
> +	    value > TLS_MAX_PAYLOAD_SIZE + 1)
> +		return -EINVAL;

The RFC is not very explicit about this, but I think this +1 for
TLS1.3 is to allow an actual payload of TLS_MAX_PAYLOAD_SIZE and save
1B of room for the content_type that gets appended.

   This value is the length of the plaintext of a protected record.  The
   value includes the content type and padding added in TLS 1.3 (that
   is, the complete length of TLSInnerPlaintext).

AFAIU we don't actually want to stuff TLS_MAX_PAYLOAD_SIZE+1 bytes of
payload into a record.

If we set tx_record_size_limit to TLS_MAX_PAYLOAD_SIZE+1, we'll end up
sending a record with a plaintext of TLS_MAX_PAYLOAD_SIZE+2 bytes
(TLS_MAX_PAYLOAD_SIZE+1 of payload, then 1B of content_type), and a
"normal" implementation will reject the record since it's too big
(ktls does that in net/tls/tls_sw.c:tls_rx_msg_size).

So we should subtract 1 from the userspace-provided value for 1.3, and
then add it back in getsockopt/tls_get_info.

Or maybe userspace should provide the desired payload limit, instead
of the raw record_size_limit it got from the extension (ie, do -1 when
needed before calling the setsockopt). Then we should rename this
"tx_payload_size_limit" (and adjust the docs) to make it clear it's
not the raw record_size_limit.

The "tx_payload_size_limit" approach is maybe a little bit simpler
(not having to add/subtract 1 in a few places - I think userspace
would only have to do it in one place).


Wilfred, Jakub, what do you think?


> +	ctx->tx_record_size_limit = value;
> +
> +	return 0;
> +}

-- 
Sabrina