Subject: Re: [PATCH net-next v4 1/6] tls: block decryption when a rekey is pending

From: Jakub Kicinski <[email-masked]>
Date: Tue, 3 Dec 2024 19:47:01 -0800

On Thu, 14 Nov 2024 16:50:48 +0100 Sabrina Dubroca wrote:
> +static int tls_check_pending_rekey(struct tls_context *ctx, struct sk_buff *skb)
> +{
> +	const struct tls_msg *tlm = tls_msg(skb);
> +	const struct strp_msg *rxm = strp_msg(skb);
> +	char hs_type;
> +	int err;
> +
> +	if (likely(tlm->control != TLS_RECORD_TYPE_HANDSHAKE))
> +		return 0;
> +
> +	if (rxm->full_len < 1)
> +		return -EINVAL;
> +
> +	err = skb_copy_bits(skb, rxm->offset, &hs_type, 1);
> +	if (err < 0)
> +		return err;
> +
> +	if (hs_type == TLS_HANDSHAKE_KEYUPDATE) {
> +		struct tls_sw_context_rx *rx_ctx = ctx->priv_ctx_rx;
> +
> +		WRITE_ONCE(rx_ctx->key_update_pending, true);
> +	}
> +
> +	return 0;
> +}
> +
>  static int tls_rx_one_record(struct sock *sk, struct msghdr *msg,
>  			     struct tls_decrypt_arg *darg)
>  {
> @@ -1739,6 +1769,10 @@ static int tls_rx_one_record(struct sock *sk, struct msghdr *msg,
>  	rxm->full_len -= prot->overhead_size;
>  	tls_advance_record_sn(sk, prot, &tls_ctx->rx);
>  
> +	err = tls_check_pending_rekey(tls_ctx, darg->skb);
> +	if (err < 0)
> +		return err;

Sorry if I already asked this, is this 100% safe to error out from here
after we decrypted the record? Normally once we successfully decrypted
and pulled the message header / trailer we always call tls_rx_rec_done()

The only reason the check_pending_rekey() can fail is if the message is
mis-formatted, I wonder if we are better off ignoring mis-formatted
rekeys? User space will see them and break the connection, anyway.
Alternatively - we could add a selftest for this.