Subject: Re: [PATCH net-next v5 12/25] ovpn: implement packet processing

From: Sabrina Dubroca <[email-masked]>
Date: Tue, 9 Jul 2024 10:51:33 +0200

2024-06-27, 15:08:30 +0200, Antonio Quartulli wrote:
> +/* removes the primary key from the crypto context */
> +void ovpn_crypto_kill_primary(struct ovpn_crypto_state *cs)
> +{
> +	struct ovpn_crypto_key_slot *ks;
> +
> +	mutex_lock(&cs->mutex);
> +	ks = rcu_replace_pointer(cs->primary, NULL,
> +				 lockdep_is_held(&cs->mutex));

Should there be a check that we're killing the key that has expired
and not some other key?  I'm wondering if this could happen:

ovpn_encrypt_one
    ovpn_aead_encrypt
        ovpn_pktid_xmit_next
            seq_num reaches threshold
            returns -ERANGE
        returns -ERANGE

                                            ovpn_crypto_key_slots_swap
                                                replaces cs->primary with cs->secondary

    ovpn_encrypt_post
        ret = -ERANGE
        ovpn_crypto_kill_primary
            kills the freshly installed primary key

> +	ovpn_crypto_key_slot_put(ks);
> +	mutex_unlock(&cs->mutex);
> +}
> +

[...]
> +static void ovpn_aead_encrypt_done(void *data, int ret)
> +{
> +	struct sk_buff *skb = data;
> +
> +	aead_request_free(ovpn_skb_cb(skb)->req);
> +	ovpn_encrypt_post(skb, ret);
> +}
> +
> +int ovpn_aead_encrypt(struct ovpn_crypto_key_slot *ks, struct sk_buff *skb,
> +		      u32 peer_id)
> +{
> +	const unsigned int tag_size = crypto_aead_authsize(ks->encrypt);
> +	const unsigned int head_size = ovpn_aead_encap_overhead(ks);
> +	struct scatterlist sg[MAX_SKB_FRAGS + 2];
> +	DECLARE_CRYPTO_WAIT(wait);

unused? (also in _decrypt)

[...]
> +
> +	req = aead_request_alloc(ks->encrypt, GFP_ATOMIC);
> +	if (unlikely(!req))
> +		return -ENOMEM;
> +
> +	/* setup async crypto operation */
> +	aead_request_set_tfm(req, ks->encrypt);
> +	aead_request_set_callback(req, 0, ovpn_aead_encrypt_done, NULL);

NULL? That should be skb, ovpn_aead_encrypt_done needs it (same for
decrypt).

I suspect you haven't triggered the async path in testing. For that,
you can use crconf:

git clone https://git.code.sf.net/p/crconf/code
cd code && make
./src/crconf add driver 'pcrypt(generic-gcm-aesni)' type 3  priority 10000

Then all packets encrypted with gcm(aes) should go through the async
code.

> +	aead_request_set_crypt(req, sg, sg, skb->len - head_size, iv);
> +	aead_request_set_ad(req, OVPN_OP_SIZE_V2 + NONCE_WIRE_SIZE);
> +
> +	ovpn_skb_cb(skb)->req = req;
> +	ovpn_skb_cb(skb)->ks = ks;
> +
> +	/* encrypt it */
> +	return crypto_aead_encrypt(req);
> +}

[...]
> @@ -77,14 +133,45 @@ static void ovpn_decrypt_post(struct sk_buff *skb, int ret)
>  /* pick next packet from RX queue, decrypt and forward it to the device */
>  void ovpn_recv(struct ovpn_peer *peer, struct sk_buff *skb)
>  {
> +	struct ovpn_crypto_key_slot *ks;
> +	u8 key_id;
> +
> +	/* get the key slot matching the key ID in the received packet */
> +	key_id = ovpn_key_id_from_skb(skb);
> +	ks = ovpn_crypto_key_id_to_slot(&peer->crypto, key_id);

This takes a reference on the keyslot (ovpn_crypto_key_slot_hold), but
I don't see it getting released in ovpn_decrypt_post. In
ovpn_encrypt_post you're adding a ovpn_crypto_key_slot_put (to match
ovpn_crypto_key_slot_primary), but nothing equivalent in
ovpn_decrypt_post?

> +	if (unlikely(!ks)) {
> +		net_info_ratelimited("%s: no available key for peer %u, key-id: %u\n",
> +				     peer->ovpn->dev->name, peer->id, key_id);
> +		dev_core_stats_rx_dropped_inc(peer->ovpn->dev);
> +		kfree_skb(skb);
> +		return;
> +	}
> +
>  	ovpn_skb_cb(skb)->peer = peer;
> -	ovpn_decrypt_post(skb, 0);
> +	ovpn_decrypt_post(skb, ovpn_aead_decrypt(ks, skb));
>  }

-- 
Sabrina