Re: [PATCH net-next v1 08/15] psp: only give time-wait sockets of psp connections a validator

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- [High] psp_twsk_init() (net/psp/psp_sock.c) now sets
  tw->tw_validate_xmit_skb = psp_validate_xmit whenever the full socket…

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psp: only give time-wait sockets of psp connections a validator

This commit changes psp_twsk_init() so that tw_validate_xmit_skb is set
only when the full socket has a psp association, not on every tcp
time-wait socket. It prepares for the upcoming tc psp action, which
treats a socket that has a validator as having its own encryption
offload.

> diff --git a/net/psp/psp_sock.c b/net/psp/psp_sock.c
> index 609b06c7af65b..664955bfb4fdb 100644
> --- a/net/psp/psp_sock.c
> +++ b/net/psp/psp_sock.c
> @@ -330,10 +330,14 @@ void psp_twsk_init(struct inet_timewait_sock *tw, const struct sock *sk)
>  {
>  	struct psp_assoc *pas = psp_sk_assoc(sk);
>  
> -	if (pas)
> +	if (pas) {
>  		refcount_inc(&pas->refcnt);
> +		/* Only for PSP connections: a validator makes the socket
> +		 * look like it has an encryption offload of its own
> +		 */
> +		tw->tw_validate_xmit_skb = psp_validate_xmit;

[Severity: High]
Should this also check pas->tx.spi? As written, an Rx-only association
(pas->tx.spi == 0) also gets the validator here.

On a full socket, the validator is installed only after a Tx key is set,
in psp_sock_set_tx_key():

    WRITE_ONCE(sk->sk_validate_xmit_skb, psp_validate_xmit);

psp_sock_assoc_set_rx() publishes sk->psp_assoc but never sets
sk->sk_validate_xmit_skb. A connection that did rx-assoc but never
tx-assoc therefore has no validator while it is a full socket. It gains
one when it enters TIME_WAIT.

Other places that decide whether a connection is PSP-encrypted test
pas && pas->tx.spi. These include psp_reply_set_decrypted() in this same
file:

    if (pas && pas->tx.spi)
        skb->decrypted = 1;

and also psp_enqueue_set_decrypted() and psp_sk_overhead() in
include/net/psp/functions.h.

With the act_psp patch later in the series, tcf_psp_sk_offload() returns
true for a TIME_WAIT socket that has a tw_validate_xmit_skb:

    return sk_is_inet(sk) && sk->sk_state == TCP_TIME_WAIT &&
           inet_twsk(sk)->tw_validate_xmit_skb;

tcf_psp_act() then drops the packet with SKB_DROP_REASON_PSP_OUTPUT. In
addition, sk_validate_xmit_skb() in net/core/dev.c drops skbs carrying a
PSP metadata dst when the time-wait socket has a validator.

TIME_WAIT replies get skb->sk = tw through skb_set_owner_edemux() from
ip_send_unicast_reply() and tcp_v6_send_response(). The resulting
sequence looks like this:

  psp_sock_assoc_set_rx()       sk->psp_assoc set, tx.spi == 0, no validator
  connection closes
  psp_twsk_init()               tw_validate_xmit_skb set since pas != NULL
  TW ACK/RST reply, skb->sk = tw
  tcf_psp_act() ENCAP
    tcf_psp_sk_offload()        true -> TC_ACT_SHOT

So an egress psp encap rule that covers such a connection encapsulates
its packets while the socket is full. Once the socket is in TIME_WAIT,
the same rule drops its ACK and RST replies. Is this the inconsistency
the commit message describes? It seems to remain for Rx-only
associations.

An Rx-only socket has no Tx offload of its own.
Documentation/networking/psp.rst lists Tx as plain text in the Rx PSP
state.

Would gating on pas && pas->tx.spi, as psp_reply_set_decrypted() does,
match the full-socket rule? Copying the validator only when
sk->sk_validate_xmit_skb == psp_validate_xmit would be another option.

> +	}
>  	rcu_assign_pointer(tw->psp_assoc, pas);
> -	tw->tw_validate_xmit_skb = psp_validate_xmit;
>  }

[ ... ]

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Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261111005533.6436-1-toddfreed%40meta.com