Subject: Re: [PATCH net-next v3 1/8] netdevsim: a basic test PSP implementation

From: Willem de Bruijn <[email-masked]>
Date: Sun, 28 Sep 2025 12:02:43 -0400

Jakub Kicinski wrote:
> Provide a PSP implementation for netdevsim.
> 
> Use psp_dev_encapsulate() and psp_dev_rcv() to do actual encapsulation
> and decapsulation on skbs, but perform no encryption or decryption. In
> order to make encryption with a bad key result in a drop on the peer's
> rx side, we stash our psd's generation number in the first byte of each
> key before handing to the peer.
> 
> Signed-off-by: Jakub Kicinski <[email-masked]>
> Co-developed-by: Daniel Zahka <[email-masked]>
> Signed-off-by: Daniel Zahka <[email-masked]>
> ---
> v3:
>  - fix init error path
>  - fix unused variable with ipv6=n (__maybe_unused, all options seem
>    equally bad here)
> ---
>  drivers/net/netdevsim/Makefile    |   4 +
>  drivers/net/netdevsim/netdevsim.h |  27 ++++
>  drivers/net/netdevsim/netdev.c    |  43 +++++-
>  drivers/net/netdevsim/psp.c       | 225 ++++++++++++++++++++++++++++++
>  net/core/skbuff.c                 |   1 +
>  5 files changed, 294 insertions(+), 6 deletions(-)
>  create mode 100644 drivers/net/netdevsim/psp.c
> 
> diff --git a/drivers/net/netdevsim/Makefile b/drivers/net/netdevsim/Makefile
> index f8de93bc5f5b..14a553e000ec 100644
> --- a/drivers/net/netdevsim/Makefile
> +++ b/drivers/net/netdevsim/Makefile
> @@ -18,6 +18,10 @@ ifneq ($(CONFIG_PSAMPLE),)
>  netdevsim-objs += psample.o
>  endif
>  
> +ifneq ($(CONFIG_INET_PSP),)
> +netdevsim-objs += psp.o
> +endif
> +
>  ifneq ($(CONFIG_MACSEC),)
>  netdevsim-objs += macsec.o
>  endif
> diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
> index bddd24c1389d..02c1c97b7008 100644
> --- a/drivers/net/netdevsim/netdevsim.h
> +++ b/drivers/net/netdevsim/netdevsim.h
> @@ -108,6 +108,12 @@ struct netdevsim {
>  
>  	int rq_reset_mode;
>  
> +	struct {
> +		struct psp_dev *dev;
> +		u32 spi;
> +		u32 assoc_cnt;
> +	} psp;
> +
>  	struct nsim_bus_dev *nsim_bus_dev;
>  
>  	struct bpf_prog	*bpf_offloaded;
> @@ -421,6 +427,27 @@ static inline void nsim_macsec_teardown(struct netdevsim *ns)
>  }
>  #endif
>  
> +#if IS_ENABLED(CONFIG_INET_PSP)
> +int nsim_psp_init(struct netdevsim *ns);
> +void nsim_psp_uninit(struct netdevsim *ns);
> +void nsim_psp_handle_ext(struct sk_buff *skb, struct skb_ext *psp_ext);
> +enum skb_drop_reason
> +nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
> +	    struct netdevsim *peer_ns, struct skb_ext **psp_ext);
> +#else
> +static inline int nsim_psp_init(struct netdevsim *ns) { return 0; }
> +static inline void nsim_psp_uninit(struct netdevsim *ns) {}
> +static inline enum skb_drop_reason
> +nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
> +	    struct netdevsim *peer_ns, struct skb_ext **psp_ext)
> +{
> +	return 0;
> +}
> +
> +static inline void
> +nsim_psp_handle_ext(struct sk_buff *skb, struct skb_ext *psp_ext) {}
> +#endif
> +
>  struct nsim_bus_dev {
>  	struct device dev;
>  	struct list_head list;
> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
> index 0178219f0db5..ebc3833e95b4 100644
> --- a/drivers/net/netdevsim/netdev.c
> +++ b/drivers/net/netdevsim/netdev.c
> @@ -103,28 +103,42 @@ static int nsim_napi_rx(struct net_device *tx_dev, struct net_device *rx_dev,
>  static int nsim_forward_skb(struct net_device *tx_dev,
>  			    struct net_device *rx_dev,
>  			    struct sk_buff *skb,
> -			    struct nsim_rq *rq)
> +			    struct nsim_rq *rq,
> +			    struct skb_ext *psp_ext)
>  {
> -	return __dev_forward_skb(rx_dev, skb) ?:
> -		nsim_napi_rx(tx_dev, rx_dev, rq, skb);
> +	int ret;
> +
> +	ret = __dev_forward_skb(rx_dev, skb);
> +	if (ret)
> +		return ret;
> +
> +	nsim_psp_handle_ext(skb, psp_ext);
> +
> +	return nsim_napi_rx(tx_dev, rx_dev, rq, skb);
>  }
>  
>  static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct netdevsim *ns = netdev_priv(dev);
> +	struct skb_ext *psp_ext = NULL;
>  	struct net_device *peer_dev;
>  	unsigned int len = skb->len;
>  	struct netdevsim *peer_ns;
>  	struct netdev_config *cfg;
>  	struct nsim_rq *rq;
>  	int rxq;
> +	int dr;
>  
>  	rcu_read_lock();
>  	if (!nsim_ipsec_tx(ns, skb))
> -		goto out_drop_free;
> +		goto out_drop_any;
>  
>  	peer_ns = rcu_dereference(ns->peer);
>  	if (!peer_ns)
> +		goto out_drop_any;
> +
> +	dr = nsim_do_psp(skb, ns, peer_ns, &psp_ext);
> +	if (dr)
>  		goto out_drop_free;
>  
>  	peer_dev = peer_ns->netdev;
> @@ -141,7 +155,8 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  		skb_linearize(skb);
>  
>  	skb_tx_timestamp(skb);
> -	if (unlikely(nsim_forward_skb(dev, peer_dev, skb, rq) == NET_RX_DROP))
> +	if (unlikely(nsim_forward_skb(dev, peer_dev,
> +				      skb, rq, psp_ext) == NET_RX_DROP))
>  		goto out_drop_cnt;
>  
>  	if (!hrtimer_active(&rq->napi_timer))
> @@ -151,8 +166,10 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	dev_dstats_tx_add(dev, len);
>  	return NETDEV_TX_OK;
>  
> +out_drop_any:
> +	dr = SKB_DROP_REASON_NOT_SPECIFIED;
>  out_drop_free:
> -	dev_kfree_skb(skb);
> +	kfree_skb_reason(skb, dr);
>  out_drop_cnt:
>  	rcu_read_unlock();
>  	dev_dstats_tx_dropped(dev);
> @@ -1002,6 +1019,7 @@ static void nsim_queue_uninit(struct netdevsim *ns)
>  
>  static int nsim_init_netdevsim(struct netdevsim *ns)
>  {
> +	struct netdevsim *peer;
>  	struct mock_phc *phc;
>  	int err;
>  
> @@ -1036,6 +1054,10 @@ static int nsim_init_netdevsim(struct netdevsim *ns)
>  		goto err_ipsec_teardown;
>  	rtnl_unlock();
>  
> +	err = nsim_psp_init(ns);
> +	if (err)
> +		goto err_unregister_netdev;
> +
>  	if (IS_ENABLED(CONFIG_DEBUG_NET)) {
>  		ns->nb.notifier_call = netdev_debug_event;
>  		if (register_netdevice_notifier_dev_net(ns->netdev, &ns->nb,
> @@ -1045,6 +1067,13 @@ static int nsim_init_netdevsim(struct netdevsim *ns)
>  
>  	return 0;
>  
> +err_unregister_netdev:
> +	rtnl_lock();
> +	peer = rtnl_dereference(ns->peer);
> +	if (peer)
> +		RCU_INIT_POINTER(peer->peer, NULL);
> +	RCU_INIT_POINTER(ns->peer, NULL);
> +	unregister_netdevice(ns->netdev);

Could maybe use a shared helper with the same logic in nsim_destroy

>  err_ipsec_teardown:
>  	nsim_ipsec_teardown(ns);
>  	nsim_macsec_teardown(ns);
> @@ -1132,6 +1161,8 @@ void nsim_destroy(struct netdevsim *ns)
>  		unregister_netdevice_notifier_dev_net(ns->netdev, &ns->nb,
>  						      &ns->nn);
>  
> +	nsim_psp_uninit(ns);
> +
>  	rtnl_lock();
>  	peer = rtnl_dereference(ns->peer);
>  	if (peer)
> diff --git a/drivers/net/netdevsim/psp.c b/drivers/net/netdevsim/psp.c
> new file mode 100644
> index 000000000000..332b5b744f01
> --- /dev/null
> +++ b/drivers/net/netdevsim/psp.c
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/ip.h>
> +#include <linux/skbuff.h>
> +#include <net/ip6_checksum.h>
> +#include <net/psp.h>
> +#include <net/sock.h>
> +
> +#include "netdevsim.h"
> +
> +void nsim_psp_handle_ext(struct sk_buff *skb, struct skb_ext *psp_ext)
> +{
> +	if (psp_ext)
> +		__skb_ext_set(skb, SKB_EXT_PSP, psp_ext);
> +}
> +
> +enum skb_drop_reason
> +nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
> +	    struct netdevsim *peer_ns, struct skb_ext **psp_ext)
> +{
> +	enum skb_drop_reason rc = 0;
> +	struct psp_assoc *pas;
> +	struct net *net;
> +	void **ptr;
> +
> +	rcu_read_lock();
> +	pas = psp_skb_get_assoc_rcu(skb);
> +	if (!pas) {
> +		rc = SKB_NOT_DROPPED_YET;
> +		goto out_unlock;
> +	}
> +
> +	if (!skb_transport_header_was_set(skb)) {
> +		rc = SKB_DROP_REASON_PSP_OUTPUT;
> +		goto out_unlock;
> +	}
> +
> +	ptr = psp_assoc_drv_data(pas);
> +	if (*ptr != ns) {
> +		rc = SKB_DROP_REASON_PSP_OUTPUT;
> +		goto out_unlock;
> +	}
> +
> +	net = sock_net(skb->sk);
> +	if (!psp_dev_encapsulate(net, skb, pas->tx.spi, pas->version, 0)) {
> +		rc = SKB_DROP_REASON_PSP_OUTPUT;
> +		goto out_unlock;
> +	}
> +
> +	/* Now pretend we just received this frame */
> +	if (peer_ns->psp.dev->config.versions & (1 << pas->version)) {
> +		bool strip_icv = false;

Here for readability? Never modified, could be dropped.

> +		u8 generation;
> +
> +		/* We cheat a bit and put the generation in the key.
> +		 * In real life if generation was too old, then decryption would
> +		 * fail. Here, we just make it so a bad key causes a bad
> +		 * generation too, and psp_sk_rx_policy_check() will fail.
> +		 */
> +		generation = pas->tx.key[0];
> +
> +		skb_ext_reset(skb);
> +		skb->mac_len = ETH_HLEN;
> +		if (psp_dev_rcv(skb, peer_ns->psp.dev->id, generation,
> +				strip_icv)) {
> +			rc = SKB_DROP_REASON_PSP_OUTPUT;
> +			goto out_unlock;
> +		}
> +
> +		*psp_ext = skb->extensions;
> +		refcount_inc(&(*psp_ext)->refcnt);
> +		skb->decrypted = 1;
> +	} else {
> +		struct ipv6hdr *ip6h __maybe_unused;
> +		struct iphdr *iph;
> +		struct udphdr *uh;
> +		__wsum csum;
> +
> +		/* Do not decapsulate. Receive the skb with the udp and psp
> +		 * headers still there as if this is a normal udp packet.
> +		 * psp_dev_encapsulate() sets udp checksum to 0, so we need to
> +		 * provide a valid checksum here, so the skb isn't dropped.
> +		 */
> +		uh = udp_hdr(skb);
> +		csum = skb_checksum(skb, skb_transport_offset(skb),
> +				    ntohs(uh->len), 0);
> +
> +		switch (skb->protocol) {
> +		case htons(ETH_P_IP):
> +			iph = ip_hdr(skb);
> +			uh->check = udp_v4_check(ntohs(uh->len), iph->saddr,
> +						 iph->daddr, csum);
> +			break;
> +#if IS_ENABLED(CONFIG_IPV6)
> +		case htons(ETH_P_IPV6):
> +			ip6h = ipv6_hdr(skb);
> +			uh->check = udp_v6_check(ntohs(uh->len), &ip6h->saddr,
> +						 &ip6h->daddr, csum);
> +			break;
> +#endif
> +		}
> +
> +		uh->check	= uh->check ?: CSUM_MANGLED_0;
> +		skb->ip_summed	= CHECKSUM_NONE;

Could just set CHECKSUM_UNNECESSARY and forgo the checksum
calculation?