Re: [PATCH net v2 1/2] net: Make PSP skb extension survive packet scrubbing

[Daniel Zahka]

On Fri Oct 30, 2026 at 12:31 AM EDT, Wei Wang wrote:
> skb_scrub_packet() drops all skb extensions unconditionally via
> skb_ext_reset(). It runs on tunnel encap/decap (ip_tunnel_rcv, vxlan_rcv,
> etc.) and virtual device forwarding/redirection (dev_forward_skb,
> netkit_xmit, skb_do_redirect).
>
> When a virtual device such as netkit is associated with a PSP device in
> another network namespace via PSP_CMD_DEV_ASSOC, incoming PSP packets
> decapsulated by psp_dev_rcv() lose their SKB_EXT_PSP extension when
> scrubbed during redirection into the peer netns. Without SKB_EXT_PSP,
> __psp_sk_rx_policy_check() rejects incoming TCP data segments at the
> socket layer with SKB_DROP_REASON_PSP_INPUT.
>
> Introduce a static skb_ext_scrub() helper in net/core/skbuff.c to
> preserve SKB_EXT_PSP across skb_scrub_packet() while dropping all other
> extensions. If SKB_EXT_PSP is not active, skb_ext_scrub() simply calls
> skb_ext_reset(). If SKB_EXT_PSP is the only extension in the skb_ext
> buffer, nothing needs to be scrubbed. Otherwise, save the PSP metadata,
> reset the extension block, and re-add a fresh SKB_EXT_PSP extension (or
> clear skb->decrypted if the allocation fails).
>
> Fixes: 06c2dce2d0f6 ("psp: add new netlink cmd for dev-assoc and dev-disassoc")
> Link: https://lore.kernel.org/r/20260910-bpf-meta-inside-skb-ext-v2-3-0b21e42180b0@xxxxxxxxxxxxxx
> Reported-by: Daniel Zahka <daniel.zahka@xxxxxxxxx>
> Closes: https://lore.kernel.org/netdev/DLODSKPY6ON6.1G6SP3CFFLCPJ@xxxxxxxxx/
> Co-developed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Signed-off-by: Wei Wang <weiwan@xxxxxxxxxx>
> ---
>  net/core/skbuff.c | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 93c2f440b9ec..5c4a2e982ebc 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6369,6 +6369,32 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  }
>  EXPORT_SYMBOL(skb_try_coalesce);
>  
> +static void skb_ext_scrub(struct sk_buff *skb)
> +{
> +#if IS_ENABLED(CONFIG_INET_PSP)
> +	struct psp_skb_ext *pse = skb_ext_find(skb, SKB_EXT_PSP);
> +
> +	if (pse) {
> +		struct skb_ext *ext = skb->extensions;
> +		struct psp_skb_ext tmp = *pse;

I would move the copy just before skb_ext_reset(skb), but it probably
won't make a difference in the generated code.

> +
> +		/* Nothing else to scrub if PSP is the only extension in ext. */
> +		if (ext->chunks == SKB_EXT_CHUNKSIZEOF(*ext) +
> +				   skb_ext_type_len[SKB_EXT_PSP])
> +			return;
> +
> +		skb_ext_reset(skb);
> +		pse = skb_ext_add(skb, SKB_EXT_PSP);
> +		if (pse)
> +			memcpy(pse, &tmp, sizeof(*pse));
> +		else
> +			skb->decrypted = 0;

Agree with sashiko not to clear decrypted. It would at least prevent the
skb from being forwarded cleartext back onto the network.

> +		return;
> +	}
> +#endif
> +	skb_ext_reset(skb);
> +}
> +
>  /**
>   * skb_scrub_packet - scrub an skb
>   *
> @@ -6380,7 +6406,8 @@ EXPORT_SYMBOL(skb_try_coalesce);
>   * operations.
>   * skb_scrub_packet can also be used to clean a skb before injecting it in
>   * another namespace (@xnet == true). We have to clear all information in the
> - * skb that could impact namespace isolation.
> + * skb that could impact namespace isolation. Note that PSP skb extension is
> + * preserved across packet scrubbing by design.
>   */
>  void skb_scrub_packet(struct sk_buff *skb, bool xnet)
>  {
> @@ -6388,7 +6415,7 @@ void skb_scrub_packet(struct sk_buff *skb, bool xnet)
>  	skb->skb_iif = 0;
>  	skb->ignore_df = 0;
>  	skb_dst_drop(skb);
> -	skb_ext_reset(skb);
> +	skb_ext_scrub(skb);
>  	nf_reset_ct(skb);
>  	nf_reset_trace(skb);
>  

The only thing I would say is that skb_ext_scrub() won't scale if a
second extension comes along and wants the same treatment as PSP. In
that case, either we could make a bitmap of "no scrub" extensions and
then use the same approach of allocating a new ext block and copying
them over, or the original approach of skb_ext_del() could be used after
changing the reference counting scheme for the special extensions in the
release path of __skb_ext_put(), but that is a more intrusive change.

I'm ok with the approach from a PSP perspective:

Reviewed-by: Daniel Zahka <daniel.zahka@xxxxxxxxx>