Re: [PATCH net v2] fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

[Hangbin Liu <hangbin.liu@linux.dev>]

On Mon, Sep 21, 2026 at 04:59:20AM +0000, Hui Peng wrote:
> Commit 7a9bc9e3f423 ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.") added
> NLA_POLICY_MIN(NLA_U8, 1) to fou_nl_policy[FOU_ATTR_IPPROTO], which
> rejects an explicitly supplied FOU_ATTR_IPPROTO == 0 attribute with
> -ERANGE.
> 
> However, FOU_ATTR_IPPROTO is an optional netlink attribute. When a user
> sends FOU_CMD_ADD with FOU_ATTR_TYPE set to FOU_ENCAP_DIRECT and omits
> FOU_ATTR_IPPROTO entirely, nla_policy validation succeeds and
> parse_nl_config() leaves cfg->protocol as 0 (from memset(cfg, 0,
> sizeof(*cfg))). fou_create() then creates a FOU_ENCAP_DIRECT socket with
> fou->protocol == 0.
> 
> In fou_udp_recv(), returning -fou->protocol to udp_queue_rcv_one_skb()
> triggers IP protocol resubmission when fou->protocol > 0, whereas
> returning 0 tells the UDP tunnel layer that the skb was consumed without
> freeing it. When fou->protocol == 0, every packet received on the socket
> returns 0 from fou_udp_recv() and leaks the sk_buff.
> 
> Reject FOU_ENCAP_DIRECT when !cfg->protocol in fou_create() so that
> creating a direct encapsulation port without FOU_ATTR_IPPROTO fails with
> -EINVAL while leaving FOU_CMD_DEL and FOU_CMD_GET (which share
> parse_nl_config()) unaffected.
> 
> Tested in QEMU against Linux 7.3.0-rc3 by sending a FOU_CMD_ADD Generic
> Netlink request with FOU_ATTR_PORT = 5555 and FOU_ATTR_TYPE =
> FOU_ENCAP_DIRECT while omitting FOU_ATTR_IPPROTO. On the unfixed kernel,
> FOU_CMD_ADD succeeds (err = 0), FOU_CMD_GET reports fou->type = 1 and
> fou->protocol = 0, and sending 4000 UDP packets to 127.0.0.1:5555 leaks
> all 4000 sk_buffs (SUnreclaim in /proc/meminfo grows from 41456 kB to
> 59008 kB, +17552 kB); with this patch applied, FOU_CMD_ADD is rejected
> with -EINVAL (-22).
> 
> Fixes: 23461551c006 ("fou: Support for foo-over-udp RX path")
> Fixes: 7a9bc9e3f423 ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v2:
> - Clarify in the commit message that fou->protocol == 0 is reached on
>   current kernels by omitting the optional FOU_ATTR_IPPROTO attribute
>   when FOU_ATTR_TYPE is FOU_ENCAP_DIRECT (whereas an explicit 0
>   attribute is rejected by commit 7a9bc9e3f423), as noted by Kuniyuki
>   Iwashima and Sashiko.
> - Move the !cfg->protocol check from parse_nl_config() into the
>   FOU_ENCAP_DIRECT branch of fou_create() so FOU_CMD_DEL and FOU_CMD_GET
>   are not affected, and drop the redundant check in fou_udp_recv(), as
>   suggested by Sashiko.
> - Fix the Fixes: commit tags to 23461551c006 and 7a9bc9e3f423, as
>   pointed out by Hangbin Liu and Kuniyuki Iwashima.
> 
>  net/ipv4/fou_core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/ipv4/fou_core.c b/net/ipv4/fou_core.c
> index 5e867f1b5c1d..f30f22389283 100644
> --- a/net/ipv4/fou_core.c
> +++ b/net/ipv4/fou_core.c
> @@ -600,6 +600,10 @@ static int fou_create(struct net *net, struct fou_cfg *cfg,
>  	/* Initial for fou type */
>  	switch (cfg->type) {
>  	case FOU_ENCAP_DIRECT:
> +		if (!cfg->protocol) {
> +			err = -EINVAL;
> +			goto error;
> +		}
>  		tunnel_cfg.encap_rcv = fou_udp_recv;
>  		tunnel_cfg.gro_receive = fou_gro_receive;
>  		tunnel_cfg.gro_complete = fou_gro_complete;
> -- 
> 2.49.0

Looks good to me.

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>