[bug report] net: ethtool: Introduce per-PHY DUMP operations

[Dan Carpenter <dan.carpenter@linaro.org>]

[ Smatch checking is paused while we raise funding.  #SadFace
  https://lore.kernel.org/all/aTaiGSbWZ9DJaGo7@stanley.mountain/ -dan ]

Hello Maxime Chevallier,

Commit 172265b44cd3 ("net: ethtool: Introduce per-PHY DUMP
operations") from May 2, 2025 (linux-next), leads to the following
Smatch static checker warning:

	net/ethtool/netlink.c:714 ethnl_perphy_start()
	error: buffer overflow 'ethnl_default_requests' 52 <= 255 user_rl='0-255' uncapped

net/ethtool/netlink.c
    700 static int ethnl_perphy_start(struct netlink_callback *cb)
    701 {
    702         struct ethnl_perphy_dump_ctx *phy_ctx = ethnl_perphy_dump_context(cb);
    703         const struct genl_dumpit_info *info = genl_dumpit_info(cb);
    704         struct ethnl_dump_ctx *ctx = &phy_ctx->ethnl_ctx;
    705         struct ethnl_reply_data *reply_data;
    706         const struct ethnl_request_ops *ops;
    707         struct ethnl_req_info *req_info;
    708         struct genlmsghdr *ghdr;
    709         int ret;
    710 
    711         BUILD_BUG_ON(sizeof(*ctx) > sizeof(cb->ctx));
    712 
    713         ghdr = nlmsg_data(cb->nlh);
--> 714         ops = ethnl_default_requests[ghdr->cmd];

Smatch thinks nlmsg_data() is untrusted data, so it could be out of bounds.
It's a u8, but there are only 52 elements in the ethnl_default_requests[]
array.

    715         if (WARN_ONCE(!ops, "cmd %u has no ethnl_request_ops\n", ghdr->cmd))
    716                 return -EOPNOTSUPP;
    717         req_info = kzalloc(ops->req_info_size, GFP_KERNEL);
    718         if (!req_info)
    719                 return -ENOMEM;
    720         reply_data = kmalloc(ops->reply_data_size, GFP_KERNEL);
    721         if (!reply_data) {
    722                 ret = -ENOMEM;
    723                 goto free_req_info;
    724         }
    725 
    726         /* Unlike per-dev dump, don't ignore dev. The dump handler
    727          * will notice it and dump PHYs from given dev. We only keep track of
    728          * the dev's ifindex, .dumpit() will grab and release the netdev itself.
    729          */
    730         ret = ethnl_default_parse(req_info, &info->info, ops, false);
    731         if (ret < 0)
    732                 goto free_reply_data;
    733         if (req_info->dev) {
    734                 phy_ctx->ifindex = req_info->dev->ifindex;
    735                 netdev_put(req_info->dev, &req_info->dev_tracker);
    736                 req_info->dev = NULL;
    737         }
    738 
    739         ctx->ops = ops;
    740         ctx->req_info = req_info;
    741         ctx->reply_data = reply_data;
    742         ctx->pos_ifindex = 0;
    743 
    744         return 0;
    745 
    746 free_reply_data:
    747         kfree(reply_data);
    748 free_req_info:
    749         kfree(req_info);
    750 
    751         return ret;
    752 }

regards,
dan carpenter