Re: [PATCH v3] netlink: Bounds-check struct nlmsgerr creation

[Jakub Kicinski]

On Thu,  1 Sep 2022 00:13:36 -0700 Kees Cook wrote:
>  	rep = __nlmsg_put(skb, NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
>  			  NLMSG_ERROR, payload, flags);

All we should need here is __nlmsg_put() -> nlmsg_put(),
that's idiomatic for netlink.

>  	errmsg = nlmsg_data(rep);
>  	errmsg->error = err;
> -	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
> +	unsafe_memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg)
> +					 ?  nlh->nlmsg_len : sizeof(*nlh),
> +		      /* "payload" was bounds checked against nlh->nlmsg_len,
> +		       * and overflow-checked as tlvlen was constructed.
> +		       */);