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

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Message-ID: <20220901143423.2abc0ab0@kernel.org>
Date:   Thu, 1 Sep 2022 14:34:23 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        syzbot <syzkaller@...glegroups.com>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] netlink: Bounds-check struct nlmsgerr creation

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.
> +		       */);