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Date:   Wed, 31 Aug 2022 20:18:25 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        syzbot <syzkaller@...glegroups.com>,
        Yajun Deng <yajun.deng@...ux.dev>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] netlink: Bounds-check nlmsg_len()

On Wed, 31 Aug 2022 20:06:09 -0700 Kees Cook wrote:
>  static inline int nlmsg_len(const struct nlmsghdr *nlh)
>  {
> -	return nlh->nlmsg_len - NLMSG_HDRLEN;
> +	u32 nlmsg_contents_len;
> +
> +	if (WARN_ON_ONCE(check_sub_overflow(nlh->nlmsg_len,
> +					    (u32)NLMSG_HDRLEN,
> +					    &nlmsg_contents_len)))
> +		return 0;
> +	if (WARN_ON_ONCE(nlmsg_contents_len > INT_MAX))
> +		return INT_MAX;
> +	return nlmsg_contents_len;

We check the messages on input, making sure the length is valid wrt
skb->len, and sane, ie > NLMSG_HDRLEN. See netlink_rcv_skb().

Can we not, pretty please? :(

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