netdev - Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user

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Date:   Wed, 7 Jun 2017 16:18:39 +0200
From:   Florian Westphal <fw@...len.de>
To:     Mateusz Jurczyk <mjurczyk@...gle.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-decnet-user@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization
 in dnrmg_receive_user_skb

Mateusz Jurczyk <mjurczyk@...gle.com> wrote:
> Verify that the length of the socket buffer is sufficient to cover the
> nlmsghdr structure before accessing the nlh->nlmsg_len field for further
> input sanitization. If the client only supplies 1-3 bytes of data in
> sk_buff, then nlh->nlmsg_len remains partially uninitialized and
> contains leftover memory from the corresponding kernel allocation.
> Operating on such data may result in indeterminate evaluation of the
> nlmsg_len < sizeof(*nlh) expression.
> 
> The bug was discovered by a runtime instrumentation designed to detect
> use of uninitialized memory in the kernel. The patch prevents this and
> other similar tools (e.g. KMSAN) from flagging this behavior in the future.

Instead of changing all the internal users wouldn't it be better
to add this check once in netlink_unicast_kernel?