Date: Tue, 27 Jun 2017 19:05:27 +0200
From: Pablo Neira Ayuso <pablo@netfilter.org>
To: Mateusz Jurczyk <mjurczyk@google.com>
Cc: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>,
        Florian Westphal <fw@strlen.de>,
        "David S. Miller" <davem@davemloft.net>,
        netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] netfilter: nfnetlink: Improve input length
 sanitization in nfnetlink_rcv
Message-ID: <20170627170527.GA13513@salvia>
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 <20170607135038.1592-1-mjurczyk@google.com>
 <20170627155825.GA4885@salvia>
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On Tue, Jun 27, 2017 at 05:58:25PM +0200, Pablo Neira Ayuso wrote:
> On Wed, Jun 07, 2017 at 03:50:38PM +0200, Mateusz Jurczyk 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 < NLMSG_HDRLEN 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.
> 
> Applied, thanks.

Wait, I keeping this back after closer look.

I think we have to remove this:

        if (nlh->nlmsg_len < NLMSG_HDRLEN || <---
            skb->len < NLMSG_HDRLEN + sizeof(struct nfgenmsg))
                return;

in nfnetlink_rcv_skb_batch()

now that we make this unfront check from nfnetlink_rcv().

P.S: Sorry I couldn't look at this any sooner, I've been busy in the
last weeks preparing things for the upcoming Netfilter Workshop.