linux-kernel - Re: [PATCH v2] netfilter: nfnetlink: Improve input length sanitization in nfnetlink

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Date:   Thu, 29 Jun 2017 18:22:40 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Mateusz Jurczyk <mjurczyk@...gle.com>
Cc:     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,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] netfilter: nfnetlink: Improve input length
 sanitization in nfnetlink_rcv

On Tue, Jun 27, 2017 at 07:05:27PM +0200, Pablo Neira Ayuso wrote:
> 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().

BTW, I can just mangle your patch here to delete such line to speed up
things. See the mangled patch that is attached to this email.

Thanks.

View attachment "x.patch" of type "text/x-diff" (829 bytes)