linux-hardening - Re: [PATCH 1/2] netlink: Bounds-check nlmsg

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Date:   Thu, 1 Sep 2022 13:54:22 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        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 <netdev@...r.kernel.org>, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, Oliver Hartkopp <socketcan@...tkopp.net>,
        Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] netlink: Bounds-check nlmsg_len()

On Thu, Sep 1, 2022 at 12:49 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 31 Aug 2022 23:27:08 -0700 Kees Cook wrote:
> > This would catch corrupted values...
> >
> > Is the concern the growth in image size? The check_sub_overflow() isn't
> > large at all -- it's just adding a single overflow bit test. The WARNs
> > are heavier, but they're all out-of-line.
>
> It turns the most obvious function into a noodle bar :(
>
> Looking at this function in particular is quite useful, because
> it clearly indicates that the nlmsg_len includes the header.
>
> How about we throw in a
>
>         WARN_ON_ONCE(nlh->nlmsg_len < NLMSG_HDRLEN ||
>                      nlh->nlmsg_len > INT_MAX);
>
> but leave the actual calculation human readable C?

This is inlined, and will add a lot of extra code. We are making the
kernel slower at each release.

What about letting fuzzers like syzbot find the potential issues ?

DEBUG_NET_WARN_ON_ONCE(...);