netdev - Re: Question about dev_validate_header used in af

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Date:   Tue, 8 Sep 2020 19:08:17 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about dev_validate_header used in af_packet.c

On Tue, Sep 8, 2020 at 6:23 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Tue, Sep 8, 2020 at 4:53 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Tue, Sep 8, 2020 at 1:04 PM Xie He <xie.he.0141@...il.com> wrote:
> > >
> > > I was recently looking at some drivers, and I felt that if af_packet.c
> > > could help me filter out the invalid RAW frames, I didn't need to
> > > check the validity of the frames myself (in the driver when
> > > transmitting). But now I guess I still need to check that.
> > >
> > > I feel this makes the dev_validate_header's variable-length header
> > > check not very useful, because drivers need to do this check again
> > > (when transmitting) anyway.
> > >
> > > I was thinking, after I saw dev_validate_header, that we could
> > > eventually make it completely take over the responsibility for a
> > > driver to validate the header when transmitting RAW frames. But now it
> > > seems we would not be able to do this.
> >
> > Agreed. As is, it is mainly useful to block users who are ns_capable,
> > but not capable.
> >
> > A third option is to move it behind a sysctl (with static_branch). Your
> > point is valid that there really is no need for testing of drivers against
> > bad packets if the data is validated directly on kernel entry.
>
> I was thinking about this again and it came to me that maybe sometimes
> people actually wanted to send invalid frames on wire (for testing the
> network device on the other end of the wire)? Having thought about
> this possibility I think it might be good to keep the ability for
> people to have 2 choices (either having their RAW frames validated, or
> not validated) through "capability" or through "sysctl" as you
> mentioned. We can keep the default to be not validating the RAW frames
> because RAW sockets are already intended for very special use and are
> not for normal use.

That offers some configurability. But really, I would just leave it as is.