linux-kernel - Re: Question about dev_validate_header used in af

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Message-ID: <CA+FuTSfgxt6uqcxy=wnOXo8HxMJ3J0HAqQNiDJBLCs22Ukb_gQ@mail.gmail.com>
Date:   Tue, 8 Sep 2020 13:52:38 +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 1:04 PM Xie He <xie.he.0141@...il.com> wrote:
>
> On Tue, Sep 8, 2020 at 1:41 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > The intent is to bypass such validation to be able to test device
> > drivers. Note that removing that may cause someone's test to start
> > failing.
> >
> > >  So there's no point in
> > > keeping the ability to test this, either.
> >
> > I don't disagree in principle, but do note the failing tests. Bar any
> > strong reasons for change, I'd leave as is.
>
> OK. I got what you mean. You don't want to make people's test cases fail.
>
> 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.