netdev - Re: [PATCH v4 bpf-next 06/14] bpf: Whitelist some fields in nf_conn for BPF

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Message-ID: <20220527113343.h3q5zmkmqm7fev7r@apollo.legion>
Date:   Fri, 27 May 2022 17:06:06 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, davem@...emloft.net, kuba@...nel.org,
        edumazet@...gle.com, pabeni@...hat.com, pablo@...filter.org,
        netfilter-devel@...r.kernel.org, lorenzo.bianconi@...hat.com,
        brouer@...hat.com, toke@...hat.com, yhs@...com
Subject: Re: [PATCH v4 bpf-next 06/14] bpf: Whitelist some fields in nf_conn
 for BPF_WRITE

On Fri, May 27, 2022 at 03:15:58AM IST, Florian Westphal wrote:
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> > From: Kumar Kartikeya Dwivedi <memxor@...il.com>
> >
> > Since we want to allow user to set some fields in nf_conn after it is
> > allocated but before it is inserted, we can permit BPF_WRITE for normal
> > nf_conn, and then mark return value as read only on insert, preventing
> > further BPF_WRITE. This way, nf_conn can be written to using normal
> > BPF instructions after allocation, but not after insertion.
> >
> > Note that we special nf_conn a bit here, inside the btf_struct_access
> > callback for XDP and TC programs. Since this is the only struct for
> > these programs requiring such adjustments, making this mechanism
> > more generic has been left as an exercise for a future patch adding
> > custom callbacks for more structs.
>
> Are you sure this is safe?
> As far as I can see this allows nf_conn->status = ~0ul.
> I'm fairly sure this isn't a good idea, see nf_ct_delete() for example.

This only allows writing to an allocated but not yet inserted nf_conn. The idea
was that insert checks whether ct->status only has permitted bits set before
making the entry visible, and then we make nf_conn pointer read only, however
the runtime check seems to be missing right now in patch 12; something to fix in
v5. With that sorted, would it be fine?

--
Kartikeya