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

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On Fri, May 27, 2022 at 03:15:58AM IST, Florian Westphal wrote:
> Lorenzo Bianconi <lorenzo@xxxxxxxxxx> wrote:
> > From: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> >
> > 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



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