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

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Date:   Thu, 26 May 2022 23:45:58 +0200
From:   Florian Westphal <fw@...len.de>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     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, fw@...len.de, netfilter-devel@...r.kernel.org,
        lorenzo.bianconi@...hat.com, brouer@...hat.com, toke@...hat.com,
        memxor@...il.com, yhs@...com
Subject: Re: [PATCH v4 bpf-next 06/14] bpf: Whitelist some fields in nf_conn
 for BPF_WRITE

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.