netdev - Re: [PATCH bpf-next v4 06/10] bpf: Track provenance for pointers formed from referenced PTR_TO_BTF

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Date:   Sat, 18 Dec 2021 18:28:39 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v4 06/10] bpf: Track provenance for pointers
 formed from referenced PTR_TO_BTF_ID

On Fri, Dec 17, 2021 at 07:20:27AM +0530, Kumar Kartikeya Dwivedi wrote:
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index b80fe5bf2a02..a6ef11db6823 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -128,6 +128,16 @@ struct bpf_reg_state {
>  	 * allowed and has the same effect as bpf_sk_release(sk).
>  	 */
>  	u32 ref_obj_id;
> +	/* This is set for pointers which are derived from referenced
> +	 * pointer (e.g. PTR_TO_BTF_ID pointer walking), so that the
> +	 * pointers obtained by walking referenced PTR_TO_BTF_ID
> +	 * are appropriately invalidated when the lifetime of their
> +	 * parent object ends.
> +	 *
> +	 * Only one of ref_obj_id and parent_ref_obj_id can be set,
> +	 * never both at once.
> +	 */
> +	u32 parent_ref_obj_id;

How would it handle parent of parent?
Did you consider map_uid approach ?
Similar uid can be added for PTR_TO_BTF_ID.
Then every such pointer will be unique. Each deref will get its own uid.
I think the advantage of parent_ref_obj_id approach is that the program
can acquire a pointer through one kernel type, do some deref, and then
release it through a deref of other type. I'm not sure how practical is that
and it feels a bit dangerous.