netdev - Re: [PATCH bpf-next v7 03/10] bpf: net_sched: Add basic bpf qdisc kfuncs

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Message-ID: <CAMB2axNeb-UzO8AOkdXPcqrwnw2J6vKVLSRVM_R+oN=SJEsx9g@mail.gmail.com>
Date: Fri, 11 Apr 2025 13:03:00 -0700
From: Amery Hung <ameryhung@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	alexei.starovoitov@...il.com, andrii@...nel.org, daniel@...earbox.net, 
	edumazet@...gle.com, kuba@...nel.org, xiyou.wangcong@...il.com, 
	jhs@...atatu.com, martin.lau@...nel.org, jiri@...nulli.us, 
	stfomichev@...il.com, toke@...hat.com, sinquersw@...il.com, 
	ekarani.silvestre@....ufcg.edu.br, yangpeihao@...u.edu.cn, 
	yepeilin.cs@...il.com, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v7 03/10] bpf: net_sched: Add basic bpf qdisc kfuncs

On Fri, Apr 11, 2025 at 11:37 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 4/11/25 10:08 AM, Kumar Kartikeya Dwivedi wrote:
> > On Fri, 11 Apr 2025 at 18:59, Amery Hung <ameryhung@...il.com> wrote:
> >>
> >> On Fri, Apr 11, 2025 at 6:32 AM Kumar Kartikeya Dwivedi
> >> <memxor@...il.com> wrote:
> >>>
> >>> On Wed, 9 Apr 2025 at 23:46, Amery Hung <ameryhung@...il.com> wrote:
> >>>>
> >>>> From: Amery Hung <amery.hung@...edance.com>
> >>>>
> >>>> Add basic kfuncs for working on skb in qdisc.
> >>>>
> >>>> Both bpf_qdisc_skb_drop() and bpf_kfree_skb() can be used to release
> >>>> a reference to an skb. However, bpf_qdisc_skb_drop() can only be called
> >>>> in .enqueue where a to_free skb list is available from kernel to defer
> >>>> the release. bpf_kfree_skb() should be used elsewhere. It is also used
> >>>> in bpf_obj_free_fields() when cleaning up skb in maps and collections.
> >>>>
> >>>> bpf_skb_get_hash() returns the flow hash of an skb, which can be used
> >>>> to build flow-based queueing algorithms.
> >>>>
> >>>> Finally, allow users to create read-only dynptr via bpf_dynptr_from_skb().
> >>>>
> >>>> Signed-off-by: Amery Hung <amery.hung@...edance.com>
> >>>> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
> >>>> ---
> >>>
> >>> How do we prevent UAF when dynptr is accessed after bpf_kfree_skb?
> >>>
> >>
> >> Good question...
> >>
> >> Maybe we can add a ref_obj_id field to bpf_reg_state->dynptr to track
> >> the ref_obj_id of the object underlying a dynptr?
> >>
> >> Then, in release_reference(), in addition to finding ref_obj_id in
> >> registers, verifier will also search stack slots and invalidate all
> >> dynptrs with the ref_obj_id.
> >>
> >> Does this sound like a feasible solution?
> >
> > Yes, though I talked with Andrii and he has better ideas for doing
> > this generically, but for now I think we can make this fix as a
> > stopgap.
>
> In case the better fix will take longer, just want to mention that an option is
> to remove the bpf_dynptr_from_skb() from bpf qdisc. I don't see an urgent need
> for the bpf qdisc to be able to directly access the skb->data. btw, I don't
> think bpf qdisc should write to the skb->data.
>
> The same goes for the bpf_kfree_skb(). I was thinking if it is useful at all
> considering there is already a bpf_qdisc_skb_drop(). I kept it there because it
> is a little more intuitive in case the .reset/.destroy wanted to do a "skb =
> bpf_kptr_xchg(&skbn->skb, NULL);" and then explicitly free the
> bpf_kfree_skb(skb). However, the bpf prog can also directly do the
> bpf_obj_drop(skbn) and then bpf_kfree_skb() is not needed, right?
>
>

My rationale for keeping two skb releasing kfuncs: bpf_kfree_skb() is
the dtor and since dtor can only have one argument, so
bpf_qdisc_skb_drop() can not replace it. Since bpf_kfree_skb() is here
to stay, I allow users to call it directly for convenience. Only
exposing bpf_qdisc_skb_drop() and calling kfree_skb() in
bpf_qdisc_skb_drop() when to_free is NULL will also do. I don’t have a
strong opinion.

Yes, bpf_kfree_skb() will not be needed if doing bpf_obj_drop(skbn).
bpf_obj_drop() internally will call the dtor of a kptr (i.e., in this
case, bpf_kfree_skb()) in an allocated object.