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

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Message-ID: <CAMB2axNbnOoHu6jdkt-59W6p59NjmO580kUw_g45rWG2TAH5mQ@mail.gmail.com>
Date: Fri, 11 Apr 2025 10:17:52 -0700
From: Amery Hung <ameryhung@...il.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: 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 10:08 AM Kumar Kartikeya Dwivedi
<memxor@...il.com> 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.

Sounds good. Just making sure I am not doing redundant work, you will
send the fix you made, right?

Thanks,
Amery

> I will add a fixes tag, asked the question because I had the same
> question when implementing a similar pattern for my patch, and was
> wondering how you solved it.
>
> I made a similar fix to what you described for now.
>
> >
> > > >  [...]