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

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Message-ID: <CAP01T76oTKg5H2nqd5ppyLhk1rNjPY0DcYVELmyZU+Du8izbbA@mail.gmail.com>
Date: Fri, 11 Apr 2025 19:08:00 +0200
From: Kumar Kartikeya Dwivedi <memxor@...il.com>
To: Amery Hung <ameryhung@...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, 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.
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.

>
> > >  [...]