netdev - Re: [PATCH bpf-next] seg6: Ensure that seg6_bpf_srh_states can only be accessed from input_action_end

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Message-ID: <20240709051817.VmyBTQ86@linutronix.de>
Date: Tue, 9 Jul 2024 07:18:17 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: syzbot <syzbot+608a2acde8c5a101d07d@...kaller.appspotmail.com>,
	netdev@...r.kernel.org, bpf@...r.kernel.org, andrii@...nel.org,
	ast@...nel.org, daniel@...earbox.net, davem@...emloft.net,
	dsahern@...nel.org, eddyz87@...il.com, edumazet@...gle.com,
	haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
	kpsingh@...nel.org, kuba@...nel.org, pabeni@...hat.com,
	sdf@...ichev.me, sdf@...gle.com, song@...nel.org,
	syzkaller-bugs@...glegroups.com, yonghong.song@...ux.dev,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH bpf-next] seg6: Ensure that seg6_bpf_srh_states can only
 be accessed from input_action_end_bpf()

On 2024-07-08 17:03:58 [-0700], Martin KaFai Lau wrote:
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 403d23faf22e1..ea5bc4a4a6a23 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -6459,6 +6459,8 @@ BPF_CALL_4(bpf_lwt_seg6_store_bytes, struct sk_buff *, skb, u32, offset,
> >   	void *srh_tlvs, *srh_end, *ptr;
> >   	int srhoff = 0;
> > +	if (!bpf_net_ctx_seg6_state_avail())
> > +		return -EINVAL;
> 
> The syzbot stack shows that the seg6local bpf_prog can be run by test_run
> like: bpf_prog_test_run_skb() => bpf_test_run(). "return -EINVAL;" will
> reject and break the existing bpf prog doing test with test_run.

But wouldn't this be the case anyway because seg6_bpf_srh_states::srh
isn't assigned?

> bpf_test_run() has already done the local_bh_disable() and
> bpf_net_ctx_set(). How about doing the
> local_[un]lock_nested_bh(&seg6_bpf_srh_states.bh_lock) in bpf_test_run()
> when the prog->type == BPF_PROG_TYPE_LWT_SEG6LOCAL?

Okay. Sure. And I assume it is limited that only those two call paths
can invoke this type of BPF program.

Sebastian