linux-kernel - Re: [PATCH net-net] tun: Assign missing bpf_net

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Message-ID: <20240912122847.x70_LgN_@linutronix.de>
Date: Thu, 12 Sep 2024 14:28:47 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Breno Leitao <leitao@...ian.org>
Cc: Jakub Kicinski <kuba@...nel.org>, andrii@...nel.org, ast@...nel.org,
	syzbot <syzbot+08811615f0e17bc6708b@...kaller.appspotmail.com>,
	bpf@...r.kernel.org, daniel@...earbox.net, davem@...emloft.net,
	eddyz87@...il.com, haoluo@...gle.com, hawk@...nel.org,
	john.fastabend@...il.com, jolsa@...nel.org, kpsingh@...nel.org,
	linux-kernel@...r.kernel.org, martin.lau@...ux.dev,
	netdev@...r.kernel.org, sdf@...ichev.me, song@...nel.org,
	syzkaller-bugs@...glegroups.com, yonghong.song@...ux.dev
Subject: Re: [PATCH net-net] tun: Assign missing bpf_net_context.

On 2024-09-12 05:06:36 [-0700], Breno Leitao wrote:
> Hello Sebastian, Jakub,
Hi,

> I've seen some crashes in 6.11-rc7 that seems related to 401cb7dae8130
> ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.").
> 
> Basically bpf_net_context is NULL, and it is being dereferenced by
> bpf_net_ctx->ri.kern_flags (offset 0x38) in the following code.
> 
> 	static inline struct bpf_redirect_info *bpf_net_ctx_get_ri(void)
> 	{
> 		struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
> 		if (!(bpf_net_ctx->ri.kern_flags & BPF_RI_F_RI_INIT)) {
> 
> That said, it means that bpf_net_ctx_get() is returning NULL.
> 
> This stack is coming from the bpf function bpf_redirect()
> 	BPF_CALL_2(bpf_redirect, u32, ifindex, u64, flags)
> 	{
> 	      struct bpf_redirect_info *ri = bpf_net_ctx_get_ri();
> 
> 
> Since I don't think there is XDP involved, I wondering if we need some
> preotection before calling bpf_redirect()

This origins in netkit_xmit(). If my memory serves me, then Daniel told
me that netkit is not doing any redirect and therefore does not need
"this". This must have been during one of the first "designs"/ versions. 

If you are saying, that this is possible then something must be done.
Either assign a context or reject the bpf program.

Sebastian