netdev - Re: [PATCH net] tun: Remove nested call to bpf_net_ctx_set() in do_xdp

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Message-Id: <20240725041352.13515-1-aha310510@gmail.com>
Date: Thu, 25 Jul 2024 13:13:52 +0900
From: Jeongjun Park <aha310510@...il.com>
To: willemdebruijn.kernel@...il.com
Cc: aha310510@...il.com,
	bigeasy@...utronix.de,
	bpf@...r.kernel.org,
	davem@...emloft.net,
	edumazet@...gle.com,
	jasowang@...hat.com,
	jiri@...nulli.us,
	kuba@...nel.org,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	pabeni@...hat.com,
	syzbot+44623300f057a28baf1e@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net] tun: Remove nested call to bpf_net_ctx_set() in do_xdp_generic()

Willem de Bruijn wrote:
> I'm no expert on this code, but commit 401cb7dae813 that introduced
> bpf_net_ctx_set explicitly states that nested calls are allowed.
>
> And the function does imply that:
>
> static inline struct bpf_net_context *bpf_net_ctx_set(struct bpf_net_context *bpf_net_ctx)
> {
>         struct task_struct *tsk = current;
>
>         if (tsk->bpf_net_context != NULL)
>                 return NULL;
>         bpf_net_ctx->ri.kern_flags = 0;
>
>         tsk->bpf_net_context = bpf_net_ctx;
>         return bpf_net_ctx;
> }

I'm not an expert on this code either. As you said, there is a 
possibility that the bug is not caused by overlapping calls, but various
memory corruptions are occurring due to the handling of bpf_net_context 
in do_xdp_generic. Therefore, it is appropriate to modify it to handle
it in the parent function rather than in do_xdp_generic.

> Is tun_xdp_one missing? That also calls do_xdp_generic.

This is no problem since tun_xdp_one is only called from tun_sendmsg 
and tun_sendmsg already does the bpf_net_context handling.

Regards,
Jeongjun Park.