netdev - Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

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Message-ID: <77354A95-4107-41A7-8936-D144F01C3CA4@fb.com>
Date:   Sat, 27 Jul 2019 18:20:17 +0000
From:   Song Liu <songliubraving@...com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-security@...r.kernel.org" <linux-security@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Jann Horn <jannh@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

Hi Andy, 

>>>> 
>>> 
>>> Well, yes. sys_bpf() is pretty powerful. 
>>> 
>>> The goal of /dev/bpf is to enable special users to call sys_bpf(). In 
>>> the meanwhile, such users should not take down the whole system easily
>>> by accident, e.g., with rm -rf /.
>> 
>> That’s easy, though — bpftool could learn to read /etc/bpfusers before allowing ruid != 0.
> 
> This is a great idea! fscaps + /etc/bpfusers should do the trick. 

After some discussions and more thinking on this, I have some concerns 
with the user space only approach.  

IIUC, your proposal for user space only approach is like: 

1. bpftool (and other tools) check /etc/bpfusers and only do 
   setuid for allowed users:

	int main()
	{
		if (/* uid in /etc/bpfusers */)
			setuid(0);
		sys_bpf(...);
	}

2. bpftool (and other tools) is installed with CAP_SETUID:

	setcap cap_setuid=e+p /bin/bpftool

3. sys admin maintains proper /etc/bpfusers. 

This approach is not ideal, because we need to trust the tool to give 
it CAP_SETUID. A hacked tool could easily bypass /etc/bpfusers check
or use other root only sys calls after setuid(0). 

Does this make sense? (Or did I misunderstand anything?)

Thanks,
Song