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

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Message-ID: <CACAyw98RvDc+i3gpgnAtnM0ojAfQ-mHvzRXFRUcgkEPr3K4G-g@mail.gmail.com>
Date:   Fri, 28 Jun 2019 10:01:26 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Song Liu <songliubraving@...com>
Cc:     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>, Jann Horn <jannh@...gle.com>,
        gregkh@...uxfoundation.org
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

On Thu, 27 Jun 2019 at 21:19, Song Liu <songliubraving@...com> wrote:
>
> This patch introduce unprivileged BPF access. The access control is
> achieved via device /dev/bpf. Users with write access to /dev/bpf are able
> to call sys_bpf().
>
> Two ioctl command are added to /dev/bpf:
>
> The two commands enable/disable permission to call sys_bpf() for current
> task. This permission is noted by bpf_permitted in task_struct. This
> permission is inherited during clone(CLONE_THREAD).

If I understand it correctly, a process would have to open /dev/bpf before
spawning other threads for this to work?

That still wouldn't work for Go I'm afraid. The runtime creates and destroys
threads on an ad-hoc basis, and there is no way to "initialize" in the
first thread.
With the API as is, any Go wrapper wishing to use this would have to do the
following _for every BPF syscall_:

1. Use runtime.LockOSThread() to prevent the scheduler from moving the
    goroutine.
2. Open /dev/bpf to set the bit in current_task
3. Execute the syscall
4. Call runtime.UnlockOSThread()

Note that calling into C code via CGo doesn't change this. Is it not possible to
set the bit on all processes in the current thread group?

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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