netdev - Re: [PATCH v5 bpf-next 2/3] bpf: implement CAP

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Date:   Mon, 11 May 2020 17:12:10 -0700
From:   sdf@...gle.com
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com,
        linux-security-module@...r.kernel.org, acme@...hat.com,
        jamorris@...ux.microsoft.com, jannh@...gle.com, kpsingh@...gle.com
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: implement CAP_BPF

On 05/08, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
[..]
> @@ -3932,7 +3977,7 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr  
> __user *, uattr, unsigned int, siz
>   	union bpf_attr attr;
>   	int err;

> -	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
> +	if (sysctl_unprivileged_bpf_disabled && !bpf_capable())
>   		return -EPERM;
This is awesome, thanks for reviving the effort!

One question I have about this particular snippet:
Does it make sense to drop bpf_capable checks for the operations
that work on a provided fd?

The use-case I have in mind is as follows:
* privileged (CAP_BPF) process loads the programs/maps and pins
   them at some known location
* unprivileged process opens up those pins and does the following:
   * prepares the maps (and will later on read them)
   * does SO_ATTACH_BPF/SO_ATTACH_REUSEPORT_EBPF which afaik don't
     require any capabilities

This essentially pushes some of the permission checks into a fs layer. So
whoever has a file descriptor (via unix sock or open) can do BPF operations
on the object that represents it.

Thoughts? Am I missing something important?