netdev - Re: [PATCH RFC v2 net-next 05/16] bpf: introduce syscall(BPF, ...) and BPF maps

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Date:	Wed, 23 Jul 2014 12:30:24 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 net-next 05/16] bpf: introduce syscall(BPF, ...)
 and BPF maps

On Wed, Jul 23, 2014 at 11:02 AM, Kees Cook <keescook@...omium.org> wrote:
>> --- a/Documentation/networking/filter.txt
>> +++ b/Documentation/networking/filter.txt
>>
>> +eBPF maps
>> +---------
>> +'maps' is a generic storage of different types for sharing data between kernel
>> +and userspace.
>> +
>> +The maps are accessed from user space via BPF syscall, which has commands:
>> +- create a map with given id, type and attributes
>> +  map_id = bpf_map_create(int map_id, map_type, struct nlattr *attr, int len)
>> +  returns positive map id or negative error
>
> Looks like these docs need updating for the fd-based approach instead
> of the map_id approach?

ohh, yes. updated it in srcs and in commit log, but forgot in docs.

>> +SYSCALL_DEFINE5(bpf, int, cmd, unsigned long, arg2, unsigned long, arg3,
>> +               unsigned long, arg4, unsigned long, arg5)
>> +{
>> +       if (!capable(CAP_SYS_ADMIN))
>> +               return -EPERM;
>
> It might be valuable to have a comment here describing why this is
> currently limited to CAP_SYS_ADMIN.

makes sense.
There are several reasons it should be limited to root initially:
- to phase changes in gradually
- verifier is not detecting pointer leaks yet
- full security audit wasn't performed
- tracing and network analytics are root only anyway
Currently eBPF is safe (non-crashing), since safety is relatively easy
to enforce by static analysis. For somebody with compiler background
it's natural to think about bounds, alignments, uninitialized access, etc.
So I'm confident that I didn't miss anything big in 'safety' aspect.
'Non-root security' is harder. I'll add pointer leak detection first
and will ask for more suggestions.

>> +       switch (cmd) {
>> +       case BPF_MAP_CREATE:
>> +               return map_create((enum bpf_map_type) arg2,
>> +                                 (struct nlattr __user *) arg3, (int) arg4);
>
> I'd recommend requiring arg5 == 0 here, just for future flexibility.

Though I expect all extensions to go through nlattr attributes,
it's indeed cleaner to enforce arg5==0 here and for all other cmds.
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