linux-kernel - Re: [PATCH net-next 1/3] bpf: introduce current->pid, tgid, uid, gid, comm accessors

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Message-ID: <CALCETrWUuoZz2XOox5FJJPAfFdBakLnY1d7LLP5Uxq9tqgkpzg@mail.gmail.com>
Date:	Fri, 12 Jun 2015 17:24:53 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Wang Nan <wangnan0@...wei.com>, Li Zefan <lizefan@...wei.com>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Daniel Borkmann <daniel@...earbox.net>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/3] bpf: introduce current->pid, tgid, uid, gid,
 comm accessors

On Fri, Jun 12, 2015 at 5:15 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> On 6/12/15 5:03 PM, Andy Lutomirski wrote:
>>
>> On Fri, Jun 12, 2015 at 4:55 PM, Alexei Starovoitov <ast@...mgrid.com>
>> wrote:
>>>
>>> On 6/12/15 4:47 PM, Andy Lutomirski wrote:
>>>>
>>>>
>>>> On Fri, Jun 12, 2015 at 4:38 PM, Alexei Starovoitov <ast@...mgrid.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> On 6/12/15 4:25 PM, Andy Lutomirski wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> It's a dangerous tool.  Also, shouldn't the returned uid match the
>>>>>> namespace of the task that installed the probe, not the task that's
>>>>>> being probed?
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> so leaking info to unprivileged apps is the concern?
>>>>> The whole thing is for root only as you know.
>>>>> The non-root is still far away. Today root needs to see the whole
>>>>> kernel. That was the goal from the beginning.
>>>>>
>>>>
>>>> This is more of a correctness issue than a security issue.  ISTM using
>>>> current_user_ns() in a kprobe is asking for trouble.  It certainly
>>>> allows any unprivilege user to show any uid it wants to the probe,
>>>> which is probably not what the installer of the probe expects.
>>>
>>>
>>>
>>> probe doesn't expect anything. it doesn't make any decisions.
>>> bpf is read only. it's _visibility_ into the kernel.
>>> It's not used for security.
>>> When we start connecting eBPF to seccomp I would agree that uid
>>> handling needs to be done carefully, but we're not there yet.
>>> I don't want to kill _visibility_ because in some distant future
>>> bpf becomes a decision making tool in security area and
>>> get_current_uid() will return numbers that shouldn't be blindly
>>> used to reject/accept a user requesting something. That's far away.
>>>
>>
>> All that is true, but the code that *installed* the bpf probe might
>> get might confused when it logs that uid 0 did such-and-such when
>> really some unprivileged userns root did it.
>
>
> so what specifically you proposing?
> Use from_kuid(&init_user_ns,...) instead?

That seems reasonable to me.  After all, you can't install one of
these probes from a non-init userns.

--Andy
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