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

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Message-ID: <CALCETrWYHkBtVr5D0zdrXswoH2gSXsxhtD23w6J7EP=HEwPt8A@mail.gmail.com>
Date:	Fri, 12 Jun 2015 17:03:47 -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 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.

Also, as you start calling more and more non-trivial functions from
bpf, you might need to start preventing bpf probe installations in
those functions.

--Andy
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