Re: piping core dump to a program escapes container

Dongsheng Yang
2015-12-10T00:35:24Z

On 12/09/2015 04:34 PM, Bruno Prémont wrote:
> On Tue, 08 Dec 2015 21:29:13 -0600 Eric W. Biederman wrote:
>> Dongsheng Yang <yangds.fnst@cn.fujitsu.com> writes:
>>
>>> On 12/09/2015 10:26 AM, Dongsheng Yang wrote:
>>>> On 10/25/2015 05:54 AM, Shayan Pooya wrote:
>>>>> I noticed the following core_pattern behavior in my linux box while
>>>>> running docker containers. I am not sure if it is bug, but it is
>>>>> inconsistent and not documented.
>>>>>
>>>>> If the core_pattern is set on the host, the containers will observe
>>>>> and use the pattern for dumping cores (there is no per cgroup
>>>>> core_pattern). According to core(5) for setting core_pattern one can:
>>>>>
>>>>> 1. echo "/tmp/cores/core.%e.%p" > /proc/sys/kernel/core_pattern
>>>>> 2. echo "|/bin/custom_core /tmp/cores/ %e %p " >
>>>>> /proc/sys/kernel/core_pattern
>>>>>
>>>>> The former pattern evaluates the /tmp/cores path in the container's
>>>>> filesystem namespace. Which means, the host does not see a core file
>>>>> in /tmp/cores.
>>>>>
>>>>> However, the latter evaluates the /bin/custom_core path in the global
>>>>> filesystem namespace. Moreover, if /bin/core decides to write the core
>>>>> to a path (/tmp/cores in this case as shown by the arg to
>>>>> custom_core), the path will be evaluated in the global filesystem
>>>>> namespace as well.
>>>>>
>>>>> The latter behaviour is counter-intuitive and error-prone as the
>>>>> container can fill up the core-file directory which it does not have
>>>>> direct access to (which means the core is also not accessible for
>>>>> debugging if someone only has access to the container).
>>
>> From a container perspective it is perhaps counter intuitive from
>> the perspective of the operator of the machine nothing works specially
>> about core_pattern and it works as designed with no unusual danages.
>>
>>>> Hi Shayan,
>>>> We found the same problem with what you described here.
>>>> Is there any document for this behaviour? I want to know is
>>>> that intentional or as you said a 'bug'. Maybe that's intentional
>>>> to provide a way for admin to collect core dumps from all containers as
>>>> Richard said. I am interested in it too.
>>>>
>>>> Anyone can help here?
>>>
>>> In addition, is that a good idea to make core_pattern to be seperated
>>> in different namespace?
>>
>> The behavior was the best we could do at the time last time this issue
>> was examined. There is enough information available to be able to
>> write a core dumping program that can reliably place your core dumps
>> in your container.
>>
>> There has not yet been an obvious namespace in which to stick
>> core_pattern, and even worse exactly how to appropriate launch a process
>> in a container has not been figured out.
>>
>> If those tricky problems can be solved we can have a core_pattern in a
>> container. What we have now is the best we have been able to figure out
>> so far.
>
> Isn't the second option dangerous if its run in global namespace and
> settable from some other namespace/container?
>
> If a process inside a container can set /proc/sys/kernel/core_pattern
> then it could e.g. set it to
> echo "|/bin/rm -rf / /tmp/cores/ %e %p " > /proc/sys/kernel/core_pattern
> and kill the host (eventually itself included).
> Other command lines could do different bad things.

Yes, if you don't give a privileged to container, that's read-only
to them. But if you give containers privilege, that true as you said.
But that's similar with if you give a privilege to any of process
running in the machine. So I think it's not a problem.

Yang
>
>
> Something that would sound reasonable is to have the core dumping
> helper process run under the namespaces the process which wrote to
> /proc/sys/kernel/core_pattern had.
> When some of those namespaces are gone, falling back to the namespaces
> of the process for which core is to be dumped might seem reasonable
> (or just not dumping core at all as is done when core_pipe_limit is
> exceeded).
>
> The value of core_pattern (and other core_* sysctls) should probably belong
> to the mount namespace the proc filesystem used for setting its value
> was in - or the matching namespace of calling process when set via syscall.
>
> Bruno
>
>
> .
>