Interaction user namespace, /proc/1 ownership & cap_set

Richard Weinberger richard at nod.at
Tue Jul 2 09:45:23 UTC 2013


Am 02.07.2013 11:25, schrieb Daniel P. Berrange:
> On Tue, Jul 02, 2013 at 10:56:37AM +0200, Richard Weinberger wrote:
>> Am 02.07.2013 10:44, schrieb Eric W. Biederman:
>>> Gao feng <gaofeng at cn.fujitsu.com> writes:
>>>
>>>> On 07/02/2013 12:16 AM, Daniel P. Berrange wrote:
>>>>> I'm struggling debugging a strange problem with interaction between user
>>>>> namespaces, cap_set and ownership of files in /proc/1/
>>>>>
>>>>
>>>> This problem is occured after we call setuid/gid.
>>>>
>>>> for example, a task whose pid is 1234 calls
>>>> setregid(10,10);
>>>> setreuid(10,10);
> 
> If seems to get reset to the right values (0:0) when we execve()
> the init binary though.  This doesn't happen if we have invoked
> the capset() syscall in between the setregid & the execve() calls.
> 
>>>>
>>>>
>>>> The uid/gid of the /proc/1234 is 10:0
>>>> ll /proc/1234 -d
>>>> dr-xr-xr-x 8 uucp wheel 0 Jul  2 10:57 /proc/1234
>>>>
>>>> the uid/gid of the files under /proc/1234 are two kinds...
>>>> ll /proc/1234
>>>> dr-xr-xr-x 2 uucp wheel 0 Jul  2 10:58 attr
>>>> -rw-r--r-- 1 root root 0 Jul  2 10:58 autogroup
>>>> ...
>>>> dr-xr-xr-x 5 uucp wheel 0 Jul  2 10:58 net
>>>> dr-x--x--x 2 root root 0 Jul  2 10:58 ns
>>>> ...
>>>> dr-xr-xr-x 3 uucp wheel 0 Jul  2 10:58 task
>>>>
>>>> I checked the pre_revalidate and found the owner of the files under /proc/<pid>
>>>> will be set to the GLOBAL_ROOT_UID if the task executed setuid/setgid(task_dumpable is false).
>>>> Is this what we expected? why? 
>>>
>>> Expected yes.  Perfect perhaps not.
>>>
>>> That piece of code has not been examined to see if it is safe to use
>>> make_kuid(task_user_ns(task), 0), instead of GLOBAL_ROOT_UID.
>>>
>>>> For user namespace,the owner of /proc/1/* is incorrect and
>>>> after task call setuid/gid in user namespace, the owner of /proc/<pid-of-this-task>/* is incorrect
>>>> too.
>>>
>>> From the current semantics of dumpable GLOBAL_ROOT_UID is correct.
>>>
>>> Please double check but I believe /proc/self should continue to work,
>>> despite this.
>>
>> /proc/self is not an option. systemd (in particular some of it's tools with pid != 1) read from /proc/1/environ to find out
>> what environment variables it got to detect LXC and other visualization environments.
>> With userns enabled this check fails and systemd goes nuts because it thinks that it lives on top of a "real" Linux.
> 
> I don't even see how /proc/self would solve this, since it
> is just a symlink pointing to /proc/1 in this scenario, so
> the ownership of files at /proc/1/XXXX would still be wrong.

Yep.

> This isn't really a systemd specific problem either, I think
> any app would expect to be able to read its own files under
> /proc/$PID/

True. I was not blaming systemd.
In my case systemd is just the program which suffers from the issue.

Thanks,
//richard


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