Re: uid=0 inside user-namespace and procfs file permissions

[Eric W. Biederman]

Aditya Kali <adityakali@xxxxxxxxxx> writes:

> On Tue, Sep 30, 2014 at 5:35 PM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>> Aditya Kali <adityakali@xxxxxxxxxx> writes:
>>
>>> Hi all,
>>>
>>> I am trying to run a process with uid=0 inside userns. But in the when
>>> I also do capset() after setresuid(0, 0, 0), I am seeing inconsistent
>>> proc file permissions. Almost all the files in /proc/<pid>/ has global
>>> 'root' as owner and group even if the actual process uid is correctly
>>> changed.
>>>
>>> I wrote a simple program that demonstrate the issue:
>>>
>>> 1. parent, as global root (uid=0 in init_user_ns) fork()s a child
>>> 2. child:
>>>     a) unshare(CLONE_NEWUSER)
>>>     b) [wait for parent to write uid_map]
>>>     c) setresgid(id, id, id) ; setresuid(0, 0, 0);
>>>     d) conditionally call capset() to clear capabilities
>>>     e) execve(/bin/sleep)
>>> 3. parent:
>>>     a) populates child's uid_map and maps some uid to 0 inside userns. ex:
>>>     0 99 1
>>>     b) waitpid()
>>>
>>> (the actual program can be found at http://pastebin.com/f4P17VFn for
>>> your reference).
>>>
>>> When there is no capset() call after setresuid(0,0,0), everything is
>>> fine. But when I do a capset() to clear all capabilities, the 'owner'
>>> and 'group' of all the files under /proc/<child_pid>/  of the child
>>> process are reverted to global 'root' user.
>>>
>>> # without capset (2.d):
>>> root@vm1# id
>>> uid=0(root) gid=0(root) groups=0(root)
>>>
>>> root@vm1# ./userns_uid0
>>> child_pid: 24277
>>> proc_file: /proc/24277/uid_map
>>> proc_file: /proc/24277/gid_map
>>> child resuming
>>>
>>> ^Z
>>> [1]+  Stopped                 ./userns_uid0
>>> root@vm1# cat /proc/24277/uid_map
>>>          0         99          1
>>> root@vm1# cat /proc/24277/status | grep -e "Uid:" -e "Gid:"
>>> Uid: 99 99 99 99
>>> Gid: 99 99 99 99
>>> root@vm1# ls -l /proc/24277/
>>> total 0
>>> dr-xr-xr-x 2 nobody nobody 0 2014-09-30 16:31 attr
>>> -r-------- 1 nobody nobody 0 2014-09-30 16:31 auxv
>>> -r--r--r-- 1 nobody nobody 0 2014-09-30 16:31 cgroup
>>> --w------- 1 nobody nobody 0 2014-09-30 16:31 clear_refs
>>> -r--r--r-- 1 nobody nobody 0 2014-09-30 16:31 cmdline
>>> -rw-r--r-- 1 nobody nobody 0 2014-09-30 16:31 comm
>>> -rw-r--r-- 1 nobody nobody 0 2014-09-30 16:31 coredump_filter
>>> -r--r--r-- 1 nobody nobody 0 2014-09-30 16:31 cpuset
>>> ...
>>> [All files have owner='nobody' and group='nobody' .. same as that of
>>> the process]
>>>
>>> With the additional capset() call, the files under /proc/<child_pid>/
>>> are now owned by global root:
>>>
>>> root@vm1# ./userns_uid0 resetcaps
>>> child_pid: 24706
>>> proc_file: /proc/24706/uid_map
>>> proc_file: /proc/24706/gid_map
>>> child resuming
>>> resetting caps
>>> ^Z
>>> [2]+  Stopped                 ./userns_uid0 resetcaps
>>> root@vm1# cat /proc/24706/uid_map
>>>          0         99          1
>>> root@vm1# cat /proc/24706/status | grep -e "Uid:" -e "Gid:"
>>> Uid: 99 99 99 99
>>> Gid: 99 99 99 99
>>>
>>> [Everything as before till now]
>>>
>>> root@vm1# ls -l /proc/24706/
>>> total 0
>>> dr-xr-xr-x 2 nobody nobody 0 2014-09-30 16:47 attr
>>> -r-------- 1 root   root   0 2014-09-30 16:47 auxv
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 cgroup
>>> --w------- 1 root   root   0 2014-09-30 16:47 clear_refs
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 cmdline
>>> -rw-r--r-- 1 root   root   0 2014-09-30 16:47 comm
>>> -rw-r--r-- 1 root   root   0 2014-09-30 16:47 coredump_filter
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 cpuset
>>> ...
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 mountinfo
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 mounts
>>> -r-------- 1 root   root   0 2014-09-30 16:47 mountstats
>>> dr-xr-xr-x 5 nobody nobody 0 2014-09-30 16:47 net
>>> dr-x--x--x 2 root   root   0 2014-09-30 16:47 ns
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 numa_maps
>>> ...
>>> -r--r--r-- 1 root   root   0 2014-09-30 16:47 status
>>> -r-------- 1 root   root   0 2014-09-30 16:47 syscall
>>> dr-xr-xr-x 3 nobody nobody 0 2014-09-30 16:47 task
>>> ..
>>>
>>> Only the directories 'attr', 'net' and 'task' are owned by the uid=99.
>>> Rest all files are owned by global root.
>>>
>>> This behavior seems inconsistent. I ran this on 3.17 kernel. Can
>>> someone with expertise in this area explain if this is expected?
>>
>> So I am not quite certain what you are seeing.
>>
>> In general proc files are expected to be owned by the euid of a process.
>> However when the task_dumpable is cleared the files become owned by the
>> global root user.  We have considered relaxing that to the namespace
>> root user but so far implementing a more granular task_dumpable has not
>> been done.
>>
>
> I tried explicitly setting PR_SET_DUMPABLE before execve(), but that
> didn't either.
>
>> The directories are world readable so they don't matter.
>>
>> What puzzles me is that you have directories owned by nobody, and you
>> are talking about uid = 99 and gid = 99.  Nobody is traditionally
>> (u16_t)-2 and there should never actually be used by anyone.   And is
>> used as the default number of unmapped uids and gids.
>>
>> It looks like you are doing something weird with nobody so I don't have
>> a clue what is actually going on.
>>
>
> The issue is not specific to uid 99 or "nobody". Its just a dummy user
> I have for testing. The issue happens with any user with non-zero uid.

But my issue with reading your directory listings of proc is.

I can't tell if you are giving me a listing of proc from a process in
the user namespace or outside of the user namespace.

If the process 24706 had uid == 99 and gid == 99 (outside of the user
namespace).  And your are listing the files from outside of the user
namespace.  And uid 99 is mapped to nobody in /etc/passwd and
gid 99 is mapped to nobody in /etc/group.  And your ls process is
not running in your user namespace.  Then this looks like proper
handling of dumpable.   Otherwise I don't have a clue what is going on
because I can't make sense of your directory listings.

Eric
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