4

Basically, What is going on here and what am I not understanding?

I have a set of subuids for my user. I want to chown a file to specific subuid which is part of this user's allocation

administrator@host:/home/administrator$ cat /etc/subuid
root:100000:65536
administrator:165536:65536
administrator:1000000:9000001

administrator@host:/home/administrator$ cat /etc/subgid
root:100000:65536
administrator:165536:65536
administrator:1000000:9000001

Trying to chown this file is failing despite this subuid being part of the allocation.

administrator@host:/home/administrator$ ls -lhat
...
-rw-rw-r-- 1 administrator administrator  229 Aug  2 13:00 file
drwxrwxr-x 7 administrator administrator 4.0K Aug  2 13:00 ..

administrator@host:/home/administrator$ chown 1500000:1500000 file
chown: changing ownership of 'file': Operation not permitted

administrator@host:/home/administrator$ stat file
  File: file
  Size: 229             Blocks: 8          IO Block: 4096   regular file
Device: 802h/2050d      Inode: 658357      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/administrator)   Gid: ( 1004/administrator)
Access: 2018-08-02 13:00:36.529197108 +0000
Modify: 2018-08-02 13:00:36.529197108 +0000
Change: 2018-08-02 13:00:36.529197108 +0000
 Birth: -
administrator@host:/home/administrator$

However, I can remove the file as user, if I use sudo to chown it - but it shows as a write protected file when I do. This indicates I do in fact have permissions to modify files with this subuid.

administrator@host:~$ touch file
administrator@host:~$ chown 1500000:1500000 file
chown: changing ownership of 'file': Operation not permitted

administrator@host:~$ sudo chown 1500000:1500000 file    
administrator@host:~$ rm file
rm: remove write-protected regular empty file 'testfile.txt'?
administrator@host:~$

Can anyone explain the inner workings which is going on here? I've probably misunderstood something basic somewhere. I can't tag this as subuid because not enough rep, so I'll use uid.

Thanks!

  • What is the use case for using subgid/subuid outside of namespaces? – anx Aug 7 '18 at 15:49
  • @anx I'm using unprivileged lxc containers. I have a set of files that are located on the host and bind mounted inside a container. If the files on the host are owned by the unprivileged user, they are owned as nobody inside the container and therefore are read only. I need to set the correct subuid on those files so the container user can modify etc. Hope that makes sense. – TechnicalChaos Aug 8 '18 at 8:04
  • Access: (0664/-rw-rw-r--) Uid: ( 1000/administrator) Gid: ( 1004/administrator) Neither administrator:165536:65536, nor administrator:1000000:9000001 owns the file. However @bgtvfr's answer tells you the answer to your "What is going on here and what am I not understanding" question. Non-privileged user cannot assign ownership of a file to person or group for which they do not have privileges. – Xalorous Aug 9 '18 at 22:02
2

There is a program lxc-usernsexec that comes along with lxc. This allows you to remap user id's using the maps /etc/subuid and /etc/subgid.

Specifically, you can do the following.

  1. lxc-usernsexec -- touch /tmp/test
  2. ls -l /tmp/test will show that the file is owner:group the same as the first subuid:subgid pair in your map.
  3. rm /tmp/test should give an error since you do not currently have the right uid/gid.
  4. lxc-usernsexec -- rm /tmp/test should remove the file.

Hope this helps! The above probably requires various things setup for unprivileged LXC container use. In particular, I think /proc/sys/kernel/unprivileged_userns_clone should be 1.

  • Thanks! This is great! I'd give you the bounty but it was autoassigned whilst I was holidaying. At first glance I'm getting an error - [pid 18428] write(2, ...failed to write mapping "newuidmap: ... Operation not permitted" and [pid 18429] setgid(0) = -1 EINVAL (Invalid argument). I've ensured that unprivileged_userns_clone is set to 1 but there's likely some other flag needs sorting. I can work with this. All in all though this seems to be the right tool for the job, which is what I was looking for, so thank you. – TechnicalChaos Aug 21 '18 at 8:49
9
+25

Subuids aren't meant to work the way you expect them to work. They are designed to map UIDs in a user namespace to different UIDs outside that namespace, which comes in handy (and actually was designed) for containers.

However, a process still can have only one UID set (user namespace), and users are not permitted to change ownership of files, for obvious security reasons. It doesn't matter, as far as the process itself is concerned, if the user is actually someone else outside the namespace.

This is why the chown command fails: it doesn't matter if you could have some other UID, should the namespace was different, at that moment, you don't have that UID, and therefore, you can't change the ownership of any files (since you're not root).

As of why can you remove the file: it has actually nothing to do with subuids, instead, it all depends on you having the ownership of the directory the file resides in. Since file deletion changes the directory, and not the file itself, if you can write the directory, you can remove any files from that (except for "sticky" directories).

3

Your problem has nothing to do with subuid .

According to https://superuser.com/questions/697608/chown-operation-not-permitted

Non-privileged users (not root) cannot chown files to other user names. To use chown, a user must have the privileges of the target user. In other words, only root can give a file to another user.

  • 2
    Setuid and setgid are always cleared on chown. – RalfFriedl Aug 6 '18 at 18:22
  • "To use chown, a user must have the privileges of the target user." - I would have assumed that with subuids, the privileges of the target user are the same as the privileges of the user that has the subuids set against it? Essentially chowning to itself, just with a different uid. – TechnicalChaos Aug 8 '18 at 8:06

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