Re: Problem in understanding container permissions

[Lennart Poettering <lennart@xxxxxxxxxxxxxx>]

On Mi, 03.07.19 12:24, Kai Bojens (kb@xxxxxxxxxx) wrote:

> Ubuntu 18.04, HWE kernel 4.18.0-25-generic, systemd 237-3ubuntu10.23
>
> I have created a nspawn container with a minimal Ubuntu and booted the container
> without any problems. There were no problems and I used the default settings.
> Now I see some strange permission errors which I can't explain:
>
>  Inside the container:
>
> root@container:/var/log# ls -alt
> total 356
> -rw-r--r--  1 root   root    203203 Jul  3 09:17 dpkg.log
> drwxr-xr-x  1 root   root        60 Jul  3 09:17 apt
> -rw-r--r--  1 root   root      9046 Jul  2 15:04 alternatives.log
> -rw-------  1 root   root      6784 Jul  2 15:04 tallylog
> -rw-r--r--  1 root   root      3392 Jul  2 15:04 faillog
> -rw-r-----  1 nobody nogroup  40658 Jul  2 10:14 syslog
> -rw-rw-r--  1 nobody nogroup  16128 Jul  2 10:14 wtmp
> -rw-r-----  1 nobody nogroup   6234 Jul  2 10:14 auth.log
> -rw-rw-r--  1 nobody nogroup  30660 Jul  2 10:06 lastlog
> -rw-rw----  1 nobody nogroup    384 Jul  1 14:02 btmp
> drwxrwxr-x  1 nobody nogroup    182 Jul  1 14:02 .
> drwxr-sr-x+ 1 nobody nogroup     64 Jul  1 14:02 journal
> -rw-r--r--  1 root   root     60952 Jul  1 13:59 bootstrap.log
> drwxr-xr-x  1 root   root        90 Jul  1 13:56 ..
> root@container:/var/log# whoami
> root
> root@container:/var/log# tail syslog
> tail: cannot open 'syslog' for reading: Permission denied
>
>
>  Outside the container:
>
> root@container:/var/lib/machines/xy-test/var/log# ls -alt
> total 356
> -rw-r--r--  1 198180864       198180864 203203 Jul  3 09:17 dpkg.log
> drwxr-xr-x  1 198180864       198180864     60 Jul  3 09:17 apt
> -rw-r--r--  1 198180864       198180864   9046 Jul  2 15:04 alternatives.log
> -rw-------  1 198180864       198180864   6784 Jul  2 15:04 tallylog
> -rw-r--r--  1 198180864       198180864   3392 Jul  2 15:04 faillog
> -rw-r-----  1 syslog    adm              40658 Jul  2 10:14 syslog
> -rw-rw-r--  1 root      utmp             16128 Jul  2 10:14 wtmp
> -rw-r-----  1 syslog    adm               6234 Jul  2 10:14 auth.log
> -rw-rw-r--  1 root      utmp             30660 Jul  2 10:06 lastlog
> -rw-rw----  1 root      utmp               384 Jul  1 14:02 btmp
> drwxrwxr-x  1 root      syslog             182 Jul  1 14:02 .
> drwxr-sr-x+ 1 root      systemd-journal     64 Jul  1 14:02 journal
> -rw-r--r--  1 198180864       198180864  60952 Jul  1 13:59 bootstrap.log
> drwxr-xr-x  1 198180864       198180864     90 Jul  1 13:56 ..
>
> I have not touched any of these files from outside of the container. Is there
> anything obvious I have failed to see? Why would the ownership of these file
> change?

This is caused by user namespacing, i.e. the -U option of
systemd-nspawn (which is default in systemd-nspawn@.service, but not
in systemd-nspawn when invoked directly).

it means the container runs under its own UID range that is somewhere
above the 65536 boundary when looking from the outside of the
container, but looks like the normal 0…65536 inside of it. Except that
some system files are always owned by the special nobody user, because
they are system files.

It appears you ran your container once with and once without user
namespacing. You can fix that without loss, see documentation:

https://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#-U

Lennart

--
Lennart Poettering, Berlin
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