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by wahern 2904 days ago
> having user, group and world is lacking in a lot of situations by today standards

A process has supplemental group membership. If you were only limited to the above, then, yes, you would need to share resources through world or utilize a special broker if you wanted multiple privilege domains for a service.

But with supplemental groups you can be as fine-grained as you want. Just create a group for each domain with a unique membership set and make that a supplemental group for each user with access to that domain. When you want to share a file, just chown and chmod appropriately.

To see how this works in practice, look at BSDAuth (OpenBSD's alternative to PAM). Rather than using dynamically loadable modules for authentication methods--which effectively means that any program that wants to use authentication services needs root permissions--BSDAuth uses setuid and setgid helpers in /usr/libexec/auth.

  $ ls -lhd /usr/libexec/auth/ 
  drwxr-x---  2 root  auth   512B Mar 24 13:23 /usr/libexec/auth/

  $ doas ls -lh /usr/libexec/auth/
  total 372
  -r-xr-sr-x  4 root  _token   21.3K Mar 24 13:23 login_activ
  -r-sr-xr-x  1 root  auth      9.0K Mar 24 13:23 login_chpass
  -r-xr-sr-x  4 root  _token   21.3K Mar 24 13:23 login_crypto
  -r-sr-xr-x  1 root  auth     17.2K Mar 24 13:23 login_lchpass
  -r-sr-xr-x  1 root  auth      9.0K Mar 24 13:23 login_passwd
  -r-xr-sr-x  1 root  _radius  17.1K Mar 24 13:23 login_radius
  -r-xr-xr-x  1 root  auth      9.0K Mar 24 13:23 login_reject
  -r-xr-sr-x  1 root  auth      9.0K Mar 24 13:23 login_skey
  -r-xr-sr-x  4 root  _token   21.3K Mar 24 13:23 login_snk
  -r-xr-sr-x  4 root  _token   21.3K Mar 24 13:23 login_token
  -r-xr-sr-x  1 root  auth     21.0K Mar 24 13:23 login_yubikey
In this scheme, for any user who you want to grant permission to test basic system authentication you add the auth group to their supplemental group. Et voila, they can now use the framework, but without root permissions. For mechanisms that you may want to grant separately you use a separate group (e.g. _radius).

Like many bad ideas, Docker basically won because you can use it to isolate and package pre-existing, broken software. And because it's easier to find examples of how to [abuse] Docker for sharing files across security domains than on how to use supplemental groups to do this.
1 comments
wahern 2904 days ago
I just recalled an interesting difference between BSD and SysV (including Linux[1]) regarding group ownership.

On BSD when you create a file the group owner is set to the group owner of the directory. From the BSD open(2) manual page:

  When a new file is created it is given the group of the
  directory which contains it.
But on Linux the [default] behavior is to set the group owner to the creating process' effective group. From the Linux open(2) man page:

  The  group  ownership  (group ID) is set either to the
  effective group ID of the process or to the group ID of the
  parent  directory  (depending  on file system type and
  mount options, and the mode of the parent directory, see
  the  mount  options  bsdgroups and sysvgroups described in
  mount(8)).
The BSD behavior is more convenient when it comes to using shared supplemental groups because you can often invoke existing programs with a umask of 0002 or 0007 and files they create (without subsequently modifying permissions) will usually have the desirable permissions--specifically, writable by any process which has a supplemental group of the directory they're located in. For example, with BSD semantics Git technically wouldn't have needed a special core.sharedRepository setting. But because of SysV semantics on Linux Git had to be modified to explicitly read the group of the parent directory and explicitly change the group owner of newly created files.
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