Re: Daemons writing into HOME_DIR

On 5/3/22 13:01, Stefan Schulze Frielinghaus wrote:
> Hi all,
> 
> In short I'm wondering what the refpolicy way is to let a daemon write into
> HOME_DIR and how those files---especially the SELinux user part---should be
> labeled?
> 
> Currently I have a daemon (systemd service) running under context
> 
>    system_u:system_r:foobar_t:s0
> 
> and the policy contains
> 
>    init_daemon_domain(foobar_t, foobar_exec_t)
> 
> The daemon reads and writes files under HOME_DIR/foobar which are labeled as
> foobar_rw_t and the policy has the following file context entry:
> 
>    HOME_DIR/foobar(/.*)? gen_context(system_u:object_r:foobar_rw_t,s0)
> 
> However, newly created files still seem to have a wrong user according to
> restorecon (the daemon runs under Linux user marge which is assigned to SELinux
> user user_u):
> 
>    $ restorecon -FRvn /home/marge/foobar
>    Would relabel /home/marge/foobar/baz from system_u:object_r:foobar_rw_t:s0 to user_u:object_r:foobar_rw_t:s0
> 
> It looks like as if user_u wins over system_u for files under HOME_DIR.  This
> does not have any effect on the functionality of the daemon, however, it still
> feels wrong to me.

This is genhomedircon setting the seuser of the files to match the seuser 
mapping in `semanage login`.  You want this behavior, especially if you have 
UBAC turned on, otherwise UBAC doesn't provide a benefit, since system_u is 
excluded from UBAC.


> So I'm wondering how to fix this and thought about:
> 
> 1) Can/Should a daemon run under a different SELinux user than system_u?

If this is a system daemon, e.g. started by systemd (pid 1) then that is not 
expected in refpolicy, not generally suggested.  If this is a daemon running out 
of a user session, such as systemd --user, then yes, it should have the user's 
seuser, e.g. user_u.


> 2) Another option, which I think is worse, would be to the change the SELinux
> user from user_u to system_u for Linux user marge under which the daemon runs.

Running an interactive user as system_u is contrary to system_u's purpose, which 
is for non-interactive system processes only.


> 3) A third option would be to keep the users as is, i.e., let the daemon run
> under system_u and let marge be assigned to user_u, but tweak the policy to keep
> the file context labels under HOME_DIR with system_u.

See my first comment.

> Any thoughts?

You could change the default_user[1] so the seuser comes from the parent 
directory, but that would change it for the entire system which may have 
unintended and worse consequences.

You're seeing the behavior I expect to see for this type of policy design.


[1] 
https://github.com/SELinuxProject/selinux-notebook/blob/main/src/default_rules.md#default_user

-- 
Chris PeBenito