Not needing run_init for password-less service management


Sven Vermeulen Tue 09 April 2013

One of the things that has been bugging me was why, even with having set in /etc/pam.d/run_init, I cannot enjoy passwordless service management without using run_init directly:

# rc-service postgresql-9.2 status
Authenticating root.

# run_init rc-service postgresql-9.2 status
Authenticating root.
 * status: started

So I decided to strace the two commands and look for the differences. I found out that there is even a SELinux permission for being able to use the rootok setting for passwords! Apparently, is SELinux-aware and does some additional checks.

Although I don't know the exact details of it, it looks for the context before the call (exec) of run_init occurred. Then it checks if this domain has the right for passwd { rootok } (unless SELinux is in permissive, in which case it just continues) and only then it allows the "rootok" to succeed.

Now why doesn't this work without using run_init? I think it has to do with how we integrate run_init in the scripts, because out of the trace I found that the previous context was also run_init_t (instead of sysadm_t):

20451 open("/proc/self/task/20451/attr/current", O_RDONLY) = 3
20451 read(3, "root:sysadm_r:run_init_t\0", 4095) = 25
20451 close(3)                          = 0
20451 gettid()                          = 20451
20451 open("/proc/self/task/20451/attr/prev", O_RDONLY) = 3
20451 read(3, "root:sysadm_r:run_init_t\0", 4095) = 25
20451 close(3)

Because there already is a transition to run_init_t upon calling the scripts, the underlying call to runscripts causes the "previous" attribute to be set to run_init_t as well, and only then is run_init called (which then causes the PAM functions to be called). But by prepending the commands with run_init (which quickly causes the PAM functions to be called) the previous context is sysadm_t.

I tested on a system with the following policy update, and this succeeds nicely.

policy_module(localruninit, 1.0)

  class passwd { passwd chfn chsh rootok };
  type run_init_t;

allow run_init_t self:passwd rootok;

I'll probably add this in Gentoo's policy.