Re: sshd and setcurrent

[Daniel J Walsh <dwalsh@redhat.com>]

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On 03/03/2014 08:37 PM, Andy Ruch wrote:
> Hello,
> 
> 
> I'm implementing a custom SELinux policy based on the reference policy for
> a secure RHEL 6.5 system. Currently, my policy does not allow 'setcurrent'
> for sshd_t and I'm seeing the appropriate AVCs associated to this. However,
> even with these AVCs, everything appears to work. I still end up with a
> bash process running as staff_t. The only difference I see using 'ps -efZ'
> is the context of parent process of the bash shell:
> 
> PERMISSIVE:
> 
> system_u:system_r:sshd_t:s0-s0:c0.c1023    root    29470    1520     ...
> sshd: myuser [priv] myuser_u:staff_r:staff_t:s0-s0:c0.c1023    myuser
> 29475    29470    ...    sshd: myuser@pts/0 
> myuser_u:staff_r:staff_t:s0-s0:c0.c1023    myuser  29476    29475    ...
> -bash
> 
> 
> 
> ENFORCING (context on 2nd line different):
> 
> system_u:system_r:sshd_t:s0-s0:c0.c1023    root    29505    1520     ...
> sshd: myuser [priv] system_u:system_r:sshd_t:s0-s0:c0.c1023    myuser
> 29510    29505    ...    sshd: myuser@pts/0 
> myuser_u:staff_r:staff_t:s0-s0:c0.c1023    myuser  29511    29510    ...
> -bash
> 
> 
> Does this second process labeled as "sshd_t" provide any usability or
> security issues?
> 
> 
> Digging into the policy code more, I see comments in domain.te along the
> lines of "setcurrent breaks process tranquility" and "don't use it if you
> don't understand it". Could someone provide some more details regarding
> when to use 'setcurrent' for a secure system?
> 
> 
> Also, I understand that the RHEL Targeted policy has a completely different
> use case than my custom policy. However, why is it allowed in the Targeted
> policy if the sshd appears to work without it?
> 
> 
> Thanks, Andy Ruch
> 
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> 
The way sshd works is to setup a process running as the user without doing a
fork/exec.  This is why we did a setcurrent in sshd.  The problem we were
trying to solve, is if you have a user type that you do not want to prevent
running as a service, like guest_t, you can get around it by using sshd tunnels.

By setting this process to guest_t rather then sshd_t, the policy works as you
would expect.  Also since this process is running as the User UID it might be
able to be influenced by other processes with the same UID.  Running as sshd_t
could be a risk.

The problem with setcurrent is that it is not as clean as a setexeccon.  It is
always better to do a fork/exec from an SELinux point of view.  But if the app
does not implement this, we think setcurrent is a decent fall back.


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