Re: permission denied without an (obvious) reason when changing directory permissions

Hi Philippe


> I'm running into a SELinux permission issue when simply changing the ownership of a directory and I've got no clue why this happens.
>
> The program in question is smokeping. It runs as root with the context of "system_u:system_r:smokeping_t" and tries to write to /var/lib/smokeping/rrd.
> When having /var/lib/smokeping (and its subfolders) owned by root, everything works fine.
> As soon as I change the ownership to apache:apache and remove permissions for other users (e.g. 0770), an EACCES pops up but no avc denied shows up in the audit log.
>
> Here's what I got so far:
>
> $ ls -dZ /var/lib/smokeping/rrd
> drwxr-xr-x. root root system_u:object_r:smokeping_var_lib_t:s0 /var/lib/smokeping/rrd
> $ runcon -t smokeping_t -r system_r smokeping --debug
> # (works fine)
>
> $ chown apache: /var/lib/smokeping/rrd
> $ chmod 770 /var/lib/smokeping/rrd
> $ ls -dZ /var/lib/smokeping/rrd
> drwxrwx---. apache apache system_u:object_r:smokeping_var_lib_t:s0 /var/lib/smokeping/rrd
> $ runcon -t smokeping_t -r system_r smokeping --debug
> # (breaks)
>
> an strace shows:
>
> $ grep -h EACCES /tmp/smokeping.pid.*
> open("/etc/shadow", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
> stat("/var/lib/smokeping/rrd/foo.rrd", 0x1219138) = -1 EACCES (Permission denied)
> open("/var/lib/smokeping/rrd/foo.rrd", O_RDWR|O_CREAT|O_TRUNC, 0666) = -1 EACCES (Permission denied)
> stat("/var/lib/smokeping/rrd/foo~bar.rrd", 0x1219138) = -1 EACCES (Permission denied)
> open("/var/lib/smokeping/rrd/foo~bar.rrd", O_RDWR|O_CREAT|O_TRUNC, 0666) = -1 EACCES (Permission denied)
> stat("/var/lib/smokeping/rrd/foo~baz.rrd", 0x1219138) = -1 EACCES (Permission denied)
> open("/var/lib/smokeping/rrd/foo~baz.rrd", O_RDWR|O_CREAT|O_TRUNC, 0666) = -1 EACCES (Permission denied)
> stat("/var/lib/smokeping/rrd/threshold", 0x1219138) = -1 EACCES (Permission denied)
> mkdir("/var/lib/smokeping/rrd/threshold", 0755) = -1 EACCES (Permission denied)

As the process runs confined, root is not the same root as if you run it 
interactively after sudo su - or the like.

Root for the somkeping process is like a normal user. It can't override 
DAC permissions. If root has no permissions to write to the folder it 
simply can't overrule that permission because DAC_OVERRIDE is denied. 
There should be some audit messages logged with dac_override .

A Dan Walsh blogpost about dac override with some details: 
https://danwalsh.livejournal.com/80232.html

- Thomas
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