RE: pipefs issue

On Thu, 2007-05-31 at 13:02 -0700, Clarkson, Mike R (US SSA) wrote:
> Thanks for the info on the 3 pipes that Java creates. I had a feeling
> that this is what was going on, but I'm still confused on one point. The
> c++ executable is not writing anything to stdout or stderr, it writes
> it's output to a file. The AVC messages below indicate that the import_t
> domain (the c++ executable) is attempting to write to the pipe. Is there
> some implicit writing that occurs between the processes?

SELinux checks permissions when a file descriptor is opened, inherited across execve or received via local IPC, so you will see attempted accesses at those times even if the descriptor is never actually used for a read or write. In this case, the pipe descriptors are inherited across execve and checked at that time (and closed and replaced with descriptors to the null device if access is denied). So if you don't actually need to allow the accesses, you can just add dontaudit rules to suppress the audit messages and SELinux will silently remap those descriptors to /dev/null for you.

> Also, it looks like SELinux is not allowing me to write up. Is that
> standard? The BLP model typically allows writing up.

Many MLS systems don't allow writing up without privilege (as that allows a low process to potentially corrupt high data), and the default mls constraints in SELinux policy are written that way. You of course can change them in your policy/mls file if you want, and just use Type Enforcement to provide integrity protection.

> The only mls attribute that I've seen that addresses writing up is
> mlsfilewritetoclr, which looks like it allows writing to the high end of
> a process range. Is that correct?

mlsfilewritetoclr is for writing up to your high/clearance, while mlsfilewrite allows arbitrary writing (up or down) by the process, and mlstrustedobject allows any process to write to the object.

mlsfilewriteinrange looks a bit odd to me.

Again, these can be customized to your needs.

-- 
Stephen Smalley
National Security Agency


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