Linux is not public domain software. It is licensed according to the GNU General Public License, which has a strict set of rules for use.

If you take the Linux kernel or any Linux utility and modify it, port it, or add features to it, you must make the source available to anyone who asks for it. If you are not careful you may give up the rights to your proprietary software unintentionally. It is best to always consult your attorney on all open-source copyright issues.