Which LaTeX fonts are open source? Someone told me they are all, but I am not sure...
You can check the license for each and every font on CTAN. Go to the fonts section: ctan.org/topic/font, find the font you are interested in and check how it is licensed. If in doubt do consult the documentation! (Another good starting place is The LaTeX Font Catalogue. Just follow the links from there to CTAN.)
An example: The license information for
EB Garamond can be found under: www.ctan.org/tex-archive/fonts/ebgaramond