Asalamu Alaykum warahmatullahi wabarakatu.
Is this salawat authentically from ibn Mashish rh, and may one just recite it or does it need permission?
Yes it is narrated from him with its chain of transmission and there is agreement as to its attribution to him. All salawats can be recited without permission but you will not understand it without a master teaching you its meanings.
Hajj Gibril Haddad