Justin Bieber --I'm Gonna Get Hannibal Lectured At the Roast

Comedy Central is pulling out the big guns for Justin Bieber's roast ... and we've learned two comedians who don't hold back have signed on.

Sources involved in the roast tell TMZ ... the network has locked down both Jeff Ross and Hannibal Buress for the big show next month. You'll recall, Ross has already said he was ready to destroy.

As for Buress ... well, you can ask Bill Cosby about that.

Kevin Hart will officiate as Roast Master, and there are some huge names producers are now gunning to get.

Never say never Justin ... about backing out.