"9 in 10 attendees" is a probable claim, as it's been said that "marijuana was the most widely used drug, with many youths maintaining that practically everyone in the audience was smoking."

The "no violence" claim originates from Dr. William Abruzzi, the chief medical officer for the festival, who was quoted as saying, "There has been no violence whatsoever, which is remarkable for a crowd of this size. These people are really beautiful."

Irrespective of the fact of whether or not there was any violence reported, the quote you bring up is an oversimplification. There's a lot that goes on in a gathering of 400,000 people over a three-day span, and the claim makes it seem as if the festival was pure bliss and peace (especially due in part, and in argument of marijuana), which wasn't the case. There is a certain romance attached to Woodstock, and that obviously has a part in this meme.

Outside of the topic of violence, there were two deaths that took place over the course of the festival – one being an overdose, the other being an incident with a tractor.

Furthermore, 4,000 people were treated for injuries, illnesses, and drug-use related matters (approximately ~400 of the 4,000 were drug related). Eighty arrests were made on drug charges, as well.

In a way, the meme is accurate. However, it purports a weak and fallacious argument (which is the sweeping statement that there being no major outbreaks of violence is consequential to the prevalent use of marijuana).

Barnard L. Collier, "Tired Rock Fans Begin Exodus." The New York Times, 1969. See the article here.