Warren Buffett's Berkshire Hathaway has been busy buying stocks, including a new stake in General Motors.

Berkshire's just-released 13-F filing with the SEC shows two new stocks were added to its portfolio during the first quarter of the year, and stakes were increased for another seven companies.

It also appears Berkshire was doing even more buying in the first three months of the year that it hasn't disclosed in this filing. It says that "confidential information has been omitted ... and filed separately" with the SEC.

That's often a sign Buffett is accumulating a large stake in a company but doesn't want copycats to drive up the price until he's done. Last year the omitted confidential information shielded Buffett's multi-month purchase of a $11 billion stake in IBM.