U.S. stocks got a much-needed boost Monday when the market had its biggest rally in a month on speculation that European leaders will act to prevent a debt crisis from getting worse. The Dow jumped 272 points, or 2.5 percent, to 11,043. The S&P 500 rose 2.3 percent. A spokesman for the European Commission said that discussions were underway to extend the euro zone’s bailout fund. Warren Buffett’s Berkshire Hathaway led the blue-chip rally after announcing that it will repurchase shares for as much as 110 percent of their book price, saying the stock was undervalued after falling 17 percent this year.