Edit: looking at the BP disclosure, they've decided to make the accounting changes that show they're going to sell the stake, they'll likely be looking for a smart way to offload the shares, probably to some large buyer (not just pressing sell on some brokerage account).
Edit 2: the $25B figure isn't really that accurate. Before this all kicked off, Rosneft had a market cap of around $70B,and BP held about 20%, or $14B. The article sums BP's carrying value for the company ($14B, coincidentally I think), and an accumulated foreign exchange loss of $11B, which had already been charged to equity. The current market cap of Rosneft is about $30B,so the actual hit to BP sharebolders will be something like $8B if they could sell at current prices.