We'll never know 100% but Ukraine controlled the air so only the rebels had motive to shoot down planes, and had bragged about it on social media.
It's possible that Ukrainian forces or someone else had a conspiracy to frame the rebels (if you have a good conspiracy theory, Russian propaganda would love to hear it). But this is very unlikely as it's a high-risk, low pay-off strategy and even Russian leaders are usually not that evil or stupid.
They'd need more than the will to frame the separatists; they'd also have to get a big lumbering BUK launcher right into the middle of a separatist stronghold, sandwiched between Donetsk and Russia, crew it, set it up, launch the missile, and then get the thing out undetected.
It's possible that Ukrainian forces or someone else had a conspiracy to frame the rebels (if you have a good conspiracy theory, Russian propaganda would love to hear it). But this is very unlikely as it's a high-risk, low pay-off strategy and even Russian leaders are usually not that evil or stupid.