Linux (Ubuntu or otherwise) can use many sources of entropy to generate random numbers, including the timing of hardware access such as key presses and mouse movements, as well as a hardware generator such as RdRand if available. Entropy is also saved from one boot to the next.
If done right (and Linux does it right — not perfect, but good enough to combat the threat of a backdoored hardware RNG), combining different sources of entropy can only make for better entropy, not worse. Indeed, it is expected that the sources of entropy are individually not good enough, and only their combination is. So even if RdRand is crippled and has less entropy than the hardware manufacturer pretends, this does not weaken Linux's random number generator.
For a more in-depth explanation, see Could RDRAND (Intel) compromise entropy?
There is an indirect threat which is that none of the sources of entropy are good enough. It isn't that RdRand is compromised in itself, but that a system might be counting on it as its sole good entropy source. This is especially likely on an embedded system which lacks other hardware that exhibits randomness. Feeding /dev/random entropy pool? has some suggestions of additional entropy sources.