This is one of two lovely side-channel vulnerabilities in Intel processors released today. Apparently they also leak RDRAND and RDSEED data across cores and took 18 months to fix it: https://www.vusec.net/projects/crosstalk/
Entire memory bus as in entire access to any kind of shared memory (shared CPU caches, RAM, ...)? If yes, wow, that's a pretty desperate mitigation. Doesn't that starve large parts of the whole CPU whenever an affected, but mitigated instruction is executed, as all other cores can't interact with memory anymore for the duration of this execution? Doesn't that also open the ability to "DoS" CPUs by constantly executing such instructions?
Fortunately the Linux kernel can kill offenders with SIGBUS: https://www.phoronix.com/scan.php?page=news_item&px=Linux-Sp...
I guess this is the only solution for Intel (apart from an hardware fix) as one core doesn't have control over the loads of another core.
And wouldn't you be able to detect RdRand instruction execution for (for example) key generation. Pinpointing when processes perform sensitive operations.
And, would it be possible to use RdRand execution as a time based exfiltration channel? Run a timing loop in one process on one core. Use RdRand reading with different frequency in a second process to (using morse code or similar) to encode information. The first process use its timing loop to recover the information.