On Fri, Dec 13, 2024 at 04:52:02PM -0800, Douglas Anderson wrote: > The code for detecting CPUs that are vulnerable to Spectre BHB was > based on a hardcoded list of CPU IDs that were known to be affected. > Unfortunately, the list mostly only contained the IDs of standard ARM > cores. The IDs for many cores that are minor variants of the standard > ARM cores (like many Qualcomm Kyro CPUs) weren't listed. This led the > code to assume that those variants were not affected. > > Flip the code on its head and instead list CPU IDs for cores that are > known to be _not_ affected. Now CPUs will be assumed vulnerable until > added to the list saying that they're safe. > > As of right now, the only CPU IDs added to the "unaffected" list are > ARM Cortex A35, A53, and A55. This list was created by looking at > older cores listed in cputype.h that weren't listed in the "affected" > list previously. There's a list of affected CPUs from Arm here: https://developer.arm.com/Arm%20Security%20Center/Spectre-BHB (obviously only covers their own designs). So it looks like A510 and A520 should be unaffected too, although I didn't check exhaustively. It also looks like A715 is affected but the whitepaper doesn't tell you what version of 'k' to use... > Unfortunately, while this solution is better than what we had before, > it's still an imperfect solution. Specifically there are two ways to > mitigate Spectre BHB and one of those ways is parameterized with a "k" > value indicating how many loops are needed to mitigate. If we have an > unknown CPU ID then we've got to guess about how to mitigate it. Since > more cores seem to be mitigated by looping (and because it's unlikely > that the needed FW code will be in place for FW mitigation for unknown > cores), we'll choose looping for unknown CPUs and choose the highest > "k" value of 32. I don't think we should guess. Just say vulnerable. > The downside of our guessing is that some CPUs may now report as > "mitigated" when in reality they should need a firmware mitigation. > We'll choose to put a WARN_ON splat in the logs in this case any time > we had to make a guess since guessing the right mitigation is pretty > awful. Hopefully this will encourage CPU vendors to add their CPU IDs > to the list. Hmm. We shouldn't have to guess here as the firmware mitigation is discoverable. So if it's unavailable and we're running an a CPU which needs it, then we're vulnerable. Will
