On Fri, Jun 16, 2023 at 12:31:50AM +0100, Phillip Potter wrote: > I've now looked at this. It is possible for cdi->capacity to be > 1, as > it is set via get_capabilities() -> cdrom_number_of_slots(), if the > device is an individual or cartridge changer. Ohk. Is there an upper limit to cdi->capacity? If not, we are left with barrier_nospec(). > Therefore, I think using CDI_MAX_CAPACITY of 1 is not the correct > approach. Jordy's V2 patch is fine therefore, but perhaps using > array_index_nospec() with cdi->capacity is still better than a > do/while loop from a performance perspective, given it would be cached > etc. at that point, so possibly quicker. Thoughts? (I'm no expert on > spectre-v1 I'll admit). array_index_nospec() can only clip the arg correctly if the upper bound is correct. Problem with array_index_nospec(arg, cdi->capacity) is cdi->capacity is not a constant, so it suffers from the same problem as arg i.e. cdi->capacity could also be speculated. Although having to control 2 loads makes the attack difficult, but does not rules out completely. barrier_nospec() makes the CPU wait for all previous loads to retire before executing following instructions speculatively. This causes the conditional branch to resolve correctly. I hope this does not fall into a hotpath.
