Thanks for the explanation Pawan, a little bit off-topic for this patch but shall I send a patch to add this to the documentation of array_index_nospec() and fix other calls to that function where the upper bound is not a constant? :) On Fri, Jun 16, 2023 at 5:15 AM Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote: > > 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.
