Re: [PATCH v2 1/1] cdrom: Fix spectre-v1 gadget

[Pawan Gupta]

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.