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

[Randy Dunlap]

On 6/16/23 02:39, Jordy Zomer wrote:
> 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? :)
> 

Yes, please. We don't want to lose that info.

Thanks.

> 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.

-- 
~Randy