Re: [PATCH 0/5] Introduce support for PSF mitigation

[Saripalli, RK]

Josh, PSF being new, may be in the future someone will find something new.

This is really extra precaution. 



On 4/9/2021 11:45 AM, Josh Poimboeuf wrote:
> On Thu, Apr 08, 2021 at 09:56:47AM -0500, Saripalli, RK wrote:
>> Josh, thank you for taking the time to review the patches.
>>
>> On 4/7/2021 5:39 PM, Josh Poimboeuf wrote:
>>> On Tue, Apr 06, 2021 at 10:49:59AM -0500, Ramakrishna Saripalli wrote:
>>>> Because PSF speculation is limited to the current program context,
>>>> the impact of bad PSF speculation is very similar to that of
>>>> Speculative Store Bypass (Spectre v4)
>>>>
>>>> Predictive Store Forwarding controls:
>>>> There are two hardware control bits which influence the PSF feature:
>>>> - MSR 48h bit 2 – Speculative Store Bypass (SSBD)
>>>> - MSR 48h bit 7 – Predictive Store Forwarding Disable (PSFD)
>>>>
>>>> The PSF feature is disabled if either of these bits are set.  These bits
>>>> are controllable on a per-thread basis in an SMT system. By default, both
>>>> SSBD and PSFD are 0 meaning that the speculation features are enabled.
>>>>
>>>> While the SSBD bit disables PSF and speculative store bypass, PSFD only
>>>> disables PSF.
>>>>
>>>> PSFD may be desirable for software which is concerned with the
>>>> speculative behavior of PSF but desires a smaller performance impact than
>>>> setting SSBD.
>>>
>>> Hi Ramakrishna,
>>>
>>> Is there a realistic scenario where an application would want to disable
>>> PSF, but not disable SSB?
>>
>> It is possible most applications have been reviewed and scrubbed for
>> SSB-type attacks but PSF-type issues may not have been looked at yet.
> 
> It's "possible", but is it realistic?  As far as I know, SSB is
> impractical to scrub an application for.
> 
> Do we know of any real-world cases where this option is needed?
>