Re: [PATCH v5 2/2] x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation

[Tim Chen <tim.c.chen@linux.intel.com>]

On 09/12/2018 10:16 AM, Tom Lendacky wrote:
> 
> 
> On 09/11/2018 04:16 PM, Thomas Gleixner wrote:
>> On Tue, 11 Sep 2018, Tim Chen wrote:
>>> On 09/10/2018 04:46 AM, Jiri Kosina wrote:
>>>> Nah, IBPB is actuall there, sorry. So I'll add reporting of STIBP + fixup 
>>>> the missing reporting of RSB_CTXSW for v6.
>>>>
>>>
>>> I anticipate that STIBP could affect workloads with a lot of indirect
>>> branches (see previous discussion with Andrea).  We should have a 
>>> knob for people to opt in or opt out of STIBP.
>>
>> Feel free to send a patch to that effect.
> 
> Tim, are you planning on sending a patch for this?  If so, what type of
> opt in/out are you thinking about, something similar to SSBD?
> 

I'm working on a patch for choosing the Spectre v2 app to app
mitigation option.

Something like the following:

enum spectre_v2_app2app_mitigation {
        SPECTRE_V2_APP2APP_NONE,
        SPECTRE_V2_APP2APP_LITE,
        SPECTRE_V2_APP2APP_IBPB,
        SPECTRE_V2_APP2APP_STIBP,
        SPECTRE_V2_APP2APP_STRICT,
};

static const char *spectre_v2_app2app_strings[] = {
        [SPECTRE_V2_APP2APP_NONE]               = "App-App Vulnerable",
        [SPECTRE_V2_APP2APP_LITE]               = "App-App Mitigation: Protect only non-dumpable process",
        [SPECTRE_V2_APP2APP_IBPB]               = "App-App Mitigation: Protect app against attack from same cpu",
        [SPECTRE_V2_APP2APP_STIBP]              = "App-App Mitigation: Protect app against attack from sibling cpu",
        [SPECTRE_V2_APP2APP_STRICT]             = "App-App Mitigation: Full app to app attack protection",
};

So the APP2APP_LITE protection's intention is to turn on STIBP and IBPB for non-dumpable
process.  But in my first version I may limit it to IBPB as choosing
STIBP based on process characteristics will require some frobbing of
the flags as what we've done in SSBD.  That will require more careful
work and tests.

The STRICT option will turn STIBP on always and IBPB always on
non-ptraceable context switches.

Is this something reasonable?

Tom, if you already have a patch, feel free to post.

Tim