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

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Date:   Wed, 12 Sep 2018 15:56:51 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v5 2/2] x86/speculation: Enable cross-hyperthread spectre
 v2 STIBP mitigation

On 09/12/2018 02:45 PM, Jiri Kosina wrote:
> On Wed, 12 Sep 2018, Tim Chen wrote:
> 
>> 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?
> 
> It's probably 100% correct, but it's also 100% super-complex at the same 
> time if you ask me.
> 
> Try to imagine you're a very advanced senior sysadmin, who has heard that 
> spectre and meltdown existed of course, but figured out that updating to 
> latest kernel/distro vendor update fixes all the security issues (and it 
> actually indeed did).
> 
> Now, all of a sudden, this new option pops up, and the poor sysadmin has 
> to make a decision again.
> 
> 	"Do you care only about security across non-dumpable process 
> 	 boundaries?"
> 
> 	"Scheduled to same CPU at the time of attack? Can you guarantee that this 
> 	 is (not) happening?"
> 
> 	"If the processess can actually ptrace/debug each other, are you okay with 
> 	 them attacking each other?"
> 
> 	 "Shared HT siblings return target buffer, do you want it or 
> 	  not?"
> 
> These are the questions that even an excellent sysadmin might not have 
> qualified answers to so far. Now, all of a sudden, he/her has to make 
> these decisions?
> 
> I don't think that's how it should work. It all should be digestible by 
> "linux end-users" (where users are also super-advanced sysadmins) easily.
> 
> We currently have "I do care about spectrev2 / I don't care about 
> spectrev2" boot-time switch, and I don't see us going any deeper / more 
> fine-grained without sacrificing clarity and sanity.
> 
> Or do you see a way how to do that nicely?
> 

How about just these options:

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_STRICT]             = "App-App Mitigation: Full app to app attack protection",
};

Tim