linux-kernel - Re: [PATCH v2 2/2] Documentation/hw-vuln: Document the interaction between IBRS and STIBP

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Date:   Sat, 25 Feb 2023 20:42:35 -0500
From:   KP Singh <kpsingh@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, pjt@...gle.com, evn@...gle.com,
        jpoimboe@...nel.org, tglx@...utronix.de, x86@...nel.org,
        hpa@...or.com, peterz@...radead.org,
        pawan.kumar.gupta@...ux.intel.com, kim.phillips@....com,
        alexandre.chartre@...cle.com, daniel.sneddon@...ux.intel.com,
        corbet@....net, bp@...e.de, linyujun809@...wei.com,
        jmattson@...gle.com
Subject: Re: [PATCH v2 2/2] Documentation/hw-vuln: Document the interaction
 between IBRS and STIBP

On Thu, Feb 23, 2023 at 9:52 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Feb 21, 2023 at 07:49:08PM +0100, KP Singh wrote:
> > ... Consequently, STIBP needs to be explicitly
> > +   enabled to guard against cross-thread attacks in userspace.
>
> needs?
>
> That sounds like something the user needs to do. But we do it by
> default. Let's rephrase:
>
> "Systems which support enhanced IBRS (eIBRS) enable IBRS protections once at
> boot and they're automatically protected against Spectre v2 variant
> attacks, including cross-thread branch target injections on SMT systems
> (STIBP). IOW, eIBRS enables STIBP too.
>
> Legacy IBRS systems clear the IBRS bit on exit to userspace and
> therefore explicitly enable STIBP for that."

+   Systems which support enhanced IBRS (eIBRS) enable IBRS protection once at
+   boot, by setting the IBRS bit, and they're automatically protected against
+   Spectre v2 variant attacks, including cross-thread branch target injections
+   on SMT systems (STIBP). In other words, eIBRS enables STIBP too.
+
+   Legacy IBRS systems clear the IBRS bit on exit to userspace and
+   therefore explicitly enable STIBP for that


I did add one phrase, we really need to stress on the IBRS bit here.
Had we been enabling KERNEL_IBRS accidentally with eIBRS, it would
still mess things up as the bit being set is important.

This is why my original patch felt "obtuse" as it focused on
KERNEL_IBRS instead of IBRS or eIBRS :).


>
> Simple.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette