linux-kernel - Re: [patch V2 18/28] x86/speculation: Prepare for per task indirect branch speculation control

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Message-ID: <alpine.DEB.2.21.1811272138290.1875@nanos.tec.linutronix.de>
Date:   Tue, 27 Nov 2018 21:39:44 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 18/28] x86/speculation: Prepare for per task indirect
 branch speculation control

On Tue, 27 Nov 2018, Lendacky, Thomas wrote:
> On 11/25/2018 12:33 PM, Thomas Gleixner wrote:
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -406,6 +406,11 @@ static __always_inline void spec_ctrl_up
> >  	if (static_cpu_has(X86_FEATURE_SSBD))
> >  		msr |= ssbd_tif_to_spec_ctrl(tifn);
> 
> I did some quick testing and found my original logic was flawed. Since
> spec_ctrl_update_msr() can now be called for STIBP, an additional check
> is needed to set the SSBD MSR bit.
> 
> Both X86_FEATURE_VIRT_SSBD and X86_FEATURE_LS_CFG_SSBD cause
> X86_FEATURE_SSBD to be set. Before this patch, spec_ctrl_update_msr() was
> only called if X86_FEATURE_SSBD was set and one of the other SSBD features
> wasn't set. But now, STIBP can cause spec_ctrl_update_msr() to get called
> and cause the SSBD MSR bit to be set when it shouldn't (could result in
> a GP fault).

The below should fix that. We have the same logic in x86_virt_spec_ctrl()

Thanks,

	tglx

8<---------------
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -403,10 +403,11 @@ static __always_inline void spec_ctrl_up
 	u64 msr = x86_spec_ctrl_base;
 
 	/*
-	 * If X86_FEATURE_SSBD is not set, the SSBD bit is not to be
-	 * touched.
+	 * If SSBD is not controlled in MSR_SPEC_CTRL, the SSBD bit has not
+	 * to be touched.
 	 */
-	if (static_cpu_has(X86_FEATURE_SSBD))
+	if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
+	    static_cpu_has(X86_FEATURE_AMD_SSBD))
 		msr |= ssbd_tif_to_spec_ctrl(tifn);
 
 	/* Only evaluate if conditional STIBP is enabled */