linux-kernel - Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

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Date:   Wed, 22 Sep 2021 23:11:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

On Wed, Sep 22, 2021 at 11:07:22PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 20, 2021 at 07:23:45PM +0000, Fenghua Yu wrote:
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index a58800973aed..a25d738ae839 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -61,6 +61,7 @@
> >  #include <asm/insn.h>
> >  #include <asm/insn-eval.h>
> >  #include <asm/vdso.h>
> > +#include <asm/iommu.h>
> >  
> >  #ifdef CONFIG_X86_64
> >  #include <asm/x86_init.h>
> > @@ -526,6 +527,14 @@ static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
> >  	return GP_CANONICAL;
> >  }
> >  
> > +static bool fixup_pasid_exception(void)
> > +{
> > +	if (!cpu_feature_enabled(X86_FEATURE_ENQCMD))
> > +		return false;
> > +
> > +	return __fixup_pasid_exception();
> > +}

That is, shouldn't the above at the very least decode the instruction
causing the #GP and check it's this ENQCMD thing?

> >  #define GPFSTR "general protection fault"
> >  
> >  DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
> > @@ -538,6 +547,9 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
> >  
> >  	cond_local_irq_enable(regs);
> >  
> > +	if (user_mode(regs) && fixup_pasid_exception())
> > +		goto exit;
> > +
> >  	if (static_cpu_has(X86_FEATURE_UMIP)) {
> >  		if (user_mode(regs) && fixup_umip_exception(regs))
> >  			goto exit;
> 
> So you're eating any random #GP that might or might not be PASID
> related. And all that witout a comment... Enlighten?