linux-kernel - Re: [PATCH v11 7/9] KVM: X86: Add userspace access interface for CET MSRs

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Date:   Sat, 25 Apr 2020 17:28:48 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        jmattson@...gle.com, yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v11 7/9] KVM: X86: Add userspace access interface for CET
 MSRs

On Fri, Apr 24, 2020 at 08:10:49AM -0700, Sean Christopherson wrote:
> On Fri, Apr 24, 2020 at 11:02:46PM +0800, Yang Weijiang wrote:
> > On Thu, Apr 23, 2020 at 11:14:06AM -0700, Sean Christopherson wrote:
> > > > +	case MSR_IA32_INT_SSP_TAB:
> > > > +		if (!cet_check_ctl_msr_access(vcpu, msr_info))
> > > > +			return 1;
> > > > +		if (!is_64_bit_mode(vcpu))
> > > 
> > > This is wrong, the SDM explicitly calls out the !64 case:
> > > 
> > >   IA32_INTERRUPT_SSP_TABLE_ADDR (64 bits; 32 bits on processors that do not
> > >   support Intel 64 architecture).
> > So the check is also unnecessary as it's natual size?
> 
> It still needs a canonical check.
> 
> Note, KVM diverges from the SDM for canonical checks in that it performs
> canonical checks even when the virtual CPU doesn't support 64-bit and/or
> the host kernel is a 32-bit kernel.  This is intentional because the
> underlying hardware will still enforce the checks, i.e. KVM needs to make
> the physical CPU happy, and the number of people running KVM on hardware
> without 64-bit support can probably be counted on one hand.
Got it, thank you!