linux-kernel - Re: [PATCH v15 07/14] KVM: VMX: Emulate reads and writes to CET MSRs

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Message-ID: <YoUb4/iP+X+xgsfQ@google.com>
Date:   Wed, 18 May 2022 16:16:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     John Allen <john.allen@....com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
        jmattson@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v15 07/14] KVM: VMX: Emulate reads and writes to CET MSRs

On Wed, May 18, 2022, John Allen wrote:
> On Wed, Feb 03, 2021 at 07:34:14PM +0800, Yang Weijiang wrote:
> > +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> > +		if (!cet_is_ssp_msr_accessible(vcpu, msr_info))
> > +			return 1;
> > +		if ((data & GENMASK(2, 0)) || is_noncanonical_address(data, vcpu))
> 
> Sorry to revive this old thread. I'm working on the corresponding SVM
> bits for shadow stack and I noticed the above check. Why isn't this
> GENMASK(1, 0)? The *SSP MSRs should be a 4-byte aligned canonical
> address meaning that just bits 1 and 0 should always be zero. I was
> looking through the previous versions of the set and found that this
> changed between versions 11 and 12, but I don't see any discussion
> related to this on the list.

Huh.  I'm not entirely sure what to make of the SDM's wording:

  The linear address written must be aligned to 8 bytes and bits 2:0 must be 0
  (hardware requires bits 1:0 to be 0).

Looking at the rest of the CET stuff, I believe requiring 8-byte alignment is
correct, and that the "hardware requires" blurb is trying to call out that the
SSP stored in hardware will always be 4-byte aligned but not necessarily 8-byte
aligned in order to play nice with 32-bit/compatibility mode.  But "base" addresses
that come from software, e.g. via MSRs and whatnot, must always be 8-byte aligned.