linux-kernel - Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

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Message-ID: <aKTD42Gma7qG3k9_@google.com>
Date: Tue, 19 Aug 2025 11:35:15 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin@...or.com>
Cc: Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	mlevitsk@...hat.com, rick.p.edgecombe@...el.com, weijiang.yang@...el.com, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v12 15/24] KVM: VMX: Emulate read and write to CET MSRs

On Tue, Aug 19, 2025, Xin Li wrote:
> On 8/19/2025 9:09 AM, Sean Christopherson wrote:
> > > +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> > > +		if (!guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK))
> > > +			return KVM_MSR_RET_UNSUPPORTED;
> > > +		if (is_noncanonical_msr_address(data, vcpu))
> > This emulation is wrong (in no small part because the architecture sucks).  From
> > the SDM:
> > 
> >    If the processor does not support Intel 64 architecture, these fields have only
> >    32 bits; bits 63:32 of the MSRs are reserved.
> > 
> >    On processors that support Intel 64 architecture this value cannot represent a
> >    non-canonical address.
> > 
> >    In protected mode, only 31:0 are loaded.
> > 
> > That means KVM needs to drop bits 63:32 if the vCPU doesn't have LM or if the vCPU
> > isn't in 64-bit mode.  The last one is especially frustrating, because software
> > can still get a 64-bit value into the MSRs while running in protected, e.g. by
> > switching to 64-bit mode, doing WRMSRs, then switching back to 32-bit mode.
> > 
> > But, there's probably no point in actually trying to correctly emulate/virtualize
> > the Protected Mode behavior, because the MSRs can be written via XRSTOR, and to
> > close that hole KVM would need to trap-and-emulate XRSTOR.  No thanks.
> > 
> > Unless someone has a better idea, I'm inclined to take an erratum for this, i.e.
> > just sweep it under the rug.
> 
> Since WRMSR (WRMSRNS) and XRSTORS are the two instructions that write to
> MSRs in CPL0, Why KVM doesn't use the XSS-exiting bitmap?

Intercepting XRSTORS would introduce (likely) unacceptable performance overhead
if a guest actually used CET in PM, I have zero desire to try and proxy/emulate
XRSTORS from guest memory in KVM, and CET already has big gaping virtualization
holes.