linux-kernel - Re: [PATCH] KVM: VMX: Flush shadow VMCS on emergency reboot

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Message-ID: <aObtM-7S0UfIRreU@google.com>
Date: Wed, 8 Oct 2025 16:01:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Flush shadow VMCS on emergency reboot

Trimmed Cc: to lists, as this is basically off-topic, but I thought you might
be amused :-)

On Thu, Apr 10, 2025, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Chao Gao wrote:
> > Ensure the shadow VMCS cache is evicted during an emergency reboot to
> > prevent potential memory corruption if the cache is evicted after reboot.
> 
> I don't suppose Intel would want to go on record and state what CPUs would actually
> be affected by this bug.  My understanding is that Intel has never shipped a CPU
> that caches shadow VMCS state.
> 
> On a very related topic, doesn't SPR+ now flush the VMCS caches on VMXOFF?  If
> that's going to be the architectural behavior going forward, will that behavior
> be enumerated to software?  Regardless of whether there's software enumeration,
> I would like to have the emergency disable path depend on that behavior.  In part
> to gain confidence that SEAM VMCSes won't screw over kdump, but also in light of
> this bug.

Apparently I completely purged it from my memory, but while poking through an
internal branch related to moving VMXON out of KVM, I came across this:

--
Author:     Sean Christopherson <seanjc@...gle.com>
AuthorDate: Wed Jan 17 16:19:28 2024 -0800
Commit:     Sean Christopherson <seanjc@...gle.com>
CommitDate: Fri Jan 26 13:16:31 2024 -0800

    KVM: VMX: VMCLEAR loaded shadow VMCSes on kexec()
    
    Add a helper to VMCLEAR _all_ loaded VMCSes in a loaded_vmcs pair, and use
    it when doing VMCLEAR before kexec() after a crash to fix a (likely benign)
    bug where KVM neglects to VMCLEAR loaded shadow VMCSes.  The bug is likely
    benign as existing Intel CPUs don't insert shadow VMCSes into the VMCS
    cache, i.e. shadow VMCSes can't be evicted since they're never cached, and
    thus won't clobber memory in the new kernel.

--

At least my reaction was more or less the same both times?

> If all past CPUs never cache shadow VMCS state, and all future CPUs flush the
> caches on VMXOFF, then this is a glorified NOP, and thus probably shouldn't be
> tagged for stable.
> 
> > This issue was identified through code inspection, as __loaded_vmcs_clear()
> > flushes both the normal VMCS and the shadow VMCS.
> > 
> > Avoid checking the "launched" state during an emergency reboot, unlike the
> > behavior in __loaded_vmcs_clear(). This is important because reboot NMIs
> > can interfere with operations like copy_shadow_to_vmcs12(), where shadow
> > VMCSes are loaded directly using VMPTRLD. In such cases, if NMIs occur
> > right after the VMCS load, the shadow VMCSes will be active but the
> > "launched" state may not be set.
> > 
> > Signed-off-by: Chao Gao <chao.gao@...el.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index b70ed72c1783..dccd1c9939b8 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -769,8 +769,11 @@ void vmx_emergency_disable_virtualization_cpu(void)
> >  		return;
> >  
> >  	list_for_each_entry(v, &per_cpu(loaded_vmcss_on_cpu, cpu),
> > -			    loaded_vmcss_on_cpu_link)
> > +			    loaded_vmcss_on_cpu_link) {
> >  		vmcs_clear(v->vmcs);
> > +		if (v->shadow_vmcs)
> > +			vmcs_clear(v->shadow_vmcs);
> > +	}
> >  
> >  	kvm_cpu_vmxoff();
> >  }
> > -- 
> > 2.46.1
> >