linux-kernel - Re: [PATCH 1/4] KVM: x86: Reject loading KVM if host.PAT[0] != WB

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Message-ID: <298d3c2601b13bf45044e92af02a28d5440e944f.camel@redhat.com>
Date:   Mon, 18 Jul 2022 12:42:52 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: x86: Reject loading KVM if host.PAT[0] != WB

On Fri, 2022-07-15 at 23:18 +0000, Sean Christopherson wrote:
> On Fri, Jul 15, 2022, Jim Mattson wrote:
> > On Fri, Jul 15, 2022 at 4:02 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > 
> > > Reject KVM if entry '0' in the host's IA32_PAT MSR is not programmed to
> > > writeback (WB) memtype.  KVM subtly relies on IA32_PAT entry '0' to be
> > > programmed to WB by leaving the PAT bits in shadow paging and NPT SPTEs
> > > as '0'.  If something other than WB is in PAT[0], at _best_ guests will
> > > suffer very poor performance, and at worst KVM will crash the system by
> > > breaking cache-coherency expecations (e.g. using WC for guest memory).
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > What if someone changes the host's PAT to violate this rule *after*
> > kvm is loaded?
> 
> Then KVM (and probably many other things in the kernel) is hosed.  The same argument
> (that KVM isn't paranoid enough) can likely be made for a number of MSRs and critical
> registers.
> 

I was thinking about the same thing and I also 100% agree with the above.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky