linux-kernel - Re: [PATCH 2/4] KVM: x86: Harden _regs accesses to guard against buggy input

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Message-ID: <Yo+hkH9Uy0eSPErf@google.com>
Date:   Thu, 26 May 2022 15:49:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Robert Dinse <nanook@...imo.com>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/4] KVM: x86: Harden _regs accesses to guard against
 buggy input

On Thu, May 26, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > ---
> >  arch/x86/kvm/emulate.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 7226a127ccb4..c58366ae4da2 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -247,6 +247,9 @@ enum x86_transfer_type {
> >  
> >  static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
> >  {
> > +	if (WARN_ON_ONCE(nr >= 16))
> > +		nr &= 16 - 1;
> 
> As the result of this is unlikely to match the expectation (and I'm
> unsure what's the expectation here in the first place :-), why not use 
> KVM_BUG_ON() here instead?

ctxt->vcpu is a 'void *' due to the (IMO futile) separation of the emulator from
regular KVM.  I.e. this doesn't have access to the 'kvm'.