linux-kernel - Re: [PATCH] KVM: allow compiling out SMM support

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Message-ID: <YzRhT6DzgDfGU7NC@google.com>
Date:   Wed, 28 Sep 2022 14:59:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: allow compiling out SMM support

On Wed, Sep 28, 2022, Paolo Bonzini wrote:
> On 9/27/22 19:59, Sean Christopherson wrote:
> > >  static int kvm_vcpu_ioctl_smi(struct kvm_vcpu *vcpu)
> > >  {
> > > -	kvm_make_request(KVM_REQ_SMI, vcpu);
> > > -
> > > +	if (IS_ENABLED(CONFIG_KVM_SMM))
> > > +		kvm_make_request(KVM_REQ_SMI, vcpu);
> > >  	return 0;
> > 
> > This should return -EINVAL, not 0.
> 
> I'm a bit wary of changing this in case userspace is relying on it not
> failing, because the paths that lead to the failing ioctl are most likely
> controlled by the guest.

But with CONFIG_KVM_SMM=n, KVM is now reporting that KVM_CAP_X86_SMM is unsupported,
so it's firmly a VMM bug if userspace is blindly firing KVM_SMI.  As long as the
Kconfig is on by default, I don't see any reason to fudge KVM's ABI to try to avoid
breaking buggy userspace.

  /* Available with KVM_CAP_X86_SMM */
  #define KVM_SMI                   _IO(KVMIO,   0xb7)