Re: [PATCH] KVM: x86: Initialize tdp_level during vCPU creation

[Sean Christopherson]

On Wed, May 27, 2020 at 06:17:57PM +0200, Paolo Bonzini wrote:
> On 27/05/20 10:54, Sean Christopherson wrote:
> > Initialize vcpu->arch.tdp_level during vCPU creation to avoid consuming
> > garbage if userspace calls KVM_RUN without first calling KVM_SET_CPUID.
> > 
> > Fixes: e93fd3b3e89e9 ("KVM: x86/mmu: Capture TDP level when updating CPUID")
> > Reported-by: syzbot+904752567107eefb728c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index b226fb8abe41b..01a6304056197 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9414,6 +9414,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >  	fx_init(vcpu);
> >  
> >  	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
> > +	vcpu->arch.tdp_level = kvm_x86_ops.get_tdp_level(vcpu);
> >  
> >  	vcpu->arch.pat = MSR_IA32_CR_PAT_DEFAULT;
> 
> Queued, it is probably a good idea to add a selftests testcase for this
> (it's even okay if it doesn't use the whole selftests infrastructure and
> invokes KVM_CREATE_VM/KVM_CREATE_VCPU/KVM_RUN manually).

Ya.  syzbot is hitting a #GP due to NULL pointer during debugfs on the exact
same sequence.  I haven't been able to reproduce that one (have yet to try
syzbot's exact config), but it's another example of a "dumb" test hitting
meaningful bugs.