Re: KVM: VMX: fix use after free of vmx->loaded_vmcs

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jan 03, 2014 at 08:27:07PM +0100, Jan Kiszka wrote:
> On 2014-01-03 20:00, Marcelo Tosatti wrote:
> > 
> > After free_loaded_vmcs executes, the "loaded_vmcs" structure 
> > is kfreed, and now vmx->loaded_vmcs points to a kfreed area. 
> > Subsequent free_loaded_vmcs then attempts to manipulate 
> > vmx->loaded_vmcs.
> 
> Cannot follow yet. How precisely do we call free_loaded_vmcs twice on
> the same loaded_vmcs? 

You don't: 

nested_free_all_saved_vmcss calls kfree(item). item is struct
vmcs02_list *, which is:

/* Used to remember the last vmcs02 used for some recently used vmcs12s
 * */
struct vmcs02_list {
        struct list_head list;
        gpa_t vmptr;
        struct loaded_vmcs vmcs02;
};

And vmx->loaded_vmcs = &item->vmcs02.

> I thought the frees triggered by free_nested ->
> nested_free_all_saved_vmcss stay away from vmx->loaded_vmcs, no?

Stays away as far as free_loaded_vmcs, yes. 
Except it frees the structure pointed to by vmx->loaded_vmcs.



The separate question is about when is vmcs01 ever allocated again 
if freed by nested_free_all_saved_vmcss (the other email).

> Jan
> 
> > 
> > Switch the order to avoid the problem.
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1047892
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index da7837e..2efa33f0 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -7332,8 +7332,8 @@ static void vmx_free_vcpu(struct kvm_vcpu *vcpu)
> >  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> >  
> >  	free_vpid(vmx);
> > -	free_nested(vmx);
> >  	free_loaded_vmcs(vmx->loaded_vmcs);
> > +	free_nested(vmx);
> >  	kfree(vmx->guest_msrs);
> >  	kvm_vcpu_uninit(vcpu);
> >  	kmem_cache_free(kvm_vcpu_cache, vmx);
> > 
> 
>