Re: nVMX: Shadowing of CPU_BASED_VM_EXEC_CONTROL broken

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/10/2014 17:07, Jan Kiszka ha scritto:
> As usual, this was a nasty race that involved some concurrent VCPUs and
> proper host load, so hard to write unit tests...
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 04fa1b8..d6bcaca 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>  	const unsigned long *fields = shadow_read_write_fields;
>  	const int num_fields = max_shadow_read_write_fields;
>  
> +	preempt_disable();
> +
>  	vmcs_load(shadow_vmcs);
>  
>  	for (i = 0; i < num_fields; i++) {
> @@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
>  
>  	vmcs_clear(shadow_vmcs);
>  	vmcs_load(vmx->loaded_vmcs->vmcs);
> +
> +	preempt_enable();
>  }
>  
>  static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
> @@ -6457,6 +6461,8 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>  	u64 field_value = 0;
>  	struct vmcs *shadow_vmcs = vmx->nested.current_shadow_vmcs;
>  
> +	preempt_disable();
> +
>  	vmcs_load(shadow_vmcs);
>  
>  	for (q = 0; q < ARRAY_SIZE(fields); q++) {
> @@ -6483,6 +6489,8 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>  
>  	vmcs_clear(shadow_vmcs);
>  	vmcs_load(vmx->loaded_vmcs->vmcs);
> +
> +	preempt_enable();
>  }
>  
> No proper patch yet because there might be a smarter approach without
> using the preempt_disable() hammer.

copy_vmcs12_to_shadow already runs with preemption disabled; for stable@
it's not that bad to do the same in copy_shadow_to_vmcs12.

For 3.18 it could be nice of course to use loaded_vmcs properly, but it
would also incur some overhead.

Paolo

> But the point is that we temporarily
> load a vmcs without updating loaded_vmcs->vmcs. Now, if some other VCPU
> is scheduling in right in the middle of this, the wrong vmcs will be
> flushed and then reloaded - e.g. a non-shadow vmcs with that interrupt
> window flag set...
> 
> Patch is currently under heavy load testing here, but it looks very good
> as the bug was quickly reproducible before I applied it.