Re: [PATCH v2 02/13] KVM: x86: Protect kvm_hv_msr_[get|set]_crash_data() from Spectre-v1/L1TF attacks

[Vitaly Kuznetsov]

Marios Pomonis <pomonis@xxxxxxxxxx> writes:

> This fixes Spectre-v1/L1TF vulnerabilities in kvm_hv_msr_get_crash_data()
> and kvm_hv_msr_set_crash_data().
> These functions contain index computations that use the
> (attacker-controlled) MSR number.

Just to educate myself,

in both cases 'index' is equal to 'msr - HV_X64_MSR_CRASH_P0' where
'msr' is constrained:
  case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
         ....

and moreover, kvm_hv_{get,set}_msr_common() is only being called for a
narrow set of MSRs. How can an atacker overcome these limitations?

>
> Fixes: commit e7d9513b60e8 ("kvm/x86: added hyper-v crash msrs into kvm hyperv context")
>
> Signed-off-by: Nick Finco <nifi@xxxxxxxxxx>
> Signed-off-by: Marios Pomonis <pomonis@xxxxxxxxxx>
> Reviewed-by: Andrew Honig <ahonig@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/x86/kvm/hyperv.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 23ff65504d7e..26408434b9bc 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -809,11 +809,12 @@ static int kvm_hv_msr_get_crash_data(struct kvm_vcpu *vcpu,
>  				     u32 index, u64 *pdata)
>  {
>  	struct kvm_hv *hv = &vcpu->kvm->arch.hyperv;
> +	size_t size = ARRAY_SIZE(hv->hv_crash_param);
>  
> -	if (WARN_ON_ONCE(index >= ARRAY_SIZE(hv->hv_crash_param)))
> +	if (WARN_ON_ONCE(index >= size))
>  		return -EINVAL;
>  
> -	*pdata = hv->hv_crash_param[index];
> +	*pdata = hv->hv_crash_param[array_index_nospec(index, size)];
>  	return 0;
>  }
>  
> @@ -852,11 +853,12 @@ static int kvm_hv_msr_set_crash_data(struct kvm_vcpu *vcpu,
>  				     u32 index, u64 data)
>  {
>  	struct kvm_hv *hv = &vcpu->kvm->arch.hyperv;
> +	size_t size = ARRAY_SIZE(hv->hv_crash_param);
>  
> -	if (WARN_ON_ONCE(index >= ARRAY_SIZE(hv->hv_crash_param)))
> +	if (WARN_ON_ONCE(index >= size))
>  		return -EINVAL;
>  
> -	hv->hv_crash_param[index] = data;
> +	hv->hv_crash_param[array_index_nospec(index, size)] = data;
>  	return 0;
>  }

-- 
Vitaly