Re: [PATCH] KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk

Re: [PATCH] KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk

[Paolo Bonzini]

On 05/11/2017 15:11, Liran Alon wrote:
> When guest passes KVM it's pvclock-page GPA via WRMSR to
> MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize
> pvclock-page to some start-values. It just requests a clock-update which
> will happen before entering to guest.
> 
> The clock-update logic will call kvm_setup_pvclock_page() to update the
> pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly*
> assumes that the version-field is initialized to an even number. This is
> wrong because at first-time write, field could be any-value.
> 
> Fix simply makes sure that if first-time version-field is odd, increment
> it once more to make it even and only then start standard logic.
> This follows same logic as done in other pvclock shared-pages (See
> kvm_write_wall_clock() and record_steal_time()).
> 
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb7fcd6..181106080e41 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1830,6 +1830,9 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
>  	 */
>  	BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
>  
> +	if (guest_hv_clock.version & 1)
> +		++guest_hv_clock.version;  /* first time write, random junk */
> +
>  	vcpu->hv_clock.version = guest_hv_clock.version + 1;
>  	kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
>  				&vcpu->hv_clock,
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org