Re: [PATCH] KVM: SVM: Issue WBINVD after deactivating an SEV guest

[Paolo Bonzini]

On 20/03/20 17:07, Tom Lendacky wrote:
> Currently, CLFLUSH is used to flush SEV guest memory before the guest is
> terminated (or a memory hotplug region is removed). However, CLFLUSH is
> not enough to ensure that SEV guest tagged data is flushed from the cache.
> 
> With 33af3a7ef9e6 ("KVM: SVM: Reduce WBINVD/DF_FLUSH invocations"), the
> original WBINVD was removed. This then exposed crashes at random times
> because of a cache flush race with a page that had both a hypervisor and
> a guest tag in the cache.
> 
> Restore the WBINVD when destroying an SEV guest and add a WBINVD to the
> svm_unregister_enc_region() function to ensure hotplug memory is flushed
> when removed. The DF_FLUSH can still be avoided at this point.
> 
> Fixes: 33af3a7ef9e6 ("KVM: SVM: Reduce WBINVD/DF_FLUSH invocations")
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/kvm/svm.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 08568ae9f7a1..d54cdca9c140 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1980,14 +1980,6 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
>  static void __unregister_enc_region_locked(struct kvm *kvm,
>  					   struct enc_region *region)
>  {
> -	/*
> -	 * The guest may change the memory encryption attribute from C=0 -> C=1
> -	 * or vice versa for this memory range. Lets make sure caches are
> -	 * flushed to ensure that guest data gets written into memory with
> -	 * correct C-bit.
> -	 */
> -	sev_clflush_pages(region->pages, region->npages);
> -
>  	sev_unpin_memory(kvm, region->pages, region->npages);
>  	list_del(&region->list);
>  	kfree(region);
> @@ -2004,6 +1996,13 @@ static void sev_vm_destroy(struct kvm *kvm)
>  
>  	mutex_lock(&kvm->lock);
>  
> +	/*
> +	 * Ensure that all guest tagged cache entries are flushed before
> +	 * releasing the pages back to the system for use. CLFLUSH will
> +	 * not do this, so issue a WBINVD.
> +	 */
> +	wbinvd_on_all_cpus();
> +
>  	/*
>  	 * if userspace was terminated before unregistering the memory regions
>  	 * then lets unpin all the registered memory.
> @@ -7247,6 +7246,13 @@ static int svm_unregister_enc_region(struct kvm *kvm,
>  		goto failed;
>  	}
>  
> +	/*
> +	 * Ensure that all guest tagged cache entries are flushed before
> +	 * releasing the pages back to the system for use. CLFLUSH will
> +	 * not do this, so issue a WBINVD.
> +	 */
> +	wbinvd_on_all_cpus();
> +
>  	__unregister_enc_region_locked(kvm, region);
>  
>  	mutex_unlock(&kvm->lock);
> 

Queued for kvm/master, thanks.

Paolo