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

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/20/20 3:34 PM, David Rientjes wrote:
> On Fri, 20 Mar 2020, 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@amd.com>
> 
> Acked-by: David Rientjes <rientjes@google.com>
> 
> Should this be marked for stable?

The Fixes tag should take care of that.

Thanks,
Tom

> 
> Cc: stable@vger.kernel.org # 5.5+
> 
>> ---
>>   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);
>> -- 
>> 2.17.1
>>
>>