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

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

On 3/21/20 4:00 AM, Greg KH wrote:
> On Fri, Mar 20, 2020 at 03:37:23PM -0500, Tom Lendacky wrote:
>> 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.
> 
> No it does not.
> Please read:
>      https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.kernel.org%2Fdoc%2Fhtml%2Flatest%2Fprocess%2Fstable-kernel-rules.html&amp;data=02%7C01%7Cthomas.lendacky%40amd.com%7C197f666080144732040108d7cd765107%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637203780365719535&amp;sdata=NKgNt6Hd7y6BGBdpI52ckCxZvIsCRuEf9FJ7GW2PqPw%3D&amp;reserved=0
> for how to do this properly.
> 
> Yes, I have had to go around and clean up after maintainers who don't
> seem to realize this, but for KVM patches I have been explicitly told to
> NOT take any patch unless it has a cc: stable on it, due to issues that
> have happened in the past.
> 
> So for this subsystem, what you suggested guaranteed it would NOT get
> picked up, please do not do that.

Thanks for clarifying that, Greg.

Then, yes, it should have the Cc: to stable that David mentioned. If it
gets applied without that, I'll follow the process to send an email to
stable to get it included in 5.5-stable.

Thanks,
Tom

> 
> greg k-h
>