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

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Message-ID: <alpine.DEB.2.21.2003201333510.205664@chino.kir.corp.google.com>
Date:   Fri, 20 Mar 2020 13:34:12 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH] KVM: SVM: Issue WBINVD after deactivating an SEV guest

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@....com>

Acked-by: David Rientjes <rientjes@...gle.com>

Should this be marked for stable?

Cc: stable@...r.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
> 
>