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Subject[PATCH 5.5 094/170] KVM: SVM: Issue WBINVD after deactivating an SEV guest
Date
From: Tom Lendacky <thomas.lendacky@amd.com>

commit 2e2409afe5f0c284c7dfe5504058e8d115806a7d upstream.

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>
Message-Id: <c8bf9087ca3711c5770bdeaafa3e45b717dc5ef4.1584720426.git.thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/svm.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1920,14 +1920,6 @@ static void sev_clflush_pages(struct pag
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);
@@ -1958,6 +1950,13 @@ static void sev_vm_destroy(struct kvm *k
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.
*/
@@ -7212,6 +7211,13 @@ static int svm_unregister_enc_region(str
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);

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