[PATCH v3 6/8] KVM: Fully serialize gfn=>pfn cache refresh via mutex

[Sean Christopherson <seanjc@google.com>]

Protect gfn=>pfn cache refresh with a mutex to fully serialize refreshes.
The refresh logic doesn't protect against concurrent refreshes with
different GPAs (which may or may not be a desired use case, but it's
allowed in the code), nor does it protect against a false negative on the
memslot generation.

If the first refresh sees a stale memslot generation, it will refresh the
hva and generation before moving on to the hva=>pfn translation.  If it
then drops gpc->lock, a different user of the cache can come along,
acquire gpc->lock, see that the memslot generation is fresh, and skip
the hva=>pfn update due to the userspace address also matching (because
it too was updated).

The refresh path can already sleep during hva=>pfn resolution, so wrap
the refresh with a mutex to ensure that any given refresh runs to
completion before other callers can start their refresh.

Cc: stable@vger.kernel.org
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/kvm_types.h |  2 ++
 virt/kvm/pfncache.c       | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index ac1ebb37a0ff..f328a01db4fe 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -19,6 +19,7 @@ struct kvm_memslots;
 enum kvm_mr_change;
 
 #include <linux/bits.h>
+#include <linux/mutex.h>
 #include <linux/types.h>
 #include <linux/spinlock_types.h>
 
@@ -69,6 +70,7 @@ struct gfn_to_pfn_cache {
 	struct kvm_vcpu *vcpu;
 	struct list_head list;
 	rwlock_t lock;
+	struct mutex refresh_lock;
 	void *khva;
 	kvm_pfn_t pfn;
 	enum pfn_cache_usage usage;
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 05cb0bcbf662..eaef31462bbe 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -157,6 +157,13 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 	if (page_offset + len > PAGE_SIZE)
 		return -EINVAL;
 
+	/*
+	 * If another task is refreshing the cache, wait for it to complete.
+	 * There is no guarantee that concurrent refreshes will see the same
+	 * gpa, memslots generation, etc..., so they must be fully serialized.
+	 */
+	mutex_lock(&gpc->refresh_lock);
+
 	write_lock_irq(&gpc->lock);
 
 	old_pfn = gpc->pfn;
@@ -248,6 +255,8 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
  out:
 	write_unlock_irq(&gpc->lock);
 
+	mutex_unlock(&gpc->refresh_lock);
+
 	gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);
 
 	return ret;
@@ -288,6 +297,7 @@ int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 
 	if (!gpc->active) {
 		rwlock_init(&gpc->lock);
+		mutex_init(&gpc->refresh_lock);
 
 		gpc->khva = NULL;
 		gpc->pfn = KVM_PFN_ERR_FAULT;
-- 
2.36.0.464.gb9c8b46e94-goog