[PATCH 5.18 0029/1095] KVM: Fully serialize gfn=>pfn cache refresh via mutex

Greg Kroah-Hartman
2022-08-15T20:16:53Z

From: Sean Christopherson <seanjc@google.com>

commit 93984f19e7bce4c18084a6ef3dacafb155b806ed upstream.

Protect gfn=>pfn cache refresh with a mutex to fully serialize refreshes.
The refresh logic doesn't protect against

- concurrent unmaps, or refreshes with different GPAs (which may or may not
happen in practice, for example if a cache is only used under vcpu->mutex;
but it's allowed in the code)

- 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>
Message-Id: <20220429210025.3293691-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/kvm_types.h | 2 ++
virt/kvm/pfncache.c | 12 ++++++++++++
2 files changed, 14 insertions(+)

--- 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;
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -157,6 +157,13 @@ int kvm_gfn_to_pfn_cache_refresh(struct
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;
@@ -250,6 +257,8 @@ int kvm_gfn_to_pfn_cache_refresh(struct
out:
write_unlock_irq(&gpc->lock);

+ mutex_unlock(&gpc->refresh_lock);
+
gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);

return ret;
@@ -261,6 +270,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct k
void *old_khva;
kvm_pfn_t old_pfn;

+ mutex_lock(&gpc->refresh_lock);
write_lock_irq(&gpc->lock);

gpc->valid = false;
@@ -276,6 +286,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct k
gpc->pfn = KVM_PFN_ERR_FAULT;

write_unlock_irq(&gpc->lock);
+ mutex_unlock(&gpc->refresh_lock);

gpc_release_pfn_and_khva(kvm, old_pfn, old_khva);
}
@@ -290,6 +301,7 @@ int kvm_gfn_to_pfn_cache_init(struct kvm

if (!gpc->active) {
rwlock_init(&gpc->lock);
+ mutex_init(&gpc->refresh_lock);

gpc->khva = NULL;
gpc->pfn = KVM_PFN_ERR_FAULT;