From 93984f19e7bce4c18084a6ef3dacafb155b806ed Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Fri, 29 Apr 2022 21:00:23 +0000
Subject: [PATCH] KVM: Fully serialize gfn=>pfn cache refresh via mutex
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>
---
include/linux/kvm_types.h | 2 ++
virt/kvm/pfncache.c | 12 ++++++++++++
2 files changed, 14 insertions(+)
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index ac1ebb37a0ffd..f328a01db4fe9 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 05cb0bcbf662a..f610d3945b69d 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;
@@ -259,6 +268,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
void *old_khva;
kvm_pfn_t old_pfn;
+ mutex_lock(&gpc->refresh_lock);
write_lock_irq(&gpc->lock);
gpc->valid = false;
@@ -274,6 +284,7 @@ void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
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);
}
@@ -288,6 +299,7 @@ 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;
--
2.49.0