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

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Message-Id: <20220815180430.572594455@linuxfoundation.org>
Date:   Mon, 15 Aug 2022 19:50:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.18 0029/1095] KVM: Fully serialize gfn=>pfn cache refresh via mutex

From: Sean Christopherson <seanjc@...gle.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@...r.kernel.org
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Message-Id: <20220429210025.3293691-7-seanjc@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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;