From: Paolo Bonzini <pbonzini@redhat.com>
Date: Wed, 2 Aug 2017 17:55:54 +0200
Subject: KVM: avoid using rcu_dereference_protected
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Patch-mainline: v4.13-rc4
Git-commit: 3898da947bbaf9e7fd5816e825978d360028bba2
References: bsc#1077761

During teardown, accesses to memslots and buses are using
rcu_dereference_protected with an always-true condition because
these accesses are done outside the usual mutexes.  This
is because the last reference is gone and there cannot be any
concurrent modifications, but rcu_dereference_protected is
ugly and unobvious.

Instead, check the refcount in kvm_get_bus and __kvm_memslots.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Acked-by: Alexander Graf <agraf@suse.de>
---
 include/linux/kvm_host.h |    6 ++++--
 virt/kvm/kvm_main.c      |   11 ++++-------
 2 files changed, 8 insertions(+), 9 deletions(-)

--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -477,7 +477,8 @@
 static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx)
 {
 	return srcu_dereference_check(kvm->buses[idx], &kvm->srcu,
-				      lockdep_is_held(&kvm->slots_lock));
+				      lockdep_is_held(&kvm->slots_lock) ||
+				      !refcount_read(&kvm->users_count));
 }
 
 static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
@@ -570,7 +571,8 @@
 static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
 {
 	return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu,
-			lockdep_is_held(&kvm->slots_lock));
+			lockdep_is_held(&kvm->slots_lock) ||
+			!refcount_read(&kvm->users_count));
 }
 
 static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -717,10 +717,9 @@
 	hardware_disable_all();
 out_err_no_disable:
 	for (i = 0; i < KVM_NR_BUSES; i++)
-		kfree(rcu_access_pointer(kvm->buses[i]));
+		kfree(kvm_get_bus(kvm, i));
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
-		kvm_free_memslots(kvm,
-			rcu_dereference_protected(kvm->memslots[i], 1));
+		kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
 	kvm_arch_free_vm(kvm);
 	mmdrop(current->mm);
 	return ERR_PTR(r);
@@ -754,9 +753,8 @@
 	spin_unlock(&kvm_lock);
 	kvm_free_irq_routing(kvm);
 	for (i = 0; i < KVM_NR_BUSES; i++) {
-		struct kvm_io_bus *bus;
+		struct kvm_io_bus *bus = kvm_get_bus(kvm, i);
 
-		bus = rcu_dereference_protected(kvm->buses[i], 1);
 		if (bus)
 			kvm_io_bus_destroy(bus);
 		kvm->buses[i] = NULL;
@@ -770,8 +768,7 @@
 	kvm_arch_destroy_vm(kvm);
 	kvm_destroy_devices(kvm);
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
-		kvm_free_memslots(kvm,
-			rcu_dereference_protected(kvm->memslots[i], 1));
+		kvm_free_memslots(kvm, __kvm_memslots(kvm, i));
 	cleanup_srcu_struct(&kvm->irq_srcu);
 	cleanup_srcu_struct(&kvm->srcu);
 	kvm_arch_free_vm(kvm);