KVM: Destroy I/O bus devices on unregister failure _after_ sync'ing SRCU

If allocating a new instance of an I/O bus fails when unregistering a
device, wait to destroy the device until after all readers are guaranteed
to see the new null bus.  Destroying devices before the bus is nullified
could lead to use-after-free since readers expect the devices on their
reference of the bus to remain valid.

Fixes: f65886606c2d ("KVM: fix memory leak in kvm_io_bus_unregister_dev()")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210412222050.876100-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 529cff1..c771d40 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4646,7 +4646,13 @@ void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
                new_bus->dev_count--;
                memcpy(new_bus->range + i, bus->range + i + 1,
                                flex_array_size(new_bus, range, new_bus->dev_count - i));
-       } else {
+       }
+
+       rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
+       synchronize_srcu_expedited(&kvm->srcu);
+
+       /* Destroy the old bus _after_ installing the (null) bus. */
+       if (!new_bus) {
                pr_err("kvm: failed to shrink bus, removing it completely\n");
                for (j = 0; j < bus->dev_count; j++) {
                        if (j == i)
@@ -4655,8 +4661,6 @@ void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
                }
        }
 
-       rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
-       synchronize_srcu_expedited(&kvm->srcu);
        kfree(bus);
        return;
 }