On Thu, 23 Mar 2017 15:34:41 +0100
David Hildenbrand <david@xxxxxxxxxx> wrote:
> No caller currently checks the return value of
> kvm_io_bus_unregister_dev(). This is evil, as all callers silently go on
> freeing their device. A stale reference will remain in the io_bus,
> getting at least used again, when the iobus gets teared down on
> kvm_destroy_vm() - leading to use after free errors.
>
> There is nothing the callers could do, except retrying over and over
> again.
>
> So let's simply remove the bus altogether, print an error and make
> sure no one can access this broken bus again (returning -ENOMEM on any
> attempt to access it).
>
> Fixes: e93f8a0f821e ("KVM: convert io_bus to SRCU")
> Cc: stable@xxxxxxxxxxxxxxx # 3.4+
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> /* Caller must hold slots_lock. */
> -int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> - struct kvm_io_device *dev)
> +void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> + struct kvm_io_device *dev)
> {
> - int i, r;
> + int i;
> struct kvm_io_bus *new_bus, *bus;
>
> bus = kvm->buses[bus_idx];
> -
> - /*
> - * It's possible the bus being released before hand. If so,
> - * we're done here.
> - */
> if (!bus)
> - return 0;
> + return;
>
> - r = -ENOENT;
> for (i = 0; i < bus->dev_count; i++)
> if (bus->range[i].dev == dev) {
> - r = 0;
> break;
> }
>
> - if (r)
> - return r;
> + if (i == bus->dev_count)
> + return;
>
> new_bus = kmalloc(sizeof(*bus) + ((bus->dev_count - 1) *
> sizeof(struct kvm_io_range)), GFP_KERNEL);
> - if (!new_bus)
> - return -ENOMEM;
> + if (!new_bus) {
> + pr_err("kvm: failed to shrink bus, removing it completely\n");
> + goto broken;
> + }
>
> memcpy(new_bus, bus, sizeof(*bus) + i * sizeof(struct kvm_io_range));
> new_bus->dev_count--;
> memcpy(new_bus->range + i, bus->range + i + 1,
> (new_bus->dev_count - i) * sizeof(struct kvm_io_range));
>
> +broken:
> rcu_assign_pointer(kvm->buses[bus_idx], new_bus);
As this may set kvm->buses[bus_idx] to NULL, don't you also need to
guard for bus == NULL in kvm_io_bus_destroy()? (I looked at the code on
kvm/queue.)
> synchronize_srcu_expedited(&kvm->srcu);
> kfree(bus);
> - return r;
> + return;
> }