Re: [PATCH] Fix a gap where acpi_pcihp_find_hotplug_bus() returns a non-hotpluggable bus

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, Aug 29, 2020 at 01:42:33PM +0530, Ani Sinha wrote:
> When ACPI hotplug for the root bus is disabled, the bsel property for that
> bus is not set. Please see the following commit:
> 
> 3d7e78aa7777f ("Introduce a new flag for i440fx to disable PCI hotplug on the root bus").
> 
> As a result, when acpi_pcihp_find_hotplug_bus() is called
> with bsel set to 0, it may return the root bus. This would be wrong since the
> root bus is not hotpluggable. In general, this can potentially happen to other
> buses as well.
> In this patch, we fix the issue in this function by checking if the bus returned
> by the function is actually hotpluggable. If not, we simply return NULL. This
> avoids the scenario where we are actually returning a non-hotpluggable bus.
> 
> Signed-off-by: Ani Sinha <ani@anisinha.ca>

What exactly are the consequences though?


> ---
>  hw/acpi/pcihp.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
> index 39b1f74442..f148e73c89 100644
> --- a/hw/acpi/pcihp.c
> +++ b/hw/acpi/pcihp.c
> @@ -147,6 +147,21 @@ static PCIBus *acpi_pcihp_find_hotplug_bus(AcpiPciHpState *s, int bsel)
>      if (!bsel && !find.bus) {
>          find.bus = s->root;
>      }
> +
> +    /*
> +     * Check if find.bus is actually hotpluggable. If bsel is set to
> +     * NULL for example on the root bus in order to make it
> +     * non-hotpluggable, find.bus will match the root bus when bsel
> +     * is 0. See acpi_pcihp_test_hotplug_bus() above. Since the
> +     * bus is not hotpluggable however, we should not select the bus.
> +     * Instead, we should set find.bus to NULL in that case. In the check
> +     * below, we generalize this case for all buses, not just the root bus.
> +     * The callers of this function check for a null return value and
> +     * handle them appropriately.
> +     */
> +    if (!qbus_is_hotpluggable(BUS(find.bus))) {
> +        find.bus = NULL;
> +    }
>      return find.bus;
>  }
>  
> -- 
> 2.17.1