Re: [PATCH 1/2] Avoid potential race with FiltersInstall

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 28/06/2021 13:58, Owen Smith wrote:
> Is certain situations, a race between XENFILT and XENBUS can lead to XENFILT
> not being loaded on the root PCI device node. This is due to XENBUS!DriverEntry
> removing the registry value just before the PnP manager determines what filters
> to load, and fails to load XENFILT on the root PCI node. This leads to XENBUS
> being unable to determine the correct ActiveDevice. Without an ActiveDevice,
> no Unplugs are issued, and emulated devices are used for boot, leading to a
> reboot prompt before XENVBD can be used as the boot device. The race appears to
> be reliable once triggered, and a reboot will follow the same sequence. This
> appears to be caused by OS upgrades which affect the order the PnP manager
> starts different driver stacks.
>
> This contains a reversion to 9d28a9e9b79, which fixed an upgrade issue that
> triggered multiple reboot requirements to reload XENFILT correctly.
> If an incompatibility is detected, which can be resolved by a reboot to
> complete the driver installation, XENFILT is inserted into the UpperFilters so
> that XENFILT is loaded on this reboot. This avoids requiring a second reboot so
> that XENFILT can load and determine the ActiveDevice.
>
> Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
Acked-by: Paul Durrant <paul@xxxxxxx>

> ---
>   src/xenbus/driver.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/src/xenbus/driver.c b/src/xenbus/driver.c
> index 1b621fa..067bb68 100644
> --- a/src/xenbus/driver.c
> +++ b/src/xenbus/driver.c
> @@ -296,6 +296,9 @@ DriverRemoveFunctionDeviceObject(
>       RemoveEntryList(&Dx->ListEntry);
>       ASSERT3U(Driver.References, !=, 0);
>       References = --Driver.References;
> +
> +    if (References == 1)
> +        FiltersUninstall();
>   }
>   
>   //
> @@ -859,16 +862,17 @@ DriverEntry(
>                         MICRO_VERSION,
>                         BUILD_NUMBER);
>       if (!NT_SUCCESS(status)) {
> -        if (status == STATUS_INCOMPATIBLE_DRIVER_BLOCKED)
> +        if (status == STATUS_INCOMPATIBLE_DRIVER_BLOCKED) {
> +            // XenBus.sys is not the same version as Xen.sys
> +            // Insert XenFilt to avoid a 2nd reboot in upgrade cases, as 
> AddDevice
> +            // will not be called to insert XenFilt.
> +            FiltersInstall();
>               __DriverRequestReboot();
> +        }
>   
>           goto done;
>       }
>   
> -    // Remove the filters from the registry. They will be re-instated by
> -    // the first successful AddDevice.
> -    FiltersUninstall();
> -
>       DriverObject->DriverExtension->AddDevice = DriverAddDevice;
>   
>       for (Index = 0; Index <= IRP_MJ_MAXIMUM_FUNCTION; Index++) {