Re: [PATCH] Platform lockdown information in sysfs (v2)

[Arnd Bergmann]

On Thu, Aug 20, 2020 at 4:51 PM Daniel Gutson
<daniel.gutson@xxxxxxxxxxxxx> wrote:
>
> This patch exports information about the platform lockdown
> firmware configuration in the sysfs filesystem.
> In this initial patch, I include some configuration attributes
> for the system SPI chip.
>
> This initial version exports the BIOS Write Enable (bioswe),
> BIOS Lock Enable (ble), and the SMM BIOS Write Protect (SMM_BWP)
> fields of the BIOS Control register. The idea is to keep adding more
> flags, not only from the BC but also from other registers in following
> versions.
>
> The goal is that the attributes are avilable to fwupd when SecureBoot
> is turned on.
>
> The patch provides a new misc driver, as proposed in the previous patch,
> that provides a registration function for HW Driver devices to register
> class_attributes.
> In this case, the intel SPI flash chip (intel-spi) registers three
> class_attributes corresponding to the fields mentioned above.
>
> This version of the patch replaces class attributes by device
> attributes.
>
> Signed-off-by: Daniel Gutson <daniel.gutson@xxxxxxxxxxxxx>

This looks much better than before, thanks for addressing the feedback.
> diff --git a/Documentation/ABI/stable/sysfs-class-platform-lockdown b/Documentation/ABI/stable/sysfs-class-platform-lockdown
> new file mode 100644
> index 000000000000..3fe75d775a42
> --- /dev/null
> +++ b/Documentation/ABI/stable/sysfs-class-platform-lockdown
> @@ -0,0 +1,23 @@
> +What:          /sys/class/platform-lockdown/bioswe

platform-lockdown is a much better name than the previous suggestions.
I'm still hoping for an even better suggestion. Like everything the term
"lockdown" is also overloaded a bit, with the other common meaning
referring to the effort to give root users less privilege than the
kernel itself,
see https://lwn.net/Articles/750730/

Shouldn't there be a device name between the class name
("platform-lockdown") and the attribute name?

> +PLATFORM LOCKDOWN ATTRIBUTES MODULE
> +M:     Daniel Gutson <daniel.gutson@xxxxxxxxxxxxx>
> +S:     Supported
> +F:     Documentation/ABI/sysfs-class-platform-lockdown
> +F:     drivers/misc/platform-lockdown-attrs.c
> +F:     include/linux/platform_data/platform-lockdown-attrs.h

include/linux/platform_data/ is not the right place for the header,
this is defined to be the place for defining properties of devices
that are created from old-style board files.

Just put the header into include/linux/ directly.
the host.
>
> +config PLATFORM_LOCKDOWN_ATTRS
> +       tristate "Platform lockdown information in the sysfs"
> +       depends on SYSFS
> +       help
> +         This kernel module is a helper driver to provide information about
> +         platform lockdown settings and configuration.
> +         This module is used by other device drivers -such as the intel-spi-
> +         to publish the information in /sys/class/platform-lockdown.

Maybe mention fwupd in the description in some form.

> +
> +static struct class platform_lockdown_class = {
> +       .name = "platform-lockdown",
> +       .owner = THIS_MODULE,
> +};
> +
> +struct device *register_platform_lockdown_data_device(struct device *parent,
> +                                                     const char *name)
> +{
> +       return device_create(&platform_lockdown_class, parent, MKDEV(0, 0),
> +                            NULL, name);
> +}
> +EXPORT_SYMBOL_GPL(register_platform_lockdown_data_device);
> +
> +void unregister_platform_lockdown_data_device(struct device *dev)
> +{
> +       device_unregister(dev);
> +}
> +EXPORT_SYMBOL_GPL(unregister_platform_lockdown_data_device);
> +
> +int register_platform_lockdown_attribute(struct device *dev,
> +                                        struct device_attribute *dev_attr)
> +{
> +       return device_create_file(dev, dev_attr);
> +}
> +EXPORT_SYMBOL_GPL(register_platform_lockdown_attribute);
> +
> +void register_platform_lockdown_attributes(struct device *dev,
> +                                          struct device_attribute dev_attrs[])
> +{
> +       u32 idx = 0;
> +
> +       while (dev_attrs[idx].attr.name != NULL) {
> +               register_platform_lockdown_attribute(dev, &dev_attrs[idx]);
> +               idx++;
> +       }

There is a bit of a race with creating the device first and then
the attributes. Generally it seems better to me to use
device_create_with_groups() instead so the device shows up
with all attributes in place already.

> +void register_platform_lockdown_custom_attributes(struct device *dev,
> +                                                 void *custom_attrs,
> +                                                 size_t dev_attr_offset,
> +                                                 size_t custom_attr_size)

This interface seems to be overly complex, I would hope it can be avoided.

> +static ssize_t cnl_spi_attr_show(struct device *dev,
> +       struct device_attribute *attr, char *buf)
> +{
> +       u32 bcr;
> +       struct cnl_spi_attr *cnl_spi_attr = container_of(attr,
> +               struct cnl_spi_attr, dev_attr);
> +
> +       if (class_child_device != dev)
> +               return -EIO;
> +
> +       if (dev->parent == NULL)
> +               return -EIO;
> +
> +       if (pci_read_config_dword(container_of(dev->parent, struct pci_dev, dev),
> +                               BCR, &bcr) != PCIBIOS_SUCCESSFUL)
> +               return -EIO;
> +
> +       return sprintf(buf, "%d\n", (int)!!(bcr & cnl_spi_attr->mask));
> +}

If I understand it right, that complexity comes from attempting to
have a single show callback for three different flags. To me that
actually feels more complicated than having an attribute group
with three similar but simpler show callbacks.

>  static void intel_spi_pci_remove(struct pci_dev *pdev)
>  {
> +       if (class_child_device != NULL) {

Please avoid the global variable here and just add a member in the
per-device data.

> +               unregister_platform_lockdown_custom_attributes(
> +                       class_child_device,
> +                       cnl_spi_attrs,
> +                       offsetof(struct cnl_spi_attr, dev_attr),
> +                       sizeof(struct cnl_spi_attr));
> +
> +               unregister_platform_lockdown_data_device(class_child_device);

It should be possible to just destroy the attributes as part of
unregister_platform_lockdown_data_device.

       Arnd