Re: [PATCH v9 01/12] platform/x86/intel/{pmt,vsec}: Prevent unbind via sysfs

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Tony,

On 7/1/26 2:35 PM, Tony Luck wrote:
> The resctrl AET code uses intel_pmt_get_regions_by_feature() (exported by the
> pmt_telemetry module) to obtain pmt_feature_group instances that are populated
> using data from the pmt_discovery module. The returned telemetry_region::addr
> values are MMIO mappings created in the telemetry driver's probe path,
> and resctrl reads from them while a mount is active.
> 
> However, a user can unbind the intel_pmt driver or its parent intel_vsec
> from their devices via sysfs "unbind" attributes. Unbinding either of these
> drivers would tear down the MMIO mappings, leaving resctrl with stale pointers
> and causing a fault on the next access.
> 
> It is safe for the discovery driver to be unbound. It does all its work at
> initial probe time and saves a copy of the features and RMID count.
> 
> Set device_driver::suppress_bind_attrs in each driver so that the "bind"
> and "unbind" sysfs attributes are not exposed and the user cannot detach
> the driver from its devices while resctrl (or any other in-kernel consumer)
> may be using the resources.

Unbinding via sysfs does not seem to be the only way in which resources that
resctrl depend on could disappear.  The "/sys/bus/pci/devices/.../remove"
interface looks to be another way? I am not aware of a way to prevent this since
this represents how devices could just be yanked from a system. I wonder if
resctrl should instead protect itself against telemetry resources disappearing
after mount with another helper that telemetry driver can call when the device
is being removed or the module is being unloaded. When telemetry driver informs AET
that resources are going away after resctrl is mounted then resctrl/AET can return
EIO on event read attempts? resctrl could possibly recover when the device/module
re-appears but if that is complicated then user space could just be forced to remount
resctrl.

With this there is no need to prevent unbind here since it creates a false
guarantee to resctrl.

> 
> Fixes: 1fb2daa60de6 ("x86/resctrl: Discover hardware telemetry events")
> Assisted-by: Claude:Opus-4.7
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> --

Please note the separator typo ("--" -> "---") above that exists in every patch in
this series that has a changelog. Please check all patches for this, I only highlight this
instance.

Reinette