Re: [PATCH] efi: expose TPM event log to userspace via sysfs

[Mikko Rapeli <mikko.rapeli@linaro.org>]

Hi,

On Thu, Apr 25, 2024 at 09:24:48AM -0400, James Bottomley wrote:
> On Thu, 2024-04-25 at 11:58 +0200, Lennart Poettering wrote:
> [...]
> > General purpose distros typically don't build all TPM drivers into
> > the kernel, but ship some in the initrd instead. Then, udev is
> > responsible for iterating all buses/devices and auto-loading the
> > necessary drivers. Each loaded bus driver might make more devices
> > available for which more drivers then need to be loaded, and so on.
> > Some of the busses are "slow" in the sense that we don't really know
> > a precise time when we know that all devices have now shown up, there
> > might always be slow devices that haven't popped up yet. Iterating
> > through the entire tree of devices in sysfs is often quite slow in
> > itself too, it's one of the most time consuming parts of the boot in
> > fact. This all is done asynchronously hence: we
> > enumerate/trigger/kmod all devices as quickly as we can, but we
> > continue doing other stuff at the same time.
> 
> So let me make a suggestion that you can use now.  Since all you
> currently care about is the EFI/ACPI device, there is always a single
> sysfs entry that corresponds to that (so you shouldn't need the log
> entry as an indicator):
> 
> /sys/bus/acpi/devices/MSFT0101\:00
> 
> That link (or a kobject uevent if you prefer to look for that) will
> always appear regardless of whether a driver has attached or not.  When
> the driver actually attaches, a driver/ directory will appear where the
> link points.
> 
> The device link is added when the acpi scan is initiated as a
> subsys_initcall, which is before all the filesystem initcalls, so it
> should run before the initrd is mounted.
> 
> Is this enough for now and we can think about a more generic indicator
> that all drivers have been probed later?

This covers EFI ACPI devices but not devices without ACPI.

Some boards have the TPM device data in devicetree.

Some boards have a firmware TPM which is not listed in devicetree
but is detected at runtime once optee drivers, tee-supplicant, etc have
loaded.

Based on the comments here, I could propose a v2 with TPMFinalLog based
sysfs file which is empty but existence of the file shows to systemd that
EFI firmware used a TPM device and that can queue in the normal ACPI,
devicetree or other mechanisms of detecting the real TPM device, loading drivers
and possibly needed userspace components like tee-supplicant for optee and fTPM.

I don't know how non-EFI firmware could be supported, unless they show the ACPI TPM
entry.

Cheers,

-Mikko