Re: [PATCH 2/2] efi: Add embedded peripheral firmware support

[Luis R. Rodriguez]

On Tue, Apr 03, 2018 at 08:07:11PM +0200, Lukas Wunner wrote:
> On Tue, Apr 03, 2018 at 10:33:25AM +0200, Hans de Goede wrote:
> > I asked Peter Jones for suggestions how to extract this during boot and
> > he suggested seeing if there was a copy of the firmware in the
> > EFI_BOOT_SERVICES_CODE memory segment, which it turns out there is.
> > 
> > My patch to add support for this contains a table of device-model (dmi
> > strings), firmware header (first 64 bits), length and crc32 and then if
> > we boot on a device-model which is in the table the code scans the
> > EFI_BOOT_SERVICES_CODE for the prefix, if found checks the crc and
> > caches the firmware for later use by request-firmware.
> > 
> > So I just do a brute-force search for the firmware, this really is hack,
> > nothing standard about it I'm afraid. But it works on 4 different x86
> > tablets I have and makes the touchscreen work OOTB on them, so I believe
> > it is a worthwhile hack to have.
> 
> The EFI Firmware Volume contains a kind of filesystem with files
> identified by GUIDs.  Those files include EFI drivers, ACPI tables,
> DMI data and so on.  It is actually quite common for vendors to
> also include device firmware on the Firmware Volume.  Apple is doing
> this to ship firmware updates e.g. for the GMUX controller found on
> dual GPU MacBook Pros.  If they want to update the controller's
> firmware, they include it in a BIOS update, and an EFI driver checks
> on boot if the firmware update for the controller is necessary and
> if so, flashes it.
> 
> The firmware files you're looking for are almost certainly included
> on the Firmware Volume as individual files.

What Hans implemented seems to have been for a specific x86 hack, best if we
confirm if indeed they are present on the Firmware Volume.

> Rather than scraping
> the EFI memory for firmware, I think it would be cleaner and more
> elegant if you just retrieve the files you're interested in from
> the Firmware Volume.
> 
> We're doing something similar with Apple EFI properties, see
> 58c5475aba67 and c9cc3aaa0281.
> 
> Basically what you need to do to implement this approach is:
> 
> * Determine the GUIDs used by vendors for the files you're interested
>   in.  Either dump the Firmware Volume or take an EFI update as
>   shipped by the vendor, then feed it to UEFIExtract:
>   https://github.com/LongSoft/UEFITool
>   
> * Add the EFI Firmware Volume Protocol to include/linux/efi.h:
>   https://www.intel.com/content/dam/doc/reference-guide/efi-firmware-file-volume-specification.pdf
> 
> * Amend arch/x86/boot/compressed/eboot.c to read the files with the
>   GUIDs you're interested in into memory and pass the files to the
>   kernel as setup_data payloads.
> 
> * Once the kernel has booted, make the files you've retrieved
>   available to device drivers as firmware blobs.

Happen to know if devices using Firmware Volumes also sign their firmware
and if hw checks the firmware at load time?

  Luis

> The end result is mostly the same as what you're doing here,
> and you'll also have a similar array of structs, but instead
> of hardcoding 8-byte signatures of firmware files, you'll be
> using GUIDs.  I envision lots of interesting use cases for
> a generic Firmware Volume file retrieval mechanism.  What you
> need to keep in mind though is that this approach only works
> if the kernel is booted via the EFI stub.
> 
> Thanks,
> 
> Lukas
> 

-- 
Do not panic