[PATCH] efi_loader: Disable devices before handing over control

[Mark Kettenis]

> From: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> Date: Wed, 21 Oct 2020 15:42:02 +0300
> 
> Hi Heinrich,
> 
> 
> On Wed, 21 Oct 2020 at 15:35, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >
> > On 21.10.20 13:41, Ilias Apalodimas wrote:
> > > Hi Heinrich,
> > >
> > > On Wed, Oct 21, 2020 at 12:17:29PM +0200, Heinrich Schuchardt wrote:
> > >> On 10/21/20 9:32 AM, Ilias Apalodimas wrote:
> > >>> U-Boot Driver Model is supposed to remove devices with either
> > >>> DM_REMOVE_ACTIVE_DMA or DM_REMOVE_OS_PREPARE flags set, before exiting.
> > >>> Our bootm command does that by explicitly calling calling
> > >>> "dm_remove_devices_flags(DM_REMOVE_ACTIVE_ALL);" and we also disable any
> > >>> USB devices.
> > >>>
> > >>> The EFI equivalent is doing none of those at the moment. As a result
> > >>> probing an fTPM driver now renders it unusable in Linux. During our
> > >>> (*probe) callback we open a session with OP-TEE, which is supposed to
> > >>> close with our (*remove) callback. Since the (*remove) is never called,
> > >>> once we boot into Linux and try to probe the device again we are getting
> > >>> a busy error response. We also never free
> > >>>
> > >>> So let's fix this by mimicking what bootm does and disconnect devices
> > >>> when efi_exit_boot_services() is called. Note that for the OP-TEE case
> > >>> and in particular any subsequent bootloader that wants to use a device
> > >>> (e.g GRUB) will need to call exit_boot_services() in order to close the
> > >>> session.
> > >>
> > >> Hello Ilias,
> > >>
> > >> thanks for the patch. Adding the function calls looks correct to me,
> > >
> > > Well the only doubt I have is what if GRUB has to extend some PCRs before
> > > calling Linux? Any idea if it's currently calling ExitBootSevices?
> > > I was considering if it would be a better idea to call the device unbinding during
> > > some kind of "exit" from U-boot's EFI code. (i.e before StartImage)
> >
> > ExitBootServices() is called by the Linux EFI stub in function
> > allocate_new_fdt_and_exit_boot().
> >
> > If GRUB would call ExitBootServices(), it would not be able to launch
> > the EFI stub via StartImage().
> 
> Yea that's my point. So with the current patch, you won't be able to
> access the fTPM driver from GRUB
> (or any other EFI application) until the Linux EFI stub calls exit
> boot services. Maybe calling those 2 functions in
> StartImage is a better idea?

Shouldn't an EFI application (such as GRUB) be using EFI protocols to
access the TPM?