Re: [PATCH v4 2/2] Support ESRT in Xen dom0

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Sep 30, 2022 at 09:11:19PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 20:21, Demi Marie Obenour
> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, Sep 30, 2022 at 06:36:11PM +0200, Ard Biesheuvel wrote:
> > > On Fri, 30 Sept 2022 at 01:02, Demi Marie Obenour
> > > <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > fwupd requires access to the EFI System Resource Table (ESRT) to
> > > > discover which firmware can be updated by the OS.  Currently, Linux does
> > > > not expose the ESRT when running as a Xen dom0.  Therefore, it is not
> > > > possible to use fwupd in a Xen dom0, which is a serious problem for e.g.
> > > > Qubes OS.
> > > >
> > > > Before Xen 4.17, this was not fixable due to hypervisor limitations.
> > > > The UEFI specification requires the ESRT to be in EfiBootServicesData
> > > > memory, which Xen will use for whatever purposes it likes.  Therefore,
> > > > Linux cannot safely access the ESRT, as Xen may have overwritten it.
> > > >
> > > > Starting with Xen 4.17, Xen checks if the ESRT is in EfiBootServicesData
> > > > or EfiRuntimeServicesData memory.  If the ESRT is in EfiBootServicesData
> > > > memory, Xen replaces the ESRT with a copy in memory that it has
> > > > reserved.  Such memory is currently of type EFI_RUNTIME_SERVICES_DATA,
> > > > but in the future it will be of type EFI_ACPI_RECLAIM_MEMORY.  This
> > > > ensures that the ESRT can safely be accessed by the OS.
> > > >
> > > > When running as a Xen dom0, use the new
> > > > xen_config_table_memory_region_max() function to determine if Xen has
> > > > reserved the ESRT and, if so, find the end of the memory region
> > > > containing it.  This allows programs such as fwupd which require the
> > > > ESRT to run under Xen, and so makes fwupd support in Qubes OS possible.
> > > >
> > > > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> > >
> > > Why do we need this patch? I'd expect esrt_table_exists() to return
> > > false when patch 1/2 is applied.
> >
> > efi_enabled(EFI_MEMMAP) is false under Xen, so there needs to be an
> > alternative way to get the end of the memory region containing the ESRT.
> > That is what this patch provides.
> 
> OK. I don't think we need that to be honest. When running under Xen,
> we should be able to assume that the ESRT does not span multiple
> memory regions arbitrarily, so we can just omit this check if
> !efi_enabled(EFI_MEMMAP)
> 
> IIRC (and Peter would know), we are trying to filter out descriptors
> that are completely bogus here: zero lenght, zero address, etc etc. I
> don't think we need that for Xen.

Xen doesn’t uninstall bogus ESRTs, so there is no less reason to worry
under Xen than on bare hardware.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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