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Date:   Fri, 30 Sep 2022 18:25:14 -0400
From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Peter Jones <pjones@redhat.com>, Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Kees Cook <keescook@chromium.org>,
        Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>,
        Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org
Subject: Re: [PATCH v4 2/2] Support ESRT in Xen dom0
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References: <cover.1664298147.git.demi@invisiblethingslab.com>
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Date: Fri, 30 Sep 2022 18:25:14 -0400
From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Peter Jones <pjones@redhat.com>, Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
	Kees Cook <keescook@chromium.org>,
	Anton Vorontsov <anton@enomsg.org>,
	Colin Cross <ccross@android.com>, Tony Luck <tony.luck@intel.com>,
	Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= <marmarek@invisiblethingslab.com>,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	linux-efi@vger.kernel.org
Subject: Re: [PATCH v4 2/2] Support ESRT in Xen dom0

On Fri, Sep 30, 2022 at 10:59:49PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 22:21, Demi Marie Obenour
> <demi@invisiblethingslab.com> wrote:
> >
> > 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@invisiblethingslab.com> 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@invisiblethingslab.com> wrote:
> > > > > >
> > > > > > fwupd requires access to the EFI System Resource Table (ESRT) to
> > > > > > discover which firmware can be updated by the OS.  Currently, L=
inux 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 limitat=
ions.
> > > > > > The UEFI specification requires the ESRT to be in EfiBootServic=
esData
> > > > > > memory, which Xen will use for whatever purposes it likes.  The=
refore,
> > > > > > Linux cannot safely access the ESRT, as Xen may have overwritte=
n it.
> > > > > >
> > > > > > Starting with Xen 4.17, Xen checks if the ESRT is in EfiBootSer=
vicesData
> > > > > > or EfiRuntimeServicesData memory.  If the ESRT is in EfiBootSer=
vicesData
> > > > > > memory, Xen replaces the ESRT with a copy in memory that it has
> > > > > > reserved.  Such memory is currently of type EFI_RUNTIME_SERVICE=
S_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 X=
en has
> > > > > > reserved the ESRT and, if so, find the end of the memory region
> > > > > > containing it.  This allows programs such as fwupd which requir=
e the
> > > > > > ESRT to run under Xen, and so makes fwupd support in Qubes OS p=
ossible.
> > > > > >
> > > > > > Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> > > > >
> > > > > Why do we need this patch? I'd expect esrt_table_exists() to retu=
rn
> > > > > 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=E2=80=99t uninstall bogus ESRTs, so there is no less reason t=
o worry
> > under Xen than on bare hardware.
>=20
> That may be true. But if Xen needs dom0 to be able to cross reference
> the EFI memory map, it should provide one (and set EFI_MEMMAP to
> enabled).

I agree, but it is also a significant amount of work compared to this
patch.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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