Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <cover.1664298147.git.demi@invisiblethingslab.com>
 <5649176eacda434267f68676f1733d06c572d19e.1664298147.git.demi@invisiblethingslab.com>
 <CAMj1kXEs-o8jvNqRiW+Ue2i52RBgg4iktg8UONCACk8-Gx4XXA@mail.gmail.com>
 <YzczpIYop5olD4hj@itl-email> <CAMj1kXHGPzy9T1LcE8LX+woGtUGTzrDgbjDBJabJ+bwDVPbTag@mail.gmail.com>
 <YzdPv4+fYX3SG9P0@itl-email> <CAMj1kXGTZZmW=QZOL1FvrsBLsvFzN1GkvZCotuZ-C-gBVGY6CQ@mail.gmail.com>
In-Reply-To: <CAMj1kXGTZZmW=QZOL1FvrsBLsvFzN1GkvZCotuZ-C-gBVGY6CQ@mail.gmail.com>
From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 30 Sep 2022 23:24:37 +0200
Message-ID: <CAMj1kXFJa4sSDEkRwTXTntJcJrtshNofU6LaZOWtzMbp56cXXQ@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] Support ESRT in Xen dom0
To:     Demi Marie Obenour <demi@invisiblethingslab.com>
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>,
        =?UTF-8?Q?Marek_Marczykowski=2DG=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Fri, 30 Sept 2022 at 22:59, Ard Biesheuvel <ardb@kernel.org> 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) t=
o
> > > > > > 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.
>
> 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).

Btw the efi_mem_reserve() for the ESRT is also redundant if it is
guaranteed to be in RT services data or ACPI reclaim memory.