Date:   Fri, 23 Sep 2022 00:14:50 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
Cc:     Jan Beulich <jbeulich@...e.com>, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Subject: Re: [PATCH v3] Support ESRT in Xen dom0

On Thu, 22 Sept 2022 at 20:12, Demi Marie Obenour
<demi@...isiblethingslab.com> wrote:
>
> On Thu, Sep 22, 2022 at 05:05:43PM +0200, Ard Biesheuvel wrote:
> > On Thu, 22 Sept 2022 at 16:56, Demi Marie Obenour
> > <demi@...isiblethingslab.com> wrote:
> > >
> > > On Thu, Sep 22, 2022 at 08:12:14AM +0200, Jan Beulich wrote:
> > > > On 22.09.2022 03:09, Demi Marie Obenour wrote:
> > > > > On Wed, Sep 21, 2022 at 10:34:04PM +0200, Jan Beulich wrote:
> > > > >> On 20.09.2022 18:09, Ard Biesheuvel wrote:
> > > > >>> On Tue, 20 Sept 2022 at 17:54, Jan Beulich <jbeulich@...e.com> wrote:
> > > > >>>>
> > > > >>>> On 20.09.2022 17:36, Ard Biesheuvel wrote:
> > > > >>>>> On Mon, 19 Sept 2022 at 21:33, Demi Marie Obenour
> > > > >>>>> <demi@...isiblethingslab.com> 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 4.16, 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 allocates some memory of type EfiRuntimeServicesData, copies
> > > > >>>>>> the ESRT to it, and finally replaces the ESRT pointer with a pointer to
> > > > >>>>>> the copy.  Since Xen will not clobber EfiRuntimeServicesData memory,
> > > > >>>>>> this ensures that the ESRT can safely be accessed by the OS.  It is safe
> > > > >>>>>> to access the ESRT under Xen if, and only if, it is in memory of type
> > > > >>>>>> EfiRuntimeServicesData.
> > > > >>>>>>
> > > > >>>>>
> > > > >>>>> Thanks for the elaborate explanation. This is really helpful.
> > > > >>>>>
> > > > >>>>> So here, you are explaining that the only way for Xen to prevent
> > > > >>>>> itself from potentially clobbering the ESRT is by creating a
> > > > >>>>> completely new allocation?
> > > > >>>>
> > > > >>>> There are surely other ways, e.g. preserving BootServices* regions
> > > > >>>> alongside RuntimeServices* ones. But as the maintainer of the EFI
> > > > >>>> code in Xen I don't view this as a reasonable approach.
> > > > >>>
> > > > >>> Why not?
> > > > >>
> > > > >> Because it's against the intentions the EFI has (or at least had)
> > > > >> for this memory type. Much more than EfiAcpiReclaimMemory this
> > > > >> type is intended for use as ordinary RAM post-boot.
> > > > >
> > > > > What about giving that memory to dom0?  dom0’s balloon driver will give
> > > > > anything dom0 doesn’t wind up using back to Xen.
> > > >
> > > > While perhaps in principle possible, this would require special casing
> > > > in Xen. Except for the memory the initrd comes in, we don't directly
> > > > hand memory to Dom0. Instead everything goes through the page allocator
> > > > first. Plus if we really were convinced boot services memory needed
> > > > retaining, then it would also need retaining across kexec (and hence
> > > > shouldn't be left to Dom0 to decide what to do with it).
> > >
> > > So how should dom0 handle the various EFI tables other than the ESRT?
> > > Right now most uses of these tables in Linux are not guarded by any
> > > checks for efi_enabled(EFI_MEMMAP) or similar.  If some of them are in
> > > EfiBootServicesData memory, they might be corrupted before Linux gets
> > > them.
> >
> > Yes, this is an annoying oversight of the EFI design: the config
> > tables are <guid, address> tuples, and the size of the table is
> > specific to each table type. So without knowing the GUID, there is no
> > way you can reserve the right size.
> >
> > Perhaps you could implement something like a hypercall in
> > efi_arch_mem_reserve(), which is called by the EFI code to reserve
> > regions that are in boot services memory but need to remain reserved?
> > This is used for all config tables that it knows or cares about.
>
> On versions of Xen that support spawning multiple domains at boot
> (instead of just dom0) this will be racy.  What about refusing to use
> tables in EfiBootServicesData when running under Xen unless a hypercall
> indicates that Xen has reserved all EfiBootServicesData memory?  Where
> could such a check be placed?

You could stick a check inside the for loop in
efi_config_parse_tables() to cross reference every table address
against the memory map when running on Xen, and disregard it if it
doesn't meet your criteria.

I take it the issue is not limited to x86?

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