linux-kernel - Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered

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Message-ID: <CAMj1kXFQNqsW5RfHGac-eGbosJHBybu6+-Fap_bi_kVxWNpGeg@mail.gmail.com>
Date:   Fri, 30 Sep 2022 18:30:57 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        linux-efi@...r.kernel.org, Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Subject: Re: [PATCH v4 1/2] Avoid using EFI tables Xen may have clobbered

On Fri, 30 Sept 2022 at 08:44, Jan Beulich <jbeulich@...e.com> wrote:
>
> On 30.09.2022 01:02, Demi Marie Obenour wrote:
> > Memory of type EFI_CONVENTIONAL_MEMORY, EFI_LOADER_CODE, EFI_LOADER_DATA,
> > EFI_BOOT_SERVICES_CODE, and EFI_BOOT_SERVICES_DATA may be clobbered by
> > Xen before Linux gets to start using it.  Therefore, Linux under Xen
> > must not use EFI tables from such memory.  Most of the remaining EFI
> > memory types are not suitable for EFI tables, leaving only
> > EFI_ACPI_RECLAIM_MEMORY, EFI_RUNTIME_SERVICES_DATA, and
> > EFI_RUNTIME_SERVICES_CODE.  When running under Xen, Linux should only
> > use tables that are located in one of these types of memory.
> >
> > This patch ensures this, and also adds a function
> > (xen_config_table_memory_region_max()) that will be used later to
> > replace the usage of the EFI memory map in esrt.c when running under
> > Xen.  This function can also be used in mokvar-table.c and efi-bgrt.c,
> > but I have not implemented this.
> >
> > Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
>
> In Xen we don't clobber EfiBootServices{Code,Data} when xen.efi was passed
> "-mapbs". Should we perhaps extend the interface such that Dom0 can then
> also use tables located in such regions, perhaps by faking
> EFI_MEMORY_RUNTIME in the attributes returned by XEN_FW_EFI_MEM_INFO?
>

I know this ship has sailed for x86, but for the sake of other
architectures, I'd strongly recommend leaving the EFI_MEMORY_RUNTIME
bits alone, for the same reasons I gave earlier. (Runtime mappings for
the firmware code itself, page table fragmentation etc etc)

I know very little about Xen, but based on the context you provided in
this thread, I'd say that the best approach from the Xen side is to
convert all EfiBootServicesData regions that have configuration tables
pointing into them into EfiAcpiReclaimMemory.

I take it XEN_FW_EFI_MEM_INFO is an existing interface? If so, you
might do the same for the returned type - EfiBootServicesData ->
EfiAcpiReclaimMemory, and not muck about with the EFI_MEMORY_RUNTIME
attribute.