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

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Date:   Fri, 30 Sep 2022 12:38:44 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     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>,
        Ard Biesheuvel <ardb@...nel.org>,
        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, Sep 30, 2022 at 08:44:21AM +0200, Jan Beulich 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 can add a check for EFI_MEMORY_RUNTIME, but only if I can require a Xen
version with https://lore.kernel.org/xen-devel/cc0fbcb4-5ea3-178c-e691-9acb7cc9a3a7@suse.com/t/#u.
This is easy in Qubes OS via RPM dependencies, but I am not sure if it
is suitable for upstream without a mechanism for dom0 to verify that the
patch has been included.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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