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

From: Demi Marie Obenour
Date: Wed Oct 05 2022 - 14:11:51 EST

On Wed, Oct 05, 2022 at 08:15:07AM +0200, Jan Beulich wrote:
> On 04.10.2022 17:46, Demi Marie Obenour wrote:
> > Linux has a function called efi_mem_reserve() that is used to reserve
> > EfiBootServicesData memory that contains e.g. EFI configuration tables.
> > This function does not work under Xen because Xen could have already
> > clobbered the memory. efi_mem_reserve() not working is the whole reason
> > for this thread, as it prevents EFI tables that are in
> > EfiBootServicesData from being used under Xen.
> >
> > A much nicer approach would be for Xen to reserve boot services memory
> > unconditionally, but provide a hypercall that dom0 could used to free
> > the parts of EfiBootServicesData memory that are no longer needed. This
> > would allow efi_mem_reserve() to work normally.
> efi_mem_reserve() actually working would be a layering violation;
> controlling the EFI memory map is entirely Xen's job.

Doing this properly would require Xen to understand all of the EFI
tables that could validly be in EfiBootServices* and which could be of
interest to dom0. It might (at least on some very buggy firmware)
require a partial ACPI and/or SMBIOS implementation too, if the firmware
decided to put an ACPI or SMBIOS table in EfiBootServices*.

> As to the hypercall you suggest - I wouldn't mind its addition, but only
> for the case when -mapbs is used. As I've indicated before, I'm of the
> opinion that default behavior should be matching the intentions of the
> spec, and the intention of EfiBootServices* is for the space to be
> reclaimed. Plus I'm sure you realize there's a caveat with Dom0 using
> that hypercall: It might use it for regions where data lives which it
> wouldn't care about itself, but which an eventual kexec-ed (or alike)
> entity would later want to consume. Code/data potentially usable by
> _anyone_ between two resets of the system cannot legitimately be freed
> (and hence imo is wrong to live in EfiBootServices* regions).

I agree, but currently some such data *is* in EfiBootServices* regions,
sadly. When -mapbs is *not* used, I recommend uninstalling all of the
configuration tables that point to EfiBootServicesData memory before
freeing that memory.

> In a way one could view the Dom0 kernel as an "or alike" entity ...

It is indeed such an entity.
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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