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

[Ard Biesheuvel <ardb@xxxxxxxxxx>]

On Thu, 6 Oct 2022 at 19:22, Demi Marie Obenour
<demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Oct 06, 2022 at 06:19:35PM +0200, Ard Biesheuvel wrote:
> > On Thu, 6 Oct 2022 at 16:43, Demi Marie Obenour
> > <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Oct 06, 2022 at 09:31:47AM +0200, Ard Biesheuvel wrote:
> > > > On Thu, 6 Oct 2022 at 03:41, Demi Marie Obenour
> > > > <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Wed, Oct 05, 2022 at 11:28:29PM +0200, Ard Biesheuvel wrote:
> > > > > > On Wed, 5 Oct 2022 at 20:11, Demi Marie Obenour
> > > > > > <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > 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.
> > > > > > >
> > > > > >
> > > > > > That seems like a reasonable approach to me. Tables like MEMATTR or
> > > > > > RT_PROP are mostly relevant for bare metal where the host kernel 
> > > > > > maps
> > > > > > the runtime services, and in general, passing on these tables 
> > > > > > without
> > > > > > knowing what they do is kind of fishy anyway. You might even argue
> > > > > > that only known table types should be forwarded in the first place,
> > > > > > regardless of the memory type.
> > > > >
> > > > > Which tables are worth handling in Xen?  I know about ACPI, SMBIOS, 
> > > > > and
> > > > > ESRT, but I am curious which others Xen should preserve.  Currently, 
> > > > > Xen
> > > > > does not know about RT_PROP or MEMATTR; could this be a cause of
> > > > > problems?
> > > >
> > > > dom0 only has access to paravirtualized EFI runtime services, so
> > > > consuming RT_PROP or MEMATTR should be up to Xen (they describe which
> > > > runtime services remain available at runtime, and which permission
> > > > attributes to use for the runtime services memory regions,
> > > > respectively)
> > >
> > > Xen does not do this right now.  I wonder if this could be the cause of
> > > compatibility issues with various firmware implementations.
> > >
> > > > Looking through the kernel code, I don't think there are any that dom0
> > > > should care about beyond ACPI, SMBIOS and ESRT. But as you suggest,
> > > > that means Xen should just mask them in the view of the EFI system
> > > > table it exposes so dom0. Otherwise, the kernel may still try to map
> > > > and parse them.
> > >
> > > What about the BGRT and MOKvar?  I agree that Xen should not expose the
> > > others.  Should it just hide the tables, or should it actually uninstall
> > > them?  My intuition is that the second would be technically more
> > > correct, but also more likely to trigger bugs in various firmware
> > > implementations.
> >
> > BGRT is a ACPI table not a EFI configuration table, so I'd assume it
> > is treated the same way as other ACPI tables
>
> That actually surprises me.  Xen definitely needs to know about its
> GUID, then.

It doesn't have a GUID. The table itself is a ACPI table that is
enumerated in the usual ACPI way. The problem is that the BGRT
describes a region in memory (where the OEM boot logo is), and this is
usually in EfiBootServicesData memory.

This data is typically used during early boot, to keep the logo intact
while the OS loader draws a nice animation below it. Not the end of
the world if it gets corrupted or removed.