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

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Date:   Thu, 6 Oct 2022 09:31:47 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>
Cc:     Jan Beulich <jbeulich@...e.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 Thu, 6 Oct 2022 at 03:41, Demi Marie Obenour
<demi@...isiblethingslab.com> 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@...isiblethingslab.com> 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)

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.