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

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Date:   Wed, 5 Oct 2022 21:40:58 -0400
From:   Demi Marie Obenour <demi@...isiblethingslab.com>
To:     Ard Biesheuvel <ardb@...nel.org>
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 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?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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