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

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Message-ID: <YzeaKjmls1YI/3ox@itl-email>
Date:   Fri, 30 Sep 2022 20:30:33 -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 Fri, Sep 30, 2022 at 08:27:09PM +0200, Ard Biesheuvel wrote:
> On Fri, 30 Sept 2022 at 19:12, Demi Marie Obenour wrote:
> > On Fri, Sep 30, 2022 at 06:30:57PM +0200, Ard Biesheuvel wrote:
> > > I know very little about Xen, but based on the context you provided in
> > > this thread, I'd say that the best approach from the Xen side is to
> > > convert all EfiBootServicesData regions that have configuration tables
> > > pointing into them into EfiAcpiReclaimMemory.
> >
> > Should Xen convert the entire region, or should it try to reserve only
> > the memory it needs?  The latter would require it to parse the
> > configuration tables.  Is there a list of configuration tables that can
> > legitimately be in EfiBootServicesData regions?
> >
> 
> Not really, no. So you would have to convert the entire region
> /unless/ Xen knows the GUID, and therefore knows how to derive the
> size of the table, allowing it to reserve memory more conservatively.
> However, I doubt whether this is worth it: splitting entries implies
> rewriting the memory map, which is a thing I'd rather avoid if I were
> in your shoes.

I actually wonder if Xen needs to reserve *all* of EfiBootServicesData.
The reason is that some (probably buggy) firmware may store ACPI tables
there, and Xen does not have an ACPI implementation.  From my
perspective, a much safer approach would be to pass all of
EfiBootServicesData memory directly to dom0, and have dom0 give Xen back
what it doesn’t wind up using.  That allows dom0’s memory reservation
code to work properly, which it currently does not.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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