linux-kernel - Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

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Message-ID: <20230116134246.soworigs56bz5v7o@box.shutemov.name>
Date:   Mon, 16 Jan 2023 16:42:46 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        x86@...nel.org, jiewen.yao@...el.com, devel@...2.groups.io,
        "Min M. Xu" <min.m.xu@...el.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2] x86/efi: Safely enable unaccepted memory in UEFI

On Mon, Jan 16, 2023 at 02:11:26PM +0100, Ard Biesheuvel wrote:
> On Mon, 16 Jan 2023 at 13:31, Kirill A. Shutemov <kirill@...temov.name> wrote:
> >
> > On Mon, Jan 16, 2023 at 11:56:48AM +0100, Gerd Hoffmann wrote:
> > > On Sat, Jan 14, 2023 at 01:20:24AM +0300, Kirill A. Shutemov wrote:
> > > > On Fri, Jan 13, 2023 at 09:29:26PM +0000, Dionna Glaze wrote:
> > > > > This patch depends on Kirill A. Shutemov's series
> > > > >
> > > > > [PATCHv8 00/14] mm, x86/cc: Implement support for unaccepted memory
> > > > >
> > > > > The UEFI v2.9 specification includes a new memory type to be used in
> > > > > environments where the OS must accept memory that is provided from its
> > > > > host. Before the introduction of this memory type, all memory was
> > > > > accepted eagerly in the firmware. In order for the firmware to safely
> > > > > stop accepting memory on the OS's behalf, the OS must affirmatively
> > > > > indicate support to the firmware.
> > > >
> > > > I think it is a bad idea.
> > > >
> > > > This approach breaks use case with a bootloader between BIOS and OS.
> > > > As the bootloader does ExitBootServices() it has to make the call on
> > > > behalf of OS when it has no idea if the OS supports unaccepted.
> > >
> > > Nothing breaks, it'll error on the safe side.  If the protocol callback
> > > is not called the firmware will simply accept all memory.  The guest OS
> > > will only see unaccepted memory if it explicitly asked for it (assuming
> > > the firmware wants know to support both cases, of course the firmware
> > > could also enforce the one or the other and just not offer the
> > > protocol).
> >
> > How bootloader suppose to know if OS will ask for unaccepted memory?
> > It can't. It means the use-case with bootloader cannot ever use
> > unaccepted memory. That's broken design.
> >
> 
> I still don't understand why we need to support every imaginable
> combination of firmware, bootloader and OS. Unaccepted memory only
> exists on a special kind of virtual machine, which provides very
> little added value unless you opt into the security and attestation
> features, which are all heavily based on firmware protocols. So why
> should care about a EFI-aware bootloader calling ExitBootServices()
> and subsequently doing a legacy boot of Linux on such systems?

Why break what works? Some users want it.

This patch adds complexity, breaks what works and the only upside will
turn into a dead weight soon.

There's alternative to add option to instruct firmware to accept all
memory from VMM side. It will serve legacy OS that doesn't know about
unaccepted memory and it is also can be use by latency-sensitive users
later on (analog of qemu -mem-prealloc).

-- 
  Kiryl Shutsemau / Kirill A. Shutemov