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

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Message-ID: <20230117102433.mawczdq3di6776bx@sirius.home.kraxel.org>
Date:   Tue, 17 Jan 2023 11:24:33 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Dionna Amalie Glaze <dionnaglaze@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 Tue, Jan 17, 2023 at 02:17:11AM +0300, Kirill A. Shutemov wrote:
> On Mon, Jan 16, 2023 at 11:43:15AM -0800, Dionna Amalie Glaze wrote:
> > > > 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.
> > >
> > 
> > The users that want legacy boot features will not be broken,
> 
> Why do you call boot with a bootloader a legacy feature?

Linux efi kernels can be booted in two ways:

  (1) old/legacy: boot loader calls ExitBootServices and jumps to the
      kernel entry point.
  (2) new/efi stub: boot loader does *not* call ExitBootServices, but
      loads the linux kernel as efi binary instead.  The linux kernel
      efi stub calls ExitBootServices then.

All kernel version relevant here (new enough to support SEV-SNP / TDX)
have efi stub support, so (1) does not really matter in practice.

the efi stub was added *exactly* to handle cases like this one: the
kernel can do efi calls needed on its own without depending on the
boot loader doing it on behalf of the kernel.

> > This means that users of a distro that has not enabled unaccepted
> > memory support cannot simply start a VM with the usual command, but
> > instead have to know a baroque extra flag to get access to all the
> > memory that they configured the machine (and for a CSP customer, paid
> > for). That's not a good experience.
> 
> New features require enabling. It is not something new.

Asking user to manually configure something which can be handled
automatically just fine is a bad design.

take care,
  Gerd