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

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Message-ID: <CAAH4kHb6-6QkMnYbcQ6MyMkwSBUN-Q3CcM3fuiStdbbnSfJv1A@mail.gmail.com>
Date:   Mon, 16 Jan 2023 11:43:15 -0800
From:   Dionna Amalie Glaze <dionnaglaze@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "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

> > 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, they'll
only get a safe view of the memory map. I don't think it's right to
choose unsafe behavior for a legacy setup.

> 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).
>

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.

With GCE at least, you can't (shouldn't) associate the boot feature
flag with a disk image because disks are mutable. If a customer
upgrades their kernel after initially starting their VM, they can't
remove the flag due to the way image annotations work.

All of this headache goes away by adopting a small patch to the kernel
that calls a 0-ary protocol interface and keeping safe acceptance
behavior in the firmware. I think Gerd is right here that we should
treat it as a transition feature that we can remove later.

> --
>   Kiryl Shutsemau / Kirill A. Shutemov

-- 
-Dionna Glaze, PhD (she/her)