linux-kernel - Re: [PATCH 0/1] x86/kexec: UKI support

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Date:   Tue, 12 Sep 2023 22:24:02 +0300
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Jan Hendrik Farr" <kernel@...rr.cc>,
        <linux-kernel@...r.kernel.org>
Cc:     <kexec@...ts.infradead.org>, <x86@...nel.org>,
        <tglx@...utronix.de>, <dhowells@...hat.com>, <vgoyal@...hat.com>,
        <keyrings@...r.kernel.org>, <akpm@...ux-foundation.org>,
        "Baoquan He" <bhe@...hat.com>, <bhelgaas@...gle.com>,
        <lennart@...ttering.net>, "Luca Boccassi" <bluca@...ian.org>
Subject: Re: [PATCH 0/1] x86/kexec: UKI support

On Tue Sep 12, 2023 at 9:56 PM EEST, Jan Hendrik Farr wrote:
> > What sort of bottleneck does the EFI stub have so that we need yet
> > another envelope?
>
> Well I can come up with a few advantages of UKI compared to normal bzImage with builtin initrd and cmdline.
>
> 1. You already identified this one. Using addons to adjust your cmdline

It is not a benefit as this is already possible today.

> 2. I can use my normal initramfs generation tooling. Just install my
> compiled kernel, my distros install script will generate the
> initramfs. Then I package it up as a UKI. This will be a lot more
> awkward with a builtin initrd.
> 3. Measured boot. You can place PCR signatures in the UKI using
> systemd-measure. This will sign the expected PCR values for booting
> this UKI. I think with normal bzImage this will be a lot more
> difficult. If you place those PCR signatures in the builtin initrd
> this will change the kernel image which means now the values you
> signed no longer match (depending on how you measure the kernel; I
> don't think the normal EFI stub even measures the kernel in first
> place, but I could be mistaken about this)
> 4. UKIs are automatically recognized by systemd-boot

These are sort of "tautological" arguments. There must be some
objective reasons why this architecture was chosen instead of
other (i.e. using what already pre-exists).

BR, Jarkko