linux-kernel - Re: [PATCH v2 0/2] x86/kexec: UKI Support

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Date:   Mon, 18 Sep 2023 17:36:07 +0200
From:   Philipp Rudo <prudo@...hat.com>
To:     "Jan Hendrik Farr" <kernel@...rr.cc>
Cc:     "Lennart Poettering" <mzxreary@...inter.de>,
        linux-kernel@...r.kernel.org, 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, "Luca Boccassi" <bluca@...ian.org>
Subject: Re: [PATCH v2 0/2] x86/kexec: UKI Support

Hi Jan,

On Thu, 14 Sep 2023 23:04:32 +0200
"Jan Hendrik Farr" <kernel@...rr.cc> wrote:

> On Thu, Sep 14, 2023, at 8:51 PM, Philipp Rudo wrote:
> > [...]
> >
> > In this context I hope it is also clear to you that when more and more
> > people rely on the spec you need a more formal process when including
> > changes. Especially when the change might break the implementation of
> > others. So no more making the .cmdline optional and allowing it to be
> > overwritten all on the same day.
> >
> > Having that said, what does "local override" exactly mean? Does that
> > mean a distro can allow a user to freely choose the cmdline without
> > checking any signatures?  
> 
> The behavior of systemd-stub is to allow the bootloader (or whatever
> called sd-stub) supplied cmdline when there is no .cmdline section in
> the UKI. That's how I understand "local override" here. For WIP v3 of
> this patch the behavior is to use the cmdline supplied by userspace to
> the kexec_file_load syscall if no .cmdline section is in the UKI.
> 
> empty .cmdline section -> empty cmdline always passed to kernel
> .cmdline section -> use bootloader/user supplied cmdline (which would
> be empty by default)
> 
> This setup does not make sense for a locked down / secure system though.
> 
> Maybe the word "override" is not ideal. There is nothing actually being
> overridden as there is no cmdline in the UKI in the first place.
> 
> sd-stub also allows the bootloader supplied cmdline if not using secure
> boot. So maybe the kernel could allow user supplied cmdline if not in
> lockdown mode for kexec maybe? If not in lockdown mode somebody can just
> kexec an unsigned kernel + unsigned cmdline using the kexec_load syscall
> anyways. For this case the word "override" makes sense.
> 
> The logic for all of this in sd-stub is in [1].
> 
> > I.e. does that mean we can get rid of this
> >      https://github.com/systemd/systemd/issues/24539  
> 
> This is a different usecase IMO.

Yeah, I expected that. The whole question was meant to be rhetorical.
The point I wanted to make was that when a spec uses terms like "local
override" it needs to explain what it means.

Thanks
Philipp

> >> Hence, seeing the spec as set in stone and as inherently low quality
> >> is the wrong way to see it I am sure. Instead, the goal here is to
> >> adjust the spec to make it work really nicely for *both* systemd and
> >> the kernel.  
> >
> > Sorry, I never wanted to intend that the spec inherently low quality.
> > Just that it doesn't meat my expectations, yet. But that is fine. The
> > spec isn't even a year old and there's only a single implementation,
> > yet. So it's more documentation rather than a spec.  
> 
> Let's make it happen.
> 
> 
> [1] https://github.com/systemd/systemd/blob/5898cef22a35ceefa068d5f46929eced2baab0ed/src/boot/efi/stub.c#L140
>