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

[Dave Young]

On Wed, 20 Sept 2023 at 20:07, Dave Young <dyoung@xxxxxxxxxx> wrote:
>
> On Wed, 20 Sept 2023 at 18:50, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> > On Wed, 20 Sept 2023 at 08:40, Dave Young <dyoung@xxxxxxxxxx> wrote:
> > >
> > > On Wed, 20 Sept 2023 at 15:43, Dave Young <dyoung@xxxxxxxxxx> wrote:
> > > >
> > > > > > In the end the only benefit this series brings is to extend the
> > > > > > signature checking on the whole UKI except of just the kernel image.
> > > > > > Everything else can also be done in user space. Compared to the
> > > > > > problems described above this is a very small gain for me.
> > > > >
> > > > > Correct. That is the benefit of pulling the UKI apart in the
> > > > > kernel. However having to sign the kernel inside the UKI defeats
> > > > > the whole point.
> > > >
> > > >
> > > > Pingfan added the zboot load support in kexec-tools, I know that he is
> > > > trying to sign the zboot image and the inside kernel twice. So
> > > > probably there are some common areas which can be discussed.
> > > > Added Ard and Pingfan in cc.
> > > > http://lists.infradead.org/pipermail/kexec/2023-August/027674.html
> > > >
> > >
> > > Here is another thread of the initial try in kernel with a few more
> > > options eg. some fake efi service helpers.
> > > https://lore.kernel.org/linux-arm-kernel/ZBvKSis+dfnqa+Vz@xxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m42abb0ad3c10126b8b3bfae8a596deb707d6f76e
> > >
> >
>
> Ard, thanks for the comments.
>
> > Currently, UKI's external interface is defined in terms of EFI
> > services, i.e., it is an executable PE/COFF binary that encapsulates
> > all the logic that performs the unpacking of the individual sections,
> > and loads the kernel as a PE/COFF binary as well (i.e., via
> > LoadImage/StartImage)
> >
> > As soon as we add support to Linux to unpack a UKI and boot the
> > encapsulated kernel using a boot protocol other than EFI, we are
> > painting ourselves into a corner, severely limiting the freedom of the
> > UKI effort to make changes to the interfaces that were implementation
> > details up to this point.
>
> Agreed, it seems UKI is more flexible and complex than the zboot,
> we do need to carefully think about a better solution.
>
> >
> > It also means that UKI handling in kexec will need to be taught about
> > every individual architecture again, which is something we are trying
> > to avoid with EFI support in general. Breaking the abstraction like
> > this lets the cat out of the bag, and will add yet another variation
> > of kexec that we will need to support and maintain forever.
> >
> > So the only way to do this properly and portably is to implement the
> > minimal set of EFI boot services [0] that Linux actually needs to run
> > its EFI stub (which is mostly identical to the set that UKI relies on
> > afaict), and expose them to the kexec image as it is being loaded.
> > This is not as bad as it sounds - I have some Rust code that could be
> > used as an inspiration [1] and which could be reused and shared
> > between architectures.
>
> Great!
>
> >
> > This would also reduce/remove the need for a purgatory: loading a EFI
> > binary in this way would run it up to the point were it calls
> > ExitBootServices(), and the actual kexec would invoke the image as if
> > it was returning from ExitBootServices().
> >
> > The only fundamental problem here is the need to allocate large chunks
> > of physical memory, which would need some kind of CMA support, I
> > imagine?
>
> Hmm, I thought that your idea is to write the efi stub code in "purgatory"
> so kexec can jump to it while rebooting then it will be able to access the
> whole usable memory, but it seems you want an efi app run under linux
> and somehow provide services to kexec?  My EFI knowledge is incomplete
> and outdated,  If my understanding of your proposal is true how can it keep
> running after switching to the new kernel stub?

Oops,  please ignore the quick reply and questioins, I apparently
forgot that this is the kexec loading
phase instead of the rebooting phase.  Yes as you said CMA might be
the only choice
for that proposal.

>
> >
> > Maybe we should do a BoF at LPC to discuss this further?
>
> It does deserve more discussion, unfortunately I will not be able to join LPC,
> Philipp Rudo (cced) planned attend the conf, so I think you guys can
> discuss together with
> other people interested. I think I will watch the recordings or
> joining virtually if possible.
>
> >
> > [0] this is not as bad as it sounds: beyond a protocol database, a
> > heap allocator and a memory map, there is actually very little needed
> > to boot Linux via the EFI stub (although UKI needs
> > LoadImage/StartImage as well)
> >
> > [1] https://github.com/ardbiesheuvel/efilite
> >