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

[Dave Young]

On Wed, 20 Sept 2023 at 20:18, Dave Young <dyoung@xxxxxxxxxx> wrote:
>
> 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.

Ok, refreshed my memory with a brief discussion with Pingfan.  My
understanding of the flow is like below:

1. kexec loads the UKI image itself without any parsing of the internal files.
2. reboot|crash -> jump to the fake stub
3. the stub parse the UKI internal kernel/initrd/cmdline
4. boot into the new kernel

With above code flow, it is still not clear for me where do we need
the large chunk
of memory for the stub.
in step 1 kexec loading does not need physical continuous big chunks as
while jumping to the stub in step 2 the UKI image will be relocated again.
the stub does need some page table setups but that is already done in current
kexec code.

Other than the boot service implementation, another issue here from my mind is
the memory map information should be passed to the stub use.  Taking x86 as an
example kexec will pass the raw e820 table to 2nd kernel according to the x86
boot protocol.  Now if we move to a chain load case, there will be similar
requirement for the stub.

Another thing is in case the stub can be more complex in the future, how can
we debug it.  The boot service conout is not usable, and since graphic kms switc
hing, I'm not sure the boot framebuffer will be usable as well.  Probably the
only way is to add serial output support.  Anyway this is something could be
hard to handle.

>
> >
> > >
> > > 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
> > >