Message-ID: <CALu+AoS86mKmeeTxSQyaA70=-5bCC3g3iO4i=A6HTZ-ewdHh=w@mail.gmail.com>
Date:   Wed, 22 Mar 2023 11:44:52 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kexec@...ts.infradead.org, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 0/6] arm64: make kexec_file able to load zboot image

On Fri, 10 Mar 2023 at 12:18, Pingfan Liu <kernelfans@...il.com> wrote:
>
> On Tue, Mar 07, 2023 at 04:08:55PM +0800, Pingfan Liu wrote:
> > Hi Ard,
> >
> > Thanks for sharing your idea. Please see the comment.
> >
> > On Mon, Mar 06, 2023 at 09:08:03AM +0100, Ard Biesheuvel wrote:
> > > (cc Mark)
> > >
> > > Hello Pingfan,
> > >
> > > Thanks for working on this.
> > >
> > > On Mon, 6 Mar 2023 at 04:03, Pingfan Liu <kernelfans@...il.com> wrote:
> > > >
> > > > After introducing zboot image, kexec_file can not load and jump to the
> > > > new style image. Hence it demands a method to load the new kernel.
> > > >
> > > > The crux of the problem lies in when and how to decompress the Image.gz.
> > > > There are three possible courses to take: -1. in user space, but hard to
> > > > achieve due to the signature verification inside the kernel.
> > >
> > > That depends. The EFI zboot image encapsulates another PE/COFF image,
> > > which could be signed as well.
> > >
> > > So there are at least three other options here:
> > > - sign the encapsulated image with the same key as the zboot image
> > > - sign the encapsulated image with a key that is only valid for kexec boot
> > > - sign the encapsulated image with an ephemeral key that is only valid
> > > for a kexec'ing an image that was produced by the same kernel build
> > >
> > > >  -2. at the
> > > > boot time, let the efi_zboot_entry() handles it, which means a simulated
> > > > EFI service should be provided to that entry, especially about how to be
> > > > aware of the memory layout.
> > >
> > > This is actually an idea I intend to explore: with the EFI runtime
> > > services regions mapped 1:1, it wouldn't be too hard to implement a
> > > minimal environment that can run the zboot image under the previous
> >
> > The idea of the minimal environment lools amazing. After digging
> > more deeply into it, I think it means to implement most of the function
> > members in efi_boot_services, besides that, some UEFI protocols due to
> > the reference of efi_call_proto(). So a clear boundary between zboot and
> > its dependent EFI service is demanded before the work.
> >
>
> Looking deeper into it. This approach may be splitted into the following
> chunks:
> -1. Estimation the memory demanded by the decompression of zboot, which
> roughly includes the size of Image, the size of the emulated service and
> the stack used by zboot. Finally we need a kexec_add_buffer() for this
> range.
>
> -2. The emulated EFI services and some initial data such as the physical
> address of dtb, the usable memory start address and size should be set
> by kexec_purgatory_get_set_symbol()
>
> -3. Set up an identity mapping of the usable memory by zboot, prepare
> stack and turn on MMU at the last point just before 'br efi_zboot_entry'
> in relocate_kernel.S, which means relocate_kernel.S should support two
> kinds of payload.
>
> -4. For efi_zboot_entry(), if jumping from kexec, limit its requirement
> to only a few boot services: e.g. allocate_pages, allocate_pool. So the
> emulated services can be deduced.

Hi Pingfan,

I'm not sure how hard it will be although Ard thinks it could be
doable.  If it is not easy I suspect it is not worth the effort.

For your current series,  my suggestion is you can try to move the
major code in the generic code path in kernel/kexec_file.c and keep
the arch code minimum so that in the future other arches can avoid
redundant code.

Otherwise a fallback solution could be using the same key to sign both
the zboot image and the internal kernel image like below:
1. sign the kernel with the same key twice (kernel image and zboot
image) in distro kernel
2. introduce a kconfig in mainline to sign the kernel image with an
ephemeral key same to kernel modules.  Distro can disable the config
option. (in this way kexec can only load the same kernel, it is not
useful if people want to load older/newer kernels)
3. patch kexec-tools to decompress the zboot image and load the kernel image


>
> > > kernel up to the point where it call ExitBootServices(), after which
> > > kexec() would take over.
> > >
> >
> > IIUC, after kexec switches to efi_zboot_entry(), it will not return,
> > right?
> >
>
> I have this assumption because letting the control path switch between
> kernel and non-kernel code is not a good idea.
>
>
> Thanks,
>
> Pingfan
>
> > > >  -3. in kernel space, during the file load
> > > > of the zboot image. At that point, the kernel masters the whole memory
> > > > information, and easily allocates a suitable memory for the decompressed
> > > > kernel image. (I think this is similar to what grub does today).
> > > >
> > >
> > > GRUB just calls LoadImage(), and the decompression code runs in the EFI context.
> > >
> >
> > Ah, thanks for the correcting. I had made an wrong assumption of grub
> > based on [1], from which, I thought that grub is the case "For
> > compatibility with non-EFI loaders, the payload can be decompressed and
> > executed by the loader as well, provided that the loader implements the
> > decompression algorithm and that non-EFI boot is supported by the
> > encapsulated image"
> >
> >
> > [1]: https://www.phoronix.com/news/Linux-6.1-Generic-EFI-Zboot
> >
> >
> > Eager to find a solution to kexec a zboot image. Hope it will come soon.
> >
> >
> > Thanks,
> >
> >       Pingfan
> > > > The core of this series is [5/6].  [3,6/6] handles the config option.
> > > > The assumption of [3/6] is kexec_file_load is independent of zboot,
> > > > especially it can load kernel images compressed with different
> > > > compression method.  [6/6] is if EFI_ZBOOT, the corresponding
> > > > decompression method should be included.
> > > >
> > > >
> > > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > > Cc: Will Deacon <will@...nel.org>
> > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > Cc: Ard Biesheuvel <ardb@...nel.org>
> > > > Cc: kexec@...ts.infradead.org
> > > > To: linux-arm-kernel@...ts.infradead.org
> > > > To: linux-kernel@...r.kernel.org
> > > >
> > > > Pingfan Liu (6):
> > > >   arm64: kexec: Rename kexec_image.c to kexec_raw_image.c
> > > >   lib/decompress: Introduce decompress_method_by_name()
> > > >   arm64: Kconfig: Pick decompressing method for kexec file load
> > > >   lib/decompress: Keep decompress routines based on selection
> > > >   arm64: kexec: Introduce zboot image loader
> > > >   init/Kconfig: Select decompressing method if compressing kernel
> > > >
> > > >  arch/arm64/Kconfig                            |  59 ++++++
> > > >  arch/arm64/include/asm/kexec.h                |   4 +-
> > > >  arch/arm64/kernel/Makefile                    |   2 +-
> > > >  .../{kexec_image.c => kexec_raw_image.c}      |   2 +-
> > > >  arch/arm64/kernel/kexec_zboot_image.c         | 186 ++++++++++++++++++
> > > >  arch/arm64/kernel/machine_kexec.c             |   1 +
> > > >  arch/arm64/kernel/machine_kexec_file.c        |   3 +-
> > > >  include/linux/decompress/generic.h            |   2 +
> > > >  include/linux/decompress/mm.h                 |   9 +-
> > > >  include/linux/zboot.h                         |  26 +++
> > > >  init/Kconfig                                  |   7 +
> > > >  lib/Kconfig                                   |   3 +
> > > >  lib/decompress.c                              |  17 +-
> > > >  13 files changed, 314 insertions(+), 7 deletions(-)
> > > >  rename arch/arm64/kernel/{kexec_image.c => kexec_raw_image.c} (98%)
> > > >  create mode 100644 arch/arm64/kernel/kexec_zboot_image.c
> > > >  create mode 100644 include/linux/zboot.h
> > > >
> > > > --
> > > > 2.31.1
> > > >
>
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