Re: [PATCH 1/2] Makefile.zboot: Sign Image before packing into EFI-STUB shell

[Pingfan Liu <piliu@redhat.com>]

On Fri, Dec 06, 2024 at 09:03:30AM +0100, Ard Biesheuvel wrote:

Also cc Jan, Philipp, who are also engaged in related topic (UKI)

> (cc Peter, Gerd)
> 
> On Fri, 6 Dec 2024 at 03:10, Pingfan Liu <piliu@redhat.com> wrote:
> >
> > At present, the kexec_file_load of either zboot or UKI kernel relies on
> > the user space to parse and extract the Image, and then pass the Image
> > through that syscall. During this process, the outmost signature on
> > zboot or UKI kernel is stripped and discarded.
> >
> > On the other hand, a secure boot platform enforces the signature
> > verfiication on the kernel image passed through the kexec_file_load
> > syscall. To cater to this requirement, this patch applies signature on
> > the PE format 'Image' before padding.
> >
> 
> The whole point of the EFI zboot format was avoiding the need to sign
> the compressed payload.
> 
> Now, we are back to signing the payload along with the full image
> using PE based tools, even though the payload is intended to be booted
> as a raw image.
> 

I remember that I sent out a zboot image parser in the kernel to tackle
with this signature issue.  But that will complicate the kernel image
parser, as a result, we defer resolving it, and finally we have it
implemented in the user space kexec-tools.

The emergence of UKI makes things more complicated. Jan introduced "UKI
format parser in linux kernel". For arm64, the UKI support in kernel
means that a UKI format parser should be followed by a zboot format
parser. 

So we tried emulator solution instead of parser. ( I have a summary on:
https://github.com/rhkdump/kexec_uefi/blob/main/overview.md)

But either of the emulator methods have their own drawback:
	-1.the purgatory-style method has trouble in the hardware scaling.
	-2.the user space emulator can not ensure the security. (also I
           think it can not resolve the hardware issue since at that time,
           it can not alter the hardware status arbitrarily)


> I'm not sure I see the point of this: EFI zboot is a trivial container
> format which records the compression type and the start and length of
> the payload in its header at a known offset.
> 
> Perhaps we should just make EFI zboot gzip-only, rather than
> supporting 7 different compression methods because that is what the
> legacy decompressors on ARM and x86 support - I struggle to see the
> point of that tbh (even though I implemented that myself)
> 
> That way, the kernel can authenticate the outer PE zboot image as
> usual, and perform the decompression itself, without having to carry
> code for all compression formats it might encounter.
> 

It is always good to keep things simple. But this seems helpless to step
around the kexec_file_load issue.

> (Apologies if we are sending you in circles, but if we get this wrong
> now, we're stuck with another kexec-related maintenance nightmare so I
> really don't want to commit to something tooo hastily)
> 

Although this issue has come full circle, we now have a clear
understanding of its solutions' limitations, advantages, and disadvantages.

Unlike the forecast of this issue about three years ago, we are now
facing real customer pressure.


Thanks,

Pingfan


> -- 
> Ard.