[edk2-devel] measurement to command-line/initrd for loading kernel via -kernel option

[James Bottomley]

[pjones added because he's done a huge amount of work to get shim to
measure stuff correctly]
On Tue, 2022-09-20 at 13:24 +0000, Lu, Ken wrote:
> > > Hi Ard, I think it better let creator to measure instead of
> > > consumer to measure
> > like today's implementation in grub[1]. The creator here means who
> > load/create it. In direct boot, it is OVMF read kernel command line
> > and initrd image. In grub boot, it is grub2.  Because the number of
> > consumer like Linux kernel could be more than 1, but the creator is
> > single.
> > 
> > I agree with this in principle.
> 
> So you are not against to do measurement in loader like current does
> in grub and OVMF, correct? I think it is OK even do twice
> measurements on cmdline and initrd for the corner case.
> In past month, I just submit patch in grub to do CC measurement at 
> https://git.savannah.gnu.org/cgit/grub.git/commit/?id=4c76565b6cb885b7e144dc27f3612066844e2d19

Wait, we have two separate cases: when the kernel boots via grub, which
is not direct boot and grub measures eveything and when we do direct
boot and grub is not involved.  Ideally, we should be able to get to a
stable PCR8,9 for measurement, but grub isn't hugely helpful there
since it dumps every grub command into the PCRs so direct boot can
never match that whatever the EFI stub does.  The TCG spec isn't very
helpful on some things, but it is very clear that once you've measured
something, you don't measure it again, so we do want to avoid measuring
the same thing twice.

> 
> > However, there are corner cases that we would like
> > to cover, such as booting Linux from the EFI shell. 
> 
> I remember Bottomley or someone mentioned to use CONFIG_CMDLINE and
> CONFIG_INITRAMFS_SOURCE, such as 
> https://blog.decentriq.com/swiss-cheese-to-cheddar-securing-amd-sev-snp-early-boot-2/
> for this corner case, especially for confidential container use case
> without grub.

Well, you know, when you talk of the devil he bites your heels ...

Part of the problem is that if you look at the protocol, the LoadImage
measurement seems not to measure the command line.  If we can fix that,
then we can get something that will work both with direct boot (cmdline
is passed to the image) as well as direct executions of the kernel from
the EFI shell.  I think that's what we should aim for.  It would be too
disruptive now to try to get grub also to measure thisorrectly.

I think the key is agreeing with TCG that the argument list of an
executable, loaded by LoadImage should be measured separately.  There
are parts of the spec that hint at this, but by and large it seems to
assume that the measurement of the boot volume entry (which does
contain the command line [usually empty] is sufficient).

James




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