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

["Ard Biesheuvel" <ardb@kernel.org>]

On Mon, 19 Sept 2022 at 04:13, Xu, Min M <min.m.xu@intel.com> wrote:
>
> On September 18, 2022 8:52 PM, Ard Biesheuvel wrote:
> > Hello Min Xu,
> >
> > On Sat, 17 Sept 2022 at 04:53, Xu, Min M <min.m.xu@intel.com> wrote:
> > >
> > > Hi, Ard
> > >
> > > I am checking the measurement behavior when loading the kernel via the
> > QEMU -kernel option. I find it is implemented by below 2 driver/lib:
> > >
> > > - OvmfPkg/QemuKernelLoaderFsDxe
> > >
> > >  This is a separate DXE driver that exposes the virtual SimpleFileSystem
> > implementation that carries the kernel and initrd passed via the QEMU
> > command line.
> > >
> > > - OvmfPkg/Library/X86QemuLoadImageLib
> > >
> > >   This is the library that consumes above driver and call
> > LoadImage/StartImage so that the kernel image gets authenticated and/or
> > measured.
> > >
> > > See https://edk2.groups.io/g/devel/message/55381
> > >
> > >
> > >
> > > I have some questions about the implementation need your help.
> > >
> > > 2. Kernel image is authenticated and/or measured in LoadImage. I am
> > wondering if “command line” is measured as well? “Command line” can be
> > treated as an external input and in my opinion it should be measured too.
> > >
> > > 3. The same question to initrd. Is it measured?
> > >
> >
> > The initrd is measured by the EFI stub in Linux, and we are currently adding
> > measurement of the load options to that as well:
> > https://lore.kernel.org/all/20220916081441.1993492-2-
> > ilias.apalodimas@linaro.org/
> >
> > The initrd is Linux specific in any case, so there, the Linux OS loader is a
> > natural place to take care of this. The load options are being added because
> > of the oversight in the TCG spec, which only covers load options if they are
> > part of a Boot#### option, but between
> > LoadImage() and StartImage, you can pass any load options you want via the
> > loaded image protocol, so it needs to be measured as well.
> >
> Thanks Ard for the explanation.
> I was told that in grub boot cmd-line/initrd will be measured as well. So my question is that will they be measured twice? One in grub.efi, the other in efi-stub?
>

The EFI stub may be the only OS loader, so the EFI stub should measure
the command line and the initrd.

Whether or not a previous loader stage exists that may or may not
measure the same pieces is not for the EFI stub to reason about. And
in any case, measuring the same thing twice is much less of an issue
than not measuring it at all.

> My understanding is that the loader should take the responsibility to do the measurement.
> For grub boot, grub.efi is the loader so it measures kernel-image/cmd-line/initrd.

If the EFI stub is invoked, the EFI stub is the OS loader. We should
not be relying on the presence of absence of GRUB (or shim) in the
boot chain.

> For direct boot, TryRunningQemuKernel() now measures kernel image (in CoreLoadImage). Shall it also measure cmd-line/initrd in the same time?
>

No, I don't think it should. This is why we are adding this to the EFI
stub instead.

If we measure the initrd and command line in the EFI stub, we don't
have to measure it anywhere else, and we can use any generic EFI
loader on a measured boot system.