Re: [RFC PATCH 0/3] efi/x86: add support for generic EFI mixed mode boot

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 13 Feb 2020 at 18:53, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Thu, Feb 13, 2020 at 03:59:25PM +0100, Ard Biesheuvel wrote:
> > This series is another part of my effort to reduce the level of knowledge
> > on the part of the bootloader or firmware of internal per-architecture
> > details regarding where/how the kernel is loaded and where its initrd and
> > other context data are passed.
> >
> > The x86 architecture has a so-called 'EFI handover protocol', which defines
> > how the bootparams struct should be populated, and how it should be
> > interpreted to figure out where to load the kernel, and at which offset in
> > the binary the entrypoint is located. This scheme allows the initrd to be
> > loaded beforehand, and allows 32-bit firmware to invoke a 64-bit kernel
> > via a special entrypoint that manages the state transitions between the
> > two execution modes.
> >
> > Due to this, x86 loaders currently do not rely on LoadImage and StartImage,
> > and therefore, are forced to re-implement things like image authentication
> > for secure boot and taking the measurements for measured boot in their open
> > coded clones of these routines.
> >
> > My previous series on this topic [0] implements a generic way to load the
> > initrd from any source supported by the loader without relying on something
> > like device trees or bootparams structures, and so native boot should not
> > need the EFI handover protocol anymore after those change are merged.
> >
> > What remains is mixed mode boot, which also needs the EFI handover protocol
> > regardless of whether an initrd is loaded or not. So let's get rid of that
> > requirement, and take advantage of the fact that EDK2 based firmware does
> > support LoadImage() for X64 binaries on IA32 firmware, which means we can
> > rely on the secure boot and measured boot checks being performed by the
> > firmware. The only thing we need to put on top is a way to discover the
> > non-native entrypoint into the binary in a way that does not rely on x86
> > specific headers and data structures.
> >
> > So let's introduce a new .compat header in the PE/COFF metadata of the
> > bzImage, and populate it with a <machine type, entrypoint> tuple, allowing
> > a generic EFI loader to decide whether the entrypoint supports its native
> > machine type, and invoke it as an ordinary EFI application entrypoint.
> > Since we will not be passing a bootparams structure, we need to discover
> > the base of the image (which contains the setup header) via the loaded
> > image protocol before we can enter the kernel in 32-bit mode at startup_32()
> >
> > A loader implementation for OVMF can be found at [1]. Note that this loader
> > code is fully generic, and could be used without modifications if other
> > architectures ever emerge that support kernels that can be invoked from a
> > non-native (but cross-type supported) loader.
> >
> > [0] https://lore.kernel.org/linux-arm-kernel/20200206140352.6300-1-ardb@kernel.org/
> > [1] https://github.com/ardbiesheuvel/edk2/commits/linux-efi-generic
> >
>
> As an alternative to the new section, how about having a CONFIG option
> to emit the 64-bit kernel with a 32-bit PE header instead, which would
> point to efi32_pe_entry? In that case it could be directly loaded by
> existing firmware already. You could even have a tool that can mangle an
> existing bzImage's header from 64-bit to 32-bit, say using the newly
> added kernel_info structure to record the existence and location of
> efi32_pe_entry.
>

That wouldn't work with, say, signed distro kernels.

> Also, the PE header can live anywhere inside the image, right? Is there
> any reason to struggle to shoehorn it into the "boot sector"?

It cannot. It must live outside a region described by the section headers.