On Thu, 13 Feb 2020 at 18:53, Arvind Sankar <nivedita@xxxxxxxxxxxx> 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@xxxxxxxxxx/ > > [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.
