Hi Ard,

On Wed, Mar 30, 2022 at 05:41:47PM +0200, Ard Biesheuvel wrote:

This is a followup to a previous series of mine [0], and it aims to
streamline the boot flow with respect to cache maintenance and redundant
copying of data in memory.

Combined with my proof-of-concept firmware for QEMU/arm64 [1], this
results in a boot where both the kernel and the initrd are loaded
straight to their final locations in memory, while the physical
placement of the kernel image is still randomized by the loader. It also
removes all memory accesses performed with the MMU and caches off
(except for instruction fetches) that are done from the moment the VM
comes out of reset.

On the kernel side, this comes down to:
- increasing the ID map to cover the entire kernel image, so we can
  build the kernel page tables with the MMU and caches enabled;
- deal with the MMU already being on at boot, and keep it on while
  building the ID map;
- ensure all stores to memory that are now done with the MMU and caches
  on are not negated by the subsequent cache invalidation.

This is on my queue to review in detail, but for now I have a couple of
high-level thoughts:

1) I like the idea of deferring/staging some work until after the MMU is on,
   and I'm in favour of doing so where we can do so in all cases. If we end up
   with infrastructure to run some MMU-on TTBR0 stub environment(s), that could
   be useful elsewhere, e.g. idmap_kpti_install_ng_mappings().

2) I do not think that we should support entering the kernel with the MMU on.

   I think that consistently using the same MMU-off boot code has saved us a
   great deal of pain thus far, and the more I think about booting with the MMU
   on, I think it opens us up to a lot of potential pain, both in the short term
   and longer term as the architecture evolves. For example, as rhetoricals
   from the top of my head:

  * How do we safely inherit whatever VMSA state the loader has left us with?
    e.g. what do we require w.r.t. TCRs, MAIRS?
    e.g. what to do when the loader uses a different granule size from the
         kernel?

  * What can we expect is mapped, and with which specific attributes and
    permissions?

  * What do we document here for loaders other than the EFI stub?
    ... and what about kexec?

  ... and generally this is another complication for maintenance and testing
  that I'd rather not open the door to.

In other words, my view is that we should *minimize* what we do with the MMU
off, but only where we can do that consistently, and we should still
consistently enter with the MMU off such that we can consistently and safely
initialize the VMSA state.

Thanks,
Mark.

Additionally, this series removes the little dance we do to create a
kernel mapping, relocate the kernel, run the KASLR init code, tear down
the old mapping and creat a new one, relocate the kernel again, and
finally enter the kernel proper. Instead, it invokes a minimal C
function 'kaslr_early_init()' while running from the ID map with a
temporary mapping of the FDT in TTBR1. This change represents a
substantial of the diffstat, as it requires some work to instantiate
code that can run safely from the wrong address. It is also the most
likely to raise objections, so it can be dropped from this series if
desired (patch #9 is the meat, and #8 is a prerequisite patch that could
be dropped in that case as well)

Changes since v1:
- Remove the dodgy handling of the KASLR seed, which was necessary to
  avoid doing two iterations of the setup/teardown of the page tables.
  This is now dealt with by creating the TTBR1 page tables while
  executing from TTBR0, and so all memory manipulations are still done
  with the MMU and caches on. (This is also the reason patch #9 is
  optional now)
- Only boot from EFI with the MMU and caches on if the image was not
  moved around in memory. Otherwise, we cannot rely on the firmware's ID
  map to have created an executable mapping for the copied code.

[0] https://lore.kernel.org/all/20220304175657.2744400-1-ardb@kernel.org/ (local)
[1] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/efilite.git/

Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>

Ard Biesheuvel (18):
  arm64: head: drop idmap_ptrs_per_pgd
  arm64: head: split off idmap creation code
  arm64: kernel: drop unnecessary PoC cache clean+invalidate
  arm64: head: cover entire kernel image in ID map
  arm64: head: factor out TTBR1 assignment into a macro
  arm64: head: populate kernel page tables with MMU and caches on
  arm64: kaslr: deal with init called with VA randomization enabled
  arm64: setup: defer R/O remapping of FDT
  arm64: head: relocate kernel only a single time if KASLR is enabled
  arm64: head: record the MMU state at primary entry
  arm64: mm: make vabits_actual a build time constant if possible
  arm64: head: avoid cache invalidation when entering with the MMU on
  arm64: head: record CPU boot mode after enabling the MMU
  arm64: head: clean the ID map page to the PoC
  arm64: lds: move idmap_pg_dir out of .rodata
  efi: libstub: pass image handle to handle_kernel_image()
  efi/arm64: libstub: run image in place if randomized by the loader
  arm64: efi/libstub: enter with the MMU on if executing in place

 arch/arm64/include/asm/kernel-pgtable.h   |   2 +-
 arch/arm64/include/asm/memory.h           |   6 +
 arch/arm64/include/asm/mmu_context.h      |   1 -
 arch/arm64/kernel/Makefile                |   2 +-
 arch/arm64/kernel/efi-entry.S             |   4 +
 arch/arm64/kernel/head.S                  | 276 +++++++++++---------
 arch/arm64/kernel/kaslr.c                 |  86 +-----
 arch/arm64/kernel/pi/Makefile             |  33 +++
 arch/arm64/kernel/pi/kaslr_early.c        | 128 +++++++++
 arch/arm64/kernel/setup.c                 |   8 +-
 arch/arm64/kernel/vmlinux.lds.S           |   9 +-
 arch/arm64/mm/mmu.c                       |  15 +-
 drivers/firmware/efi/libstub/arm32-stub.c |   3 +-
 drivers/firmware/efi/libstub/arm64-stub.c |  15 +-
 drivers/firmware/efi/libstub/efi-stub.c   |   2 +-
 drivers/firmware/efi/libstub/efistub.h    |   3 +-
 drivers/firmware/efi/libstub/riscv-stub.c |   3 +-
 include/linux/efi.h                       |  11 +
 18 files changed, 380 insertions(+), 227 deletions(-)
 create mode 100644 arch/arm64/kernel/pi/Makefile
 create mode 100644 arch/arm64/kernel/pi/kaslr_early.c

-- 
2.30.2

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel