[PATCH 0/4] arm64: Unmap linear alias of kernel data/bss

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

One of the reasons the lack of randomization of the linear map on arm64
is considered problematic is the fact that bootloaders adhering to the
original arm64 boot protocol may place the kernel at the base of DRAM,
and therefore at the base of the non-randomized linear map. This puts a
writable alias of the kernel's data and bss regions at a predictable
location, removing the need for an attacker to guess where KASLR mapped
the kernel.

Let's unmap this linear, writable alias entirely, so that knowing the
location of the linear alias does not give write access to the kernel's
data and bss regions.

Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Liz Prucka <lizprucka@google.com>
Cc: Seth Jenkins <sethjenkins@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: linux-hardening@vger.kernel.org

Ard Biesheuvel (4):
  arm64: Move fixmap page tables to end of kernel image
  arm64: Map the kernel data/bss read-only in the linear map
  arm64: Move the zero page to rodata
  arm64: Unmap kernel data/bss entirely from the linear map

 arch/arm64/include/asm/mmu.h    |  2 +-
 arch/arm64/kernel/smp.c         |  2 +-
 arch/arm64/kernel/vmlinux.lds.S |  5 +++
 arch/arm64/mm/fixmap.c          |  7 +--
 arch/arm64/mm/mmu.c             | 46 ++++++++++++++++++--
 5 files changed, 54 insertions(+), 8 deletions(-)

-- 
2.52.0.457.g6b5491de43-goog