[PATCH] arm64: align randomized TEXT_OFFSET on 4 kB boundary

[Mark Rutland]

Hi Ard,

On Wed, Aug 13, 2014 at 06:53:03PM +0100, Ard Biesheuvel wrote:
> When booting via UEFI, the kernel Image is loaded at a 4 kB boundary and
> the embedded EFI stub is executed in place. The EFI stub relocates the
> Image to reside TEXT_OFFSET bytes above a 2 MB boundary, and jumps into
> the kernel proper.
> 
> In AArch64, PC relative symbol references are emitted using adrp/add or
> adrp/ldr pairs, where the offset into a 4 kB page is resolved using a
> separate :lo12: relocation. This implicitly assumes that the code will
> always be executed at the same relative offset with respect to a 4 kB
> boundary, or the references will point to the wrong address.
> 
> This means we should link the kernel at a 4 kB aligned base address in
> order to remain compatible with the base address the UEFI loader uses
> when doing the initial load of Image. So update the code that generates
> TEXT_OFFSET to choose a multiple of 4 kB.
> 
> At the same time, update the code so it chooses from the interval [0..2MB)
> as the author originally intended.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/Makefile      | 2 +-
>  arch/arm64/kernel/head.S | 8 ++++----
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 57833546bf00..2df5e5daeebe 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -39,7 +39,7 @@ head-y		:= arch/arm64/kernel/head.o
>  
>  # The byte offset of the kernel image in RAM from the start of RAM.
>  ifeq ($(CONFIG_ARM64_RANDOMIZE_TEXT_OFFSET), y)
> -TEXT_OFFSET := $(shell awk 'BEGIN {srand(); printf "0x%04x0\n", int(65535 * rand())}')
> +TEXT_OFFSET := $(shell awk 'BEGIN {srand(); printf "0x%03x000\n", int(512 * rand())}')
>  else
>  TEXT_OFFSET := 0x00080000
>  endif
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index b6ca95aee348..ae055a18b04a 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -38,11 +38,11 @@
>  
>  #define KERNEL_RAM_VADDR	(PAGE_OFFSET + TEXT_OFFSET)
>  
> -#if (TEXT_OFFSET & 0xf) != 0
> -#error TEXT_OFFSET must be at least 16B aligned
> -#elif (PAGE_OFFSET & 0xfffff) != 0
> +#if (TEXT_OFFSET & 0xfff) != 0
> +#error TEXT_OFFSET must be at least 4kB aligned

Minor nit, but can we have "KB" here to match "MB"?

> +#elif (PAGE_OFFSET & 0x1fffff) != 0
>  #error PAGE_OFFSET must be at least 2MB aligned
> -#elif TEXT_OFFSET > 0xfffff
> +#elif TEXT_OFFSET > 0x1fffff
>  #error TEXT_OFFSET must be less than 2MB
>  #endif

I thought I'd left wording in Kconfig.debug, but that doesn't seem to be
the case. This looks sane to me, so:

Reviewed-by: Mark Rutland <mark.rutland at arm.com>

Catalin, can you pick this up? Without it UEFI boot can be broken by
ARM64_RANDOMIZE_TEXT_OFFSET.

Cheers,
Mark.