[PATCH] efi/libstub/arm64: handle randomized TEXT_OFFSET

[Ard Biesheuvel]

Hi Mark,

On 24 April 2018 at 13:00, Mark Rutland <mark.rutland at arm.com> wrote:
> When CONFIG_RANDOMIZE_TEXT_OFFSET is selected, TEXT_OFFSET is an
> arbitrary multiple of PAGE_SIZE in the interval [0, 2MB).
>
> The EFI stub doesn't accuont for this, and only handles the case where

'account'

If you agree, I will add something here to clarify that the newly
chosen offset should retain the misalignment of TEXT_OFFSET relative
to EFI_KIMG_ALIGN, because it took me a while to figure that out.

Other than that,

Reviewed-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

I will queue this as a fix

Thanks,
Ard.



> TEXT_OFFSET is multiple of EFI_KIMG_ALIGN. This can result in the kernel
> being loaded to an erroneous physical alignment. This has been observed
> to result in spurious stack overflow reports and failure to make use of
> the IRQ stacks, and theoretically could result in a number of other
> issues.
>
> We can OR in the low bits of TEXT_OFFSET to ensure that we have the
> necessary offset (and hence have the necessary alignment), so let's do
> that.
>
> Fixes: 6f26b3671184c36d ("arm64: kaslr: increase randomization granularity")
> Reported-by: Kim Phillips <kim.phillips at arm.com>
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: linux-efi at vger.kernel.org
> ---
>  drivers/firmware/efi/libstub/arm64-stub.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index b9bd827caa22..541b82fdc8a2 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -98,6 +98,13 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table_arg,
>                              (phys_seed >> 32) & mask : TEXT_OFFSET;
>
>                 /*
> +                * With CONFIG_RANDOMIZE_TEXT_OFFSET, TEXT_OFFSET may not be a
> +                * multiple of EFI_KIMG_ALIGN, and we must ensure that we apply
> +                * the offset below EFI_KIMG_ALIGN.
> +                */
> +               offset |= (TEXT_OFFSET % EFI_KIMG_ALIGN);
> +
> +               /*
>                  * If KASLR is enabled, and we have some randomness available,
>                  * locate the kernel at a randomized offset in physical memory.
>                  */
> --
> 2.11.0
>