[kernel-hardening] Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

[James Morse <james.morse@arm.com>]

Hi Ard!

On 30/12/15 15:26, Ard Biesheuvel wrote:
> This adds support for runtime relocation of the kernel Image, by
> building it as a PIE (ET_DYN) executable and applying the dynamic
> relocations in the early boot code.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---

> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 01a33e42ed70..ab582ee58b58 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -243,6 +253,16 @@ ENDPROC(stext)
>  preserve_boot_args:
>  	mov	x21, x0				// x21=FDT
>  
> +#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
> +	/*
> +	 * Mask off the bits of the random value supplied in x1 so it can serve
> +	 * as a KASLR displacement value which will move the kernel image to a
> +	 * random offset in the lower half of the VMALLOC area.
> +	 */
> +	mov	x23, #(1 << (VA_BITS - 2)) - 1
> +	and	x23, x23, x1, lsl #SWAPPER_BLOCK_SHIFT
> +#endif

I've managed to make this fail to boot by providing a seed that caused
the kernel to overlap a 1G boundary on a 4K system.

(It looks like your v3 may have the same issue - but I haven't tested it.)


> +
>  	adr_l	x0, boot_args			// record the contents of
>  	stp	x21, x1, [x0]			// x0 .. x3 at kernel entry
>  	stp	x2, x3, [x0, #16]


Thanks!

James