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Date: Mon, 4 Sep 2017 14:44:15 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc: linux-arm-kernel@...ts.infradead.org, kernel-hardening@...ts.openwall.com, 
    Arnd Bergmann <arnd@...db.de>, Russell King <linux@...linux.org.uk>, 
    Kees Cook <keescook@...omium.org>, Thomas Garnier <thgarnie@...gle.com>, 
    Marc Zyngier <marc.zyngier@....com>, Mark Rutland <mark.rutland@....com>, 
    Tony Lindgren <tony@...mide.com>, Matt Fleming <matt@...eblueprint.co.uk>, 
    Dave Martin <dave.martin@....com>
Subject: Re: [PATCH v2 24/29] ARM: kernel: implement randomization of the
 kernel load address

On Sun, 3 Sep 2017, Ard Biesheuvel wrote:

> This implements randomization of the placement of the kernel image
> inside the lowmem region. It is intended to work together with the
> decompressor to place the kernel at an offset in physical memory
> that is a multiple of 2 MB, and to take the same offset into account
> when creating the virtual mapping.
> 
> This uses runtime relocation of the kernel built as a PIE binary, to
> fix up all absolute symbol references to refer to their runtime virtual
> address. The physical-to-virtual mapping remains unchanged.
> 
> In order to allow the decompressor to hand over to the core kernel
> without making assumptions that are not guaranteed to hold when
> invoking the core kernel directly using bootloaders that are not
> KASLR aware, the KASLR offset is expected to be placed in r3 when
> entering the kernel 4 bytes past the entry point, skipping the first
> instruction.
> 
> Cc: Russell King <linux@...linux.org.uk>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  arch/arm/Kconfig       |  15 +++
>  arch/arm/kernel/head.S | 103 ++++++++++++++++++--
>  2 files changed, 109 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 300add3b8023..fe4a2cd1f15c 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1825,6 +1825,21 @@ config XEN
>  	help
>  	  Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
>  
> +config RANDOMIZE_BASE
> +	bool "Randomize the address of the kernel image"
> +	depends on MMU && AUTO_ZRELADDR
> +	depends on !XIP_KERNEL && !ZBOOT_ROM

This should work even if ZBOOT_ROM is selected. The ZBOOT_ROM option 
allows for the decompressor executing directly from ROM and 
decompressing the kernel anywhere in RAM.


> +	select RELOCATABLE
> +	help
> +	  Randomizes the virtual and physical address at which the kernel
> +	  image is loaded, as a security feature that deters exploit attempts
> +	  relying on knowledge of the location of kernel internals.
> +
> +	  This relies on the UEFI stub to invoke the EFI_RNG_PROTOCOL to
> +	  randomize the load address of the decompressed kernel in the
> +	  physical space. The same offset is applied to the virtual mapping
> +	  of the kernel in the virtual space by the kernel proper.
> +
>  endmenu
>  
>  menu "Boot options"
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 71bc0d037bc9..0795da990dde 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -48,6 +48,28 @@
>  #define PMD_ORDER	2
>  #endif
>  
> +	.macro	get_kaslr_offset, reg
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	ldr_l			\reg, __kaslr_offset
> +#else
> +	mov			\reg, #0
> +#endif
> +	.endm
> +
> +	.macro	add_kaslr_offset, reg, tmp
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	get_kaslr_offset	\tmp
> +	add			\reg, \reg, \tmp
> +#endif
> +	.endm
> +
> +	.macro	sub_kaslr_offset, reg, tmp
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	get_kaslr_offset	\tmp
> +	sub			\reg, \reg, \tmp
> +#endif
> +	.endm
> +
>  /*
>   * Kernel startup entry point.
>   * ---------------------------
> @@ -73,6 +95,7 @@
>  	.equ	swapper_pg_dir, . - PG_DIR_SIZE
>  
>  ENTRY(stext)
> +	mov	r3, #0			@ normal entry point - clear r3
>   ARM_BE8(setend	be )			@ ensure we are in BE8 mode
>  
>   THUMB(	badr	r9, 1f		)	@ Kernel is always entered in ARM.
> @@ -80,6 +103,16 @@ ENTRY(stext)
>   THUMB(	.thumb			)	@ switch to Thumb now.
>   THUMB(1:			)
>  
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	str_l	r3, __kaslr_offset, r9	@ offset in r3 if entered via kaslr ep
> +
> +	.section ".bss", "aw", %nobits
> +	.align	2
> +__kaslr_offset:
> +	.long	0			@ will be wiped before entering C code
> +	.previous
> +#endif
> +
>  #ifdef CONFIG_ARM_VIRT_EXT
>  	bl	__hyp_stub_install
>  #endif
> @@ -103,6 +136,7 @@ ENTRY(stext)
>  #ifndef CONFIG_XIP_KERNEL
>  	adr_l	r8, _text			@ __pa(_text)
>  	sub	r8, r8, #TEXT_OFFSET		@ PHYS_OFFSET
> +	sub_kaslr_offset r8, r12
>  #else
>  	ldr	r8, =PLAT_PHYS_OFFSET		@ always constant in this case
>  #endif
> @@ -139,8 +173,8 @@ ENTRY(stext)
>  	 * r0 will hold the CPU control register value, r1, r2, r4, and
>  	 * r9 will be preserved.  r5 will also be preserved if LPAE.
>  	 */
> -	ldr	r13, =__mmap_switched		@ address to jump to after
> -						@ mmu has been enabled
> +	adr_l	lr, __primary_switch		@ address to jump to after
> +	mov	r13, lr				@ mmu has been enabled
>  	badr	lr, 1f				@ return (PIC) address
>  #ifdef CONFIG_ARM_LPAE
>  	mov	r5, #0				@ high TTBR0
> @@ -151,7 +185,8 @@ ENTRY(stext)
>  	ldr	r12, [r10, #PROCINFO_INITFUNC]
>  	add	r12, r12, r10
>  	ret	r12
> -1:	b	__enable_mmu
> +1:	get_kaslr_offset r12			@ get before turning MMU on
> +	b	__enable_mmu
>  ENDPROC(stext)
>  	.ltorg
>  
> @@ -230,9 +265,14 @@ __create_page_tables:
>  	/*
>  	 * Map our RAM from the start to the end of the kernel .bss section.
>  	 */
> -	add	r0, r4, #PAGE_OFFSET >> (SECTION_SHIFT - PMD_ORDER)
> -	ldr	r6, =(_end - 1)
> -	orr	r3, r8, r7
> +	get_kaslr_offset r3
> +	add	r0, r3, #PAGE_OFFSET
> +	add	r0, r4, r0, lsr #(SECTION_SHIFT - PMD_ORDER)
> +	adr_l	r6, _end - 1
> +	sub	r6, r6, r8
> +	add	r6, r6, #PAGE_OFFSET
> +	add	r3, r3, r8
> +	orr	r3, r3, r7
>  	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
>  1:	str	r3, [r0], #1 << PMD_ORDER
>  	add	r3, r3, #1 << SECTION_SHIFT
> @@ -376,7 +416,7 @@ ENTRY(secondary_startup)
>  	 * Use the page tables supplied from  __cpu_up.
>  	 */
>  	adr_l	r3, secondary_data
> -	mov_l	r12, __secondary_switched
> +	adr_l	r12, __secondary_switch
>  	ldrd	r4, [r3, #0]			@ get secondary_data.pgdir
>  ARM_BE8(eor	r4, r4, r5)			@ Swap r5 and r4 in BE:
>  ARM_BE8(eor	r5, r4, r5)			@ it can be done in 3 steps
> @@ -414,6 +454,7 @@ ENDPROC(__secondary_switched)
>   *  r4  = TTBR pointer (low word)
>   *  r5  = TTBR pointer (high word if LPAE)
>   *  r9  = processor ID
> + *  r12 = KASLR offset
>   *  r13 = *virtual* address to jump to upon completion
>   */
>  __enable_mmu:
> @@ -451,6 +492,7 @@ ENDPROC(__enable_mmu)
>   *  r1  = machine ID
>   *  r2  = atags or dtb pointer
>   *  r9  = processor ID
> + *  r12 = KASLR offset
>   *  r13 = *virtual* address to jump to upon completion
>   *
>   * other registers depend on the function called upon completion
> @@ -466,10 +508,52 @@ ENTRY(__turn_mmu_on)
>  	mov	r3, r3
>  	mov	r3, r13
>  	ret	r3
> -__turn_mmu_on_end:
>  ENDPROC(__turn_mmu_on)
> -	.popsection
>  
> +__primary_switch:
> +#ifdef CONFIG_RELOCATABLE
> +	adr_l	r7, _text			@ r7 := __pa(_text)
> +	sub	r7, r7, #TEXT_OFFSET		@ r7 := PHYS_OFFSET
> +
> +	adr_l	r5, __rel_begin
> +	adr_l	r6, __rel_end
> +	sub	r5, r5, r7
> +	sub	r6, r6, r7
> +
> +	add	r5, r5, #PAGE_OFFSET
> +	add	r6, r6, #PAGE_OFFSET
> +	add	r5, r5, r12
> +	add	r6, r6, r12
> +
> +	adr_l	r3, __stubs_start		@ __pa(__stubs_start)
> +	sub	r3, r3, r7			@ offset of __stubs_start
> +	add	r3, r3, #PAGE_OFFSET		@ __va(__stubs_start)
> +	sub	r3, r3, #0xffff1000		@ subtract VA of stubs section
> +
> +0:	cmp	r5, r6
> +	bge	1f
> +	ldm	r5!, {r7, r8}			@ load next relocation entry
> +	cmp	r8, #23				@ R_ARM_RELATIVE
> +	bne	0b
> +	cmp	r7, #0xff000000			@ vector page?
> +	addgt	r7, r7, r3			@ fix up VA offset
> +	ldr	r8, [r7, r12]
> +	add	r8, r8, r12
> +	str	r8, [r7, r12]
> +	b	0b
> +1:
> +#endif
> +	ldr	pc, =__mmap_switched
> +ENDPROC(__primary_switch)
> +
> +#ifdef CONFIG_SMP
> +__secondary_switch:
> +	ldr	pc, =__secondary_switched
> +ENDPROC(__secondary_switch)
> +#endif
> +	.ltorg
> +__turn_mmu_on_end:
> +	.popsection
>  
>  #ifdef CONFIG_SMP_ON_UP
>  	__HEAD
> @@ -570,6 +654,7 @@ __fixup_pv_table:
>  	adr_l	r6, __pv_phys_pfn_offset
>  	adr_l	r7, __pv_offset			@ __pa(__pv_offset)
>  	mov_l	r3, __pv_offset			@ __va(__pv_offset)
> +	add_kaslr_offset r3, ip
>  	mvn	ip, #0
>  	subs	r3, r7, r3	@ PHYS_OFFSET - PAGE_OFFSET
>  	mov	r0, r8, lsr #PAGE_SHIFT	@ convert to PFN
> -- 
> 2.11.0
> 
> 

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