[PATCH 23/30] ARM: kernel: implement randomization of the kernel load address
Ard Biesheuvel
ard.biesheuvel at linaro.org
Mon Aug 14 05:54:04 PDT 2017
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 at armlinux.org.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.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..0d7b40f1061d 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 && !HAVE_TCM
+ 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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