arm64: add support for kernel ASLR

This adds support for KASLR is implemented, based on entropy provided by
the bootloader in the /chosen/kaslr-seed DT property. Depending on the size
of the address space (VA_BITS) and the page size, the entropy in the
virtual displacement is up to 13 bits (16k/2 levels) and up to 25 bits (all
4 levels), with the sidenote that displacements that result in the kernel
image straddling a 1GB/32MB/512MB alignment boundary (for 4KB/16KB/64KB
granule kernels, respectively) are not allowed, and will be rounded up to
an acceptable value.

If CONFIG_RANDOMIZE_MODULE_REGION_FULL is enabled, the module region is
randomized independently from the core kernel. This makes it less likely
that the location of core kernel data structures can be determined by an
adversary, but causes all function calls from modules into the core kernel
to be resolved via entries in the module PLTs.

If CONFIG_RANDOMIZE_MODULE_REGION_FULL is not enabled, the module region is
randomized by choosing a page aligned 128 MB region inside the interval
[_etext - 128 MB, _stext + 128 MB). This gives between 10 and 14 bits of
entropy (depending on page size), independently of the kernel randomization,
but still guarantees that modules are within the range of relative branch
and jump instructions (with the caveat that, since the module region is
shared with other uses of the vmalloc area, modules may need to be loaded
further away if the module region is exhausted)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 51 insertions, 8 deletions

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 4e69412..319f896 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -210,6 +210,7 @@ section_table:
 ENTRY(stext)
 	bl	preserve_boot_args
 	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
+	mov	x23, xzr			// KASLR offset, defaults to 0
 	adrp	x24, __PHYS_OFFSET
 	bl	set_cpu_boot_mode_flag
 	bl	__create_page_tables		// x25=TTBR0, x26=TTBR1
@@ -313,7 +314,7 @@ ENDPROC(preserve_boot_args)
 __create_page_tables:
 	adrp	x25, idmap_pg_dir
 	adrp	x26, swapper_pg_dir
-	mov	x27, lr
+	mov	x28, lr
 
 	/*
 	 * Invalidate the idmap and swapper page tables to avoid potential
@@ -392,6 +393,7 @@ __create_page_tables:
 	 */
 	mov	x0, x26				// swapper_pg_dir
 	ldr	x5, =KIMAGE_VADDR
+	add	x5, x5, x23			// add KASLR displacement
 	create_pgd_entry x0, x5, x3, x6
 	ldr	w6, kernel_img_size
 	add	x6, x6, x5
@@ -408,8 +410,7 @@ __create_page_tables:
 	dmb	sy
 	bl	__inval_cache_range
 
-	mov	lr, x27
-	ret
+	ret	x28
 ENDPROC(__create_page_tables)
 
 kernel_img_size:
@@ -421,6 +422,7 @@ kernel_img_size:
  */
 	.set	initial_sp, init_thread_union + THREAD_START_SP
 __mmap_switched:
+	mov	x28, lr				// preserve LR
 	adr_l	x8, vectors			// load VBAR_EL1 with virtual
 	msr	vbar_el1, x8			// vector table address
 	isb
@@ -449,19 +451,26 @@ __mmap_switched:
 	ldr	x13, [x9, #-8]
 	cmp	w12, #R_AARCH64_RELATIVE
 	b.ne	1f
-	str	x13, [x11]
+	add	x13, x13, x23			// relocate
+	str	x13, [x11, x23]
 	b	0b
 
 1:	cmp	w12, #R_AARCH64_ABS64
 	b.ne	0b
 	add	x12, x12, x12, lsl #1		// symtab offset: 24x top word
 	add	x12, x8, x12, lsr #(32 - 3)	// ... shifted into bottom word
+	ldrsh	w14, [x12, #6]			// Elf64_Sym::st_shndx
 	ldr	x15, [x12, #8]			// Elf64_Sym::st_value
+	cmp	w14, #-0xf			// SHN_ABS (0xfff1) ?
+	add	x14, x15, x23			// relocate
+	csel	x15, x14, x15, ne
 	add	x15, x13, x15
-	str	x15, [x11]
+	str	x15, [x11, x23]
 	b	0b
 
-2:
+2:	adr_l	x8, kimage_vaddr		// make relocated kimage_vaddr
+	dc	cvac, x8			// value visible to secondaries
+	dsb	sy				// with MMU off
 #endif
 
 	adr_l	sp, initial_sp, x4
@@ -470,7 +479,7 @@ __mmap_switched:
 	msr	sp_el0, x4			// Save thread_info
 	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
 
-	ldr	x4, =KIMAGE_VADDR		// Save the offset between
+	ldr_l	x4, kimage_vaddr		// Save the offset between
 	sub	x4, x4, x24			// the kernel virtual and
 	str_l	x4, kimage_voffset, x5		// physical mappings
 
@@ -478,6 +487,16 @@ __mmap_switched:
 #ifdef CONFIG_KASAN
 	bl	kasan_early_init
 #endif
+#ifdef CONFIG_RANDOMIZE_BASE
+	cbnz	x23, 0f				// already running randomized?
+	mov	x0, x21				// pass FDT address in x0
+	bl	kaslr_early_init		// parse FDT for KASLR options
+	cbz	x0, 0f				// KASLR disabled? just proceed
+	mov	x23, x0				// record KASLR offset
+	ret	x28				// we must enable KASLR, return
+						// to __enable_mmu()
+0:
+#endif
 	b	start_kernel
 ENDPROC(__mmap_switched)
 
@@ -486,6 +505,10 @@ ENDPROC(__mmap_switched)
  * hotplug and needs to have the same protections as the text region
  */
 	.section ".text","ax"
+
+ENTRY(kimage_vaddr)
+	.quad		_text - TEXT_OFFSET
+
 /*
  * If we're fortunate enough to boot at EL2, ensure that the world is
  * sane before dropping to EL1.
@@ -651,7 +674,7 @@ ENTRY(secondary_startup)
 	adrp	x26, swapper_pg_dir
 	bl	__cpu_setup			// initialise processor
 
-	ldr	x8, =KIMAGE_VADDR
+	ldr	x8, kimage_vaddr
 	ldr	w9, 0f
 	sub	x27, x8, w9, sxtw		// address to jump to after enabling the MMU
 	b	__enable_mmu
@@ -684,6 +707,7 @@ ENDPROC(__secondary_switched)
  */
 	.section	".idmap.text", "ax"
 __enable_mmu:
+	mrs	x18, sctlr_el1			// preserve old SCTLR_EL1 value
 	mrs	x1, ID_AA64MMFR0_EL1
 	ubfx	x2, x1, #ID_AA64MMFR0_TGRAN_SHIFT, 4
 	cmp	x2, #ID_AA64MMFR0_TGRAN_SUPPORTED
@@ -701,6 +725,25 @@ __enable_mmu:
 	ic	iallu
 	dsb	nsh
 	isb
+#ifdef CONFIG_RANDOMIZE_BASE
+	mov	x19, x0				// preserve new SCTLR_EL1 value
+	blr	x27
+
+	/*
+	 * If we return here, we have a KASLR displacement in x23 which we need
+	 * to take into account by discarding the current kernel mapping and
+	 * creating a new one.
+	 */
+	msr	sctlr_el1, x18			// disable the MMU
+	isb
+	bl	__create_page_tables		// recreate kernel mapping
+
+	msr	sctlr_el1, x19			// re-enable the MMU
+	isb
+	ic	ialluis				// flush instructions fetched
+	isb					// via old mapping
+	add	x27, x27, x23			// relocated __mmap_switched
+#endif
 	br	x27
 ENDPROC(__enable_mmu)