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

Ard Biesheuvel
2015-12-30T15:27:18Z

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>
---
Documentation/arm64/booting.txt | 3 +-
arch/arm64/Kconfig | 13 ++++
arch/arm64/Makefile | 6 +-
arch/arm64/include/asm/memory.h | 3 +
arch/arm64/kernel/head.S | 75 +++++++++++++++++++-
arch/arm64/kernel/setup.c | 22 +++---
arch/arm64/kernel/vmlinux.lds.S | 9 +++
scripts/sortextable.c | 4 +-
8 files changed, 117 insertions(+), 18 deletions(-)

diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 03e02ebc1b0c..b17181eb4a43 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -109,7 +109,8 @@ Header notes:
1 - 4K
2 - 16K
3 - 64K
- Bits 3-63: Reserved.
+ Bit 3: Relocatable kernel.
+ Bits 4-63: Reserved.

- When image_size is zero, a bootloader should attempt to keep as much
memory as possible free for use by the kernel immediately after the
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 54eeab140bca..f458fb9e0dce 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -363,6 +363,7 @@ config ARM64_ERRATUM_843419
bool "Cortex-A53: 843419: A load or store might access an incorrect address"
depends on MODULES
default y
+ select ARM64_MODULE_CMODEL_LARGE
help
This option builds kernel modules using the large memory model in
order to avoid the use of the ADRP instruction, which can cause
@@ -709,6 +710,18 @@ config ARM64_MODULE_PLTS
bool
select HAVE_MOD_ARCH_SPECIFIC

+config ARM64_MODULE_CMODEL_LARGE
+ bool
+
+config ARM64_RELOCATABLE_KERNEL
+ bool "Kernel address space layout randomization (KASLR)"
+ select ARM64_MODULE_PLTS
+ select ARM64_MODULE_CMODEL_LARGE
+ help
+ This feature randomizes the virtual address of the kernel image, to
+ harden against exploits that rely on knowledge about the absolute
+ addresses of certain kernel data structures.
+
endmenu

menu "Boot options"
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index d4654830e536..75dc477d45f5 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -15,6 +15,10 @@ CPPFLAGS_vmlinux.lds = -DTEXT_OFFSET=$(TEXT_OFFSET)
OBJCOPYFLAGS :=-O binary -R .note -R .note.gnu.build-id -R .comment -S
GZFLAGS :=-9

+ifneq ($(CONFIG_ARM64_RELOCATABLE_KERNEL),)
+LDFLAGS_vmlinux += -pie
+endif
+
KBUILD_DEFCONFIG := defconfig

# Check for binutils support for specific extensions
@@ -41,7 +45,7 @@ endif

CHECKFLAGS += -D__aarch64__

-ifeq ($(CONFIG_ARM64_ERRATUM_843419), y)
+ifeq ($(CONFIG_ARM64_MODULE_CMODEL_LARGE), y)
KBUILD_CFLAGS_MODULE += -mcmodel=large
endif

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 557228658666..afab3e669e19 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -121,6 +121,9 @@ extern phys_addr_t memstart_addr;
/* PHYS_OFFSET - the physical address of the start of memory. */
#define PHYS_OFFSET ({ memstart_addr; })

+/* the virtual base of the kernel image (minus TEXT_OFFSET) */
+extern u64 kimage_vaddr;
+
/* the offset between the kernel virtual and physical mappings */
extern u64 kimage_voffset;

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
@@ -59,8 +59,15 @@

#define __HEAD_FLAG_PAGE_SIZE ((PAGE_SHIFT - 10) / 2)

+#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
+#define __HEAD_FLAG_RELOC 1
+#else
+#define __HEAD_FLAG_RELOC 0
+#endif
+
#define __HEAD_FLAGS ((__HEAD_FLAG_BE << 0) | \
- (__HEAD_FLAG_PAGE_SIZE << 1))
+ (__HEAD_FLAG_PAGE_SIZE << 1) | \
+ (__HEAD_FLAG_RELOC << 3))

/*
* Kernel startup entry point.
@@ -231,6 +238,9 @@ ENTRY(stext)
*/
ldr x27, 0f // address to jump to after
// MMU has been enabled
+#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
+ add x27, x27, x23 // add KASLR displacement
+#endif
adr_l lr, __enable_mmu // return (PIC) address
b __cpu_setup // initialise processor
ENDPROC(stext)
@@ -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
+
adr_l x0, boot_args // record the contents of
stp x21, x1, [x0] // x0 .. x3 at kernel entry
stp x2, x3, [x0, #16]
@@ -402,6 +422,9 @@ __create_page_tables:
*/
mov x0, x26 // swapper_pg_dir
ldr x5, =KIMAGE_VADDR
+#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
+ add x5, x5, x23 // add KASLR displacement
+#endif
create_pgd_entry x0, x5, x3, x6
ldr w6, kernel_img_size
add x6, x6, x5
@@ -443,10 +466,52 @@ __mmap_switched:
str xzr, [x6], #8 // Clear BSS
b 1b
2:
+
+#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
+
+#define R_AARCH64_RELATIVE 0x403
+#define R_AARCH64_ABS64 0x101
+
+ /*
+ * Iterate over each entry in the relocation table, and apply the
+ * relocations in place.
+ */
+ adr_l x8, __dynsym_start // start of symbol table
+ adr_l x9, __reloc_start // start of reloc table
+ adr_l x10, __reloc_end // end of reloc table
+
+0: cmp x9, x10
+ b.hs 2f
+ ldp x11, x12, [x9], #24
+ ldr x13, [x9, #-8]
+ cmp w12, #R_AARCH64_RELATIVE
+ b.ne 1f
+ 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, x23]
+ b 0b
+
+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
str_l x21, __fdt_pointer, x5 // Save FDT pointer

- ldr x0, =KIMAGE_VADDR // Save the offset between
+ ldr_l x0, kimage_vaddr // Save the offset between
sub x24, x0, x24 // the kernel virtual and
str_l x24, kimage_voffset, x0 // physical mappings

@@ -462,6 +527,10 @@ 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.
@@ -622,7 +691,7 @@ 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
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 96177a7c0f05..2faee6042e99 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -292,16 +292,15 @@ u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };

void __init setup_arch(char **cmdline_p)
{
- static struct vm_struct vmlinux_vm __initdata = {
- .addr = (void *)KIMAGE_VADDR,
- .size = 0,
- .flags = VM_IOREMAP,
- .caller = setup_arch,
- };
-
- vmlinux_vm.size = round_up((unsigned long)_end - KIMAGE_VADDR,
- 1 << SWAPPER_BLOCK_SHIFT);
- vmlinux_vm.phys_addr = __pa(KIMAGE_VADDR);
+ static struct vm_struct vmlinux_vm __initdata;
+
+ vmlinux_vm.addr = (void *)kimage_vaddr;
+ vmlinux_vm.size = round_up((u64)_end - kimage_vaddr,
+ SWAPPER_BLOCK_SIZE);
+ vmlinux_vm.phys_addr = __pa(kimage_vaddr);
+ vmlinux_vm.flags = VM_IOREMAP;
+ vmlinux_vm.caller = setup_arch;
+
vm_area_add_early(&vmlinux_vm);

pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());
@@ -367,7 +366,8 @@ void __init setup_arch(char **cmdline_p)
conswitchp = &dummy_con;
#endif
#endif
- if (boot_args[1] || boot_args[2] || boot_args[3]) {
+ if ((!IS_ENABLED(CONFIG_ARM64_RELOCATABLE_KERNEL) && boot_args[1]) ||
+ boot_args[2] || boot_args[3]) {
pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"
"\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n"
"This indicates a broken bootloader or old kernel\n",
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index f935f082188d..cc1486039338 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -148,6 +148,15 @@ SECTIONS
.altinstr_replacement : {
*(.altinstr_replacement)
}
+ .rela : ALIGN(8) {
+ __reloc_start = .;
+ *(.rela .rela*)
+ __reloc_end = .;
+ }
+ .dynsym : ALIGN(8) {
+ __dynsym_start = .;
+ *(.dynsym)
+ }

. = ALIGN(PAGE_SIZE);
__init_end = .;
diff --git a/scripts/sortextable.c b/scripts/sortextable.c
index af247c70fb66..5ecbedefdb0f 100644
--- a/scripts/sortextable.c
+++ b/scripts/sortextable.c
@@ -266,9 +266,9 @@ do_file(char const *const fname)
break;
} /* end switch */
if (memcmp(ELFMAG, ehdr->e_ident, SELFMAG) != 0
- || r2(&ehdr->e_type) != ET_EXEC
+ || (r2(&ehdr->e_type) != ET_EXEC && r2(&ehdr->e_type) != ET_DYN)
|| ehdr->e_ident[EI_VERSION] != EV_CURRENT) {
- fprintf(stderr, "unrecognized ET_EXEC file %s\n", fname);
+ fprintf(stderr, "unrecognized ET_EXEC/ET_DYN file %s\n", fname);
fail_file();
}

--
2.5.0