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

Kees Cook
2016-01-05T19:51:50Z

On Wed, Dec 30, 2015 at 7:26 AM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> 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>
> ---
> 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

Should this be called "CONFIG_RELOCATABLE" instead, just to keep
naming the same across x86, powerpw, and arm64?

> + bool "Kernel address space layout randomization (KASLR)"

Strictly speaking, this enables KASLR, but doesn't provide it,
correct? It still relies on the boot loader for the randomness?

> + 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
>

-Kees

--
Kees Cook
Chrome OS & Brillo Security