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

Ard Biesheuvel
2016-01-06T07:51:27Z

On 5 January 2016 at 20:51, Kees Cook <keescook@chromium.org> wrote:
> 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?
>

Yes, I will change that.

>> + 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?
>

Indeed.

>> + 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