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

Ard Biesheuvel ard.biesheuvel at linaro.org
Tue Jan 5 23:51:22 PST 2016


On 5 January 2016 at 20:51, Kees Cook <keescook at chromium.org> wrote:
> On Wed, Dec 30, 2015 at 7:26 AM, Ard Biesheuvel
> <ard.biesheuvel at 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 at 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



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