Re: [PATCH v2] x86, relocs: move ELF relocation handling to C

From: Zhang Yanfei
Date: Mon Jul 08 2013 - 13:35:22 EST

On 07/09/2013 12:15 AM, Kees Cook wrote:
> Moves the relocation handling into C, after decompression. This requires
> that the decompressed size is passed to the decompression routine as
> well so that relocations can be found. Only kernels that need relocation
> support will use the code (currently just x86_32), but this is laying
> the ground work for 64-bit using it in support of KASLR.
>
> Based on work by Neill Clift and Michael Davidson.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>

> ---
> v2:
> - several suggestions from Zhang Yanfei:
> - adjusted Kconfig ranges to be per-architecture, updated help text.
> - used ALIGN macro
> ---
> arch/x86/Kconfig | 8 +++-
> arch/x86/Makefile | 8 ++--
> arch/x86/boot/compressed/head_32.S | 31 ++------------
> arch/x86/boot/compressed/head_64.S | 1 +
> arch/x86/boot/compressed/misc.c | 77 +++++++++++++++++++++++++++++++++-
> arch/x86/include/asm/page_32_types.h | 2 +
> arch/x86/include/asm/page_64_types.h | 5 ---
> arch/x86/include/asm/page_types.h | 5 +++
> 8 files changed, 97 insertions(+), 40 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 265c672..7598b55 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1715,9 +1715,10 @@ config X86_NEED_RELOCS
> depends on X86_32 && RELOCATABLE
>
> config PHYSICAL_ALIGN
> - hex "Alignment value to which kernel should be aligned" if X86_32
> + hex "Alignment value to which kernel should be aligned"
> default "0x1000000"
> - range 0x2000 0x1000000
> + range 0x2000 0x1000000 if X86_32
> + range 0x200000 0x1000000 if X86_64
> ---help---
> This value puts the alignment restrictions on physical address
> where kernel is loaded and run from. Kernel is compiled for an
> @@ -1735,6 +1736,9 @@ config PHYSICAL_ALIGN
> end result is that kernel runs from a physical address meeting
> above alignment restrictions.
>
> + On 32-bit this value must be a multiple of 0x2000. On 64-bit
> + this value must be a multiple of 0x200000.
> +
> Don't change this unless you know what you are doing.
>
> config HOTPLUG_CPU
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 07639c6..41250fb 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -16,6 +16,10 @@ endif
> # e.g.: obj-y += foo_$(BITS).o
> export BITS
>
> +ifdef CONFIG_X86_NEED_RELOCS
> + LDFLAGS_vmlinux := --emit-relocs
> +endif
> +
> ifeq ($(CONFIG_X86_32),y)
> BITS := 32
> UTS_MACHINE := i386
> @@ -25,10 +29,6 @@ ifeq ($(CONFIG_X86_32),y)
> KBUILD_AFLAGS += $(biarch)
> KBUILD_CFLAGS += $(biarch)
>
> - ifdef CONFIG_RELOCATABLE
> - LDFLAGS_vmlinux := --emit-relocs
> - endif
> -
> KBUILD_CFLAGS += -msoft-float -mregparm=3 -freg-struct-return
>
> # Never want PIC in a 32-bit kernel, prevent breakage with GCC built
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 1e3184f..5d6f689 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -181,8 +181,9 @@ relocated:
> /*
> * Do the decompression, and jump to the new kernel..
> */
> - leal z_extract_offset_negative(%ebx), %ebp
> /* push arguments for decompress_kernel: */
> + pushl $z_output_len /* decompressed length */
> + leal z_extract_offset_negative(%ebx), %ebp
> pushl %ebp /* output address */
> pushl $z_input_len /* input_len */
> leal input_data(%ebx), %eax
> @@ -191,33 +192,7 @@ relocated:
> pushl %eax /* heap area */
> pushl %esi /* real mode pointer */
> call decompress_kernel
> - addl $20, %esp
> -
> -#if CONFIG_RELOCATABLE
> -/*
> - * Find the address of the relocations.
> - */
> - leal z_output_len(%ebp), %edi
> -
> -/*
> - * Calculate the delta between where vmlinux was compiled to run
> - * and where it was actually loaded.
> - */
> - movl %ebp, %ebx
> - subl $LOAD_PHYSICAL_ADDR, %ebx
> - jz 2f /* Nothing to be done if loaded at compiled addr. */
> -/*
> - * Process relocations.
> - */
> -
> -1: subl $4, %edi
> - movl (%edi), %ecx
> - testl %ecx, %ecx
> - jz 2f
> - addl %ebx, -__PAGE_OFFSET(%ebx, %ecx)
> - jmp 1b
> -2:
> -#endif
> + addl $24, %esp
>
> /*
> * Jump to the decompressed kernel.
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 06e71c2..c337422 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -338,6 +338,7 @@ relocated:
> leaq input_data(%rip), %rdx /* input_data */
> movl $z_input_len, %ecx /* input_len */
> movq %rbp, %r8 /* output target address */
> + movq $z_output_len, %r9 /* decompressed length */
> call decompress_kernel
> popq %rsi
>
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 7cb56c6..b756a04 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -267,6 +267,79 @@ static void error(char *x)
> asm("hlt");
> }
>
> +#if CONFIG_X86_NEED_RELOCS
> +static void handle_relocations(void *output, unsigned long output_len)
> +{
> + int *reloc;
> + unsigned long delta, map, ptr;
> + unsigned long min_addr = (unsigned long)output;
> + unsigned long max_addr = min_addr + output_len;
> +
> + /*
> + * Calculate the delta between where vmlinux was linked to load
> + * and where it was actually loaded.
> + */
> + delta = min_addr - LOAD_PHYSICAL_ADDR;
> + if (!delta) {
> + debug_putstr("No relocation needed... ");
> + return;
> + }
> + debug_putstr("Performing relocations... ");
> +
> + /*
> + * The kernel contains a table of relocation addresses. Those
> + * addresses have the final load address of the kernel in virtual
> + * memory. We are currently working in the self map. So we need to
> + * create an adjustment for kernel memory addresses to the self map.
> + * This will involve subtracting out the base address of the kernel.
> + */
> + map = delta - __START_KERNEL_map;
> +
> + /*
> + * Process relocations: 32 bit relocations first then 64 bit after.
> + * Two sets of binary relocations are added to the end of the kernel
> + * before compression. Each relocation table entry is the kernel
> + * address of the location which needs to be updated stored as a
> + * 32-bit value which is sign extended to 64 bits.
> + *
> + * Format is:
> + *
> + * kernel bits...
> + * 0 - zero terminator for 64 bit relocations
> + * 64 bit relocation repeated
> + * 0 - zero terminator for 32 bit relocations
> + * 32 bit relocation repeated
> + *
> + * So we work backwards from the end of the decompressed image.
> + */
> + for (reloc = output + output_len - sizeof(*reloc); *reloc; reloc--) {
> + int extended = *reloc;
> + extended += map;
> +
> + ptr = (unsigned long)extended;
> + if (ptr < min_addr || ptr > max_addr)
> + error("32-bit relocation outside of kernel!\n");
> +
> + *(uint32_t *)ptr += delta;
> + }
> +#ifdef CONFIG_X86_64
> + for (reloc--; *reloc; reloc--) {
> + long extended = *reloc;
> + extended += map;
> +
> + ptr = (unsigned long)extended;
> + if (ptr < min_addr || ptr > max_addr)
> + error("64-bit relocation outside of kernel!\n");
> +
> + *(uint64_t *)ptr += delta;
> + }
> +#endif
> +}
> +#else
> +static inline void handle_relocations(void *output, unsigned long output_len)
> +{ }
> +#endif
> +
> static void parse_elf(void *output)
> {
> #ifdef CONFIG_X86_64
> @@ -321,7 +394,8 @@ static void parse_elf(void *output)
> asmlinkage void decompress_kernel(void *rmode, memptr heap,
> unsigned char *input_data,
> unsigned long input_len,
> - unsigned char *output)
> + unsigned char *output,
> + unsigned long output_len)
> {
> real_mode = rmode;
>
> @@ -361,6 +435,7 @@ asmlinkage void decompress_kernel(void *rmode, memptr heap,
> debug_putstr("\nDecompressing Linux... ");
> decompress(input_data, input_len, NULL, NULL, output, NULL, error);
> parse_elf(output);
> + handle_relocations(output, output_len);
> debug_putstr("done.\nBooting the kernel.\n");
> return;
> }
> diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
> index ef17af0..f48b17d 100644
> --- a/arch/x86/include/asm/page_32_types.h
> +++ b/arch/x86/include/asm/page_32_types.h
> @@ -15,6 +15,8 @@
> */
> #define __PAGE_OFFSET _AC(CONFIG_PAGE_OFFSET, UL)
>
> +#define __START_KERNEL_map __PAGE_OFFSET
> +
> #define THREAD_SIZE_ORDER 1
> #define THREAD_SIZE (PAGE_SIZE << THREAD_SIZE_ORDER)
>
> diff --git a/arch/x86/include/asm/page_64_types.h b/arch/x86/include/asm/page_64_types.h
> index 6c896fb..43dcd80 100644
> --- a/arch/x86/include/asm/page_64_types.h
> +++ b/arch/x86/include/asm/page_64_types.h
> @@ -32,11 +32,6 @@
> */
> #define __PAGE_OFFSET _AC(0xffff880000000000, UL)
>
> -#define __PHYSICAL_START ((CONFIG_PHYSICAL_START + \
> - (CONFIG_PHYSICAL_ALIGN - 1)) & \
> - ~(CONFIG_PHYSICAL_ALIGN - 1))
> -
> -#define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START)
> #define __START_KERNEL_map _AC(0xffffffff80000000, UL)
>
> /* See Documentation/x86/x86_64/mm.txt for a description of the memory map. */
> diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h
> index 54c9787..f97fbe3 100644
> --- a/arch/x86/include/asm/page_types.h
> +++ b/arch/x86/include/asm/page_types.h
> @@ -33,6 +33,11 @@
> (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0 ) | \
> VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
>
> +#define __PHYSICAL_START ALIGN(CONFIG_PHYSICAL_START, \
> + CONFIG_PHYSICAL_ALIGN)
> +
> +#define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START)
> +
> #ifdef CONFIG_X86_64
> #include <asm/page_64_types.h>
> #else


--
Thanks.
Zhang Yanfei
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