[PATCH 1/6] x86: kaslr: move ELF relocation handling to C

Kees Cook
2013-04-25T21:55:34Z

Moves the relocation handling into C, after decompression. Only
kernels that need relocation support will use the code. The new
CONFIG_RANDOMIZE_BASE does not yet do anything except turn on this logic
for 64-bit kernels.

Based on work by Neill Clift and Michael Davidson.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
v2:
- made CONFIG_RANDOMIZE_BASE interactively selectable.
---
arch/x86/Kconfig | 11 +++--
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 | 6 +++
8 files changed, 100 insertions(+), 41 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 15b5cef..6f59afe 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1699,13 +1699,18 @@ config RELOCATABLE
it has been loaded at and the compile time physical address
(CONFIG_PHYSICAL_START) is ignored.

-# Relocation on x86-32 needs some additional build support
+config RANDOMIZE_BASE
+ bool "Enable 64-bit relocation support (for KASLR)"
+ depends on RELOCATABLE
+ default n
+
+# Relocation on x86 needs some additional build support
config X86_NEED_RELOCS
def_bool y
- depends on X86_32 && RELOCATABLE
+ depends on RANDOMIZE_BASE || (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
---help---
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5c47726..43f8cef 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 c1d383d..81ca174 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -340,6 +340,7 @@ 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 8b491e6..203e98a 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..086c2fa 100644
--- a/arch/x86/include/asm/page_types.h
+++ b/arch/x86/include/asm/page_types.h
@@ -33,6 +33,12 @@
(((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0 ) | \
VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)

+#define __PHYSICAL_START ((CONFIG_PHYSICAL_START + \
+ (CONFIG_PHYSICAL_ALIGN - 1)) & \
+ ~(CONFIG_PHYSICAL_ALIGN - 1))
+
+#define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START)
+
#ifdef CONFIG_X86_64
#include <asm/page_64_types.h>
#else
--
1.7.9.5