[PATCH 3/6] x86: kaslr: return location from decompress_kernel

Kees Cook
2013-04-25T21:56:54Z

This allows decompress_kernel to return a new location for the kernel to
be relocated to. With CONFIG_RANDOMIZE_BASE, the choose_kernel_location
routine will select a new location to decompress the kernel, and is
presently a no-op. The logic for bypassing this routine with "noaslr"
on the kernel command line is handled.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
Documentation/kernel-parameters.txt | 4 ++++
arch/x86/boot/compressed/Makefile | 2 +-
arch/x86/boot/compressed/aslr.c | 21 +++++++++++++++++++++
arch/x86/boot/compressed/cmdline.c | 2 +-
arch/x86/boot/compressed/head_32.S | 2 +-
arch/x86/boot/compressed/head_64.S | 2 +-
arch/x86/boot/compressed/misc.c | 7 +++++--
arch/x86/boot/compressed/misc.h | 22 ++++++++++++++++------
8 files changed, 50 insertions(+), 12 deletions(-)
create mode 100644 arch/x86/boot/compressed/aslr.c

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 8ccbf27..eb1c62c 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1862,6 +1862,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
noapic [SMP,APIC] Tells the kernel to not make use of any
IOAPICs that may be present in the system.

+ noaslr [X86]
+ Disable kernel base offset ASLR (Address Space
+ Layout Randomization) if built into the kernel.
+
noautogroup Disable scheduler automatic task group creation.

nobats [PPC] Do not use BATs for mapping kernel lowmem
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index d854390..fa2629b 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -26,7 +26,7 @@ HOST_EXTRACFLAGS += -I$(srctree)/tools/include

VMLINUX_OBJS = $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
$(obj)/string.o $(obj)/cmdline.o $(obj)/early_serial_console.o \
- $(obj)/piggy.o $(obj)/cpuflags.o
+ $(obj)/piggy.o $(obj)/cpuflags.o $(obj)/aslr.o

$(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone

diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
new file mode 100644
index 0000000..d5331ee
--- /dev/null
+++ b/arch/x86/boot/compressed/aslr.c
@@ -0,0 +1,21 @@
+#include "misc.h"
+
+#ifdef CONFIG_RANDOMIZE_BASE
+
+unsigned char *choose_kernel_location(unsigned char *hint, unsigned long size)
+{
+ unsigned char *choice = hint;
+ unsigned long random;
+
+ if (cmdline_find_option_bool("noaslr")) {
+ debug_putstr("KASLR disabled...\n");
+ goto out;
+ }
+
+ /* XXX: choose random location. */
+
+out:
+ return choice;
+}
+
+#endif /* CONFIG_RANDOMIZE_BASE */
diff --git a/arch/x86/boot/compressed/cmdline.c b/arch/x86/boot/compressed/cmdline.c
index bffd73b..b68e303 100644
--- a/arch/x86/boot/compressed/cmdline.c
+++ b/arch/x86/boot/compressed/cmdline.c
@@ -1,6 +1,6 @@
#include "misc.h"

-#ifdef CONFIG_EARLY_PRINTK
+#if CONFIG_EARLY_PRINTK || CONFIG_RANDOMIZE_BASE

static unsigned long fs;
static inline void set_fs(unsigned long seg)
diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 5d6f689..a48631a 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -198,7 +198,7 @@ relocated:
* Jump to the decompressed kernel.
*/
xorl %ebx, %ebx
- jmp *%ebp
+ jmp *%eax

/*
* Stack and heap for uncompression
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 81ca174..bfd9be8 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -347,7 +347,7 @@ relocated:
/*
* Jump to the decompressed kernel.
*/
- jmp *%rbp
+ jmp *%rax

.code32
no_longmode:
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b756a04..15a3ea8 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -391,7 +391,7 @@ static void parse_elf(void *output)
free(phdrs);
}

-asmlinkage void decompress_kernel(void *rmode, memptr heap,
+asmlinkage void *decompress_kernel(void *rmode, memptr heap,
unsigned char *input_data,
unsigned long input_len,
unsigned char *output,
@@ -418,6 +418,9 @@ asmlinkage void decompress_kernel(void *rmode, memptr heap,
free_mem_ptr = heap; /* Heap */
free_mem_end_ptr = heap + BOOT_HEAP_SIZE;

+ output = choose_kernel_location(output, output_len);
+
+ /* Validate memory location choices. */
if ((unsigned long)output & (MIN_KERNEL_ALIGN - 1))
error("Destination address inappropriately aligned");
#ifdef CONFIG_X86_64
@@ -437,5 +440,5 @@ asmlinkage void decompress_kernel(void *rmode, memptr heap,
parse_elf(output);
handle_relocations(output, output_len);
debug_putstr("done.\nBooting the kernel.\n");
- return;
+ return output;
}
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 674019d..2a5c7aa 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -39,23 +39,33 @@ static inline void debug_putstr(const char *s)

#endif

-#ifdef CONFIG_EARLY_PRINTK
-
+#if CONFIG_EARLY_PRINTK || CONFIG_RANDOMIZE_BASE
/* cmdline.c */
int cmdline_find_option(const char *option, char *buffer, int bufsize);
int cmdline_find_option_bool(const char *option);
+#endif
+
+#if CONFIG_RANDOMIZE_BASE
+/* aslr.c */
+unsigned char *choose_kernel_location(unsigned char *hint, unsigned long size);
+/* cpuflags.c */
+bool has_cpuflag(int flag);
+#else
+static inline unsigned char *choose_kernel_location(unsigned char *hint,
+ unsigned long size)
+{
+ return hint;
+}
+#endif

+#ifdef CONFIG_EARLY_PRINTK
/* early_serial_console.c */
extern int early_serial_base;
void console_init(void);
-
#else
-
-/* early_serial_console.c */
static const int early_serial_base;
static inline void console_init(void)
{ }
-
#endif

#endif
--
1.7.9.5