[PATCH 4/6] x86: kaslr: select random base offset

Kees Cook
2013-04-25T21:55:37Z

Select a random location when CONFIG_RANDOMIZE_BASE is used, bounded
by CONFIG_RANDOMIZE_BASE_MAX_OFFSET. Sources of randomness currently
include RDRAND and RDTSC.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
arch/x86/Kconfig | 29 +++++++++++++--
arch/x86/boot/compressed/aslr.c | 75 +++++++++++++++++++++++++++++++++++++--
2 files changed, 100 insertions(+), 4 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 6f59afe..78db42d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1700,9 +1700,34 @@ config RELOCATABLE
(CONFIG_PHYSICAL_START) is ignored.

config RANDOMIZE_BASE
- bool "Enable 64-bit relocation support (for KASLR)"
+ bool "Randomize the address of the kernel image"
depends on RELOCATABLE
+ depends on !HIBERNATION
default n
+ ---help---
+ Randomizes the physical and virtual address at which the
+ kernel image is decompressed, as a security feature that
+ deters exploit attempts relying on knowledge of the location
+ of kernel internals.
+
+ Entropy is generated using the RDRAND instruction if it
+ is supported. If not, then RDTSC is used, if supported. If
+ neither RDRAND nor RDTSC are supported, then no randomness
+ is introduced.
+
+ The kernel will be offset by up to RANDOMIZE_BASE_MAX_OFFSET,
+ and aligned according to PHYSICAL_ALIGN.
+
+config RANDOMIZE_BASE_MAX_OFFSET
+ hex "Maximum ASLR offset allowed"
+ depends on RANDOMIZE_BASE
+ default "0x10000000"
+ range 0x0 0x10000000
+ ---help---
+ Determines the maximal offset in bytes that will be applied to the
+ kernel when Address Space Layout Randomization (ASLR) is active.
+ Must be less than or equal to the actual physical memory on the
+ system. This must be a power of two.

# Relocation on x86 needs some additional build support
config X86_NEED_RELOCS
@@ -1711,7 +1736,7 @@ config X86_NEED_RELOCS

config PHYSICAL_ALIGN
hex "Alignment value to which kernel should be aligned"
- default "0x1000000"
+ default "0x200000"
range 0x2000 0x1000000
---help---
This value puts the alignment restrictions on physical address
diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
index d5331ee..11a91c6 100644
--- a/arch/x86/boot/compressed/aslr.c
+++ b/arch/x86/boot/compressed/aslr.c
@@ -2,18 +2,89 @@

#ifdef CONFIG_RANDOMIZE_BASE

+#include <asm/archrandom.h>
+static inline int rdrand(unsigned long *v)
+{
+ int ok;
+ asm volatile("1: " RDRAND_LONG "\n\t"
+ "jc 2f\n\t"
+ "decl %0\n\t"
+ "jnz 1b\n\t"
+ "2:"
+ : "=r" (ok), "=a" (*v)
+ : "0" (RDRAND_RETRY_LOOPS));
+ return ok;
+}
+
+static inline uint32_t rdtsc(void)
+{
+ uint32_t timer;
+
+ asm volatile("rdtsc\n" : "=a" (timer));
+
+ return timer;
+}
+
+static unsigned long get_random_long(void)
+{
+ if (has_cpuflag(X86_FEATURE_RDRAND)) {
+ unsigned long random;
+
+ debug_putstr("KASLR using RDRAND...\n");
+ if (rdrand(&random))
+ return random;
+ }
+
+ if (has_cpuflag(X86_FEATURE_TSC)) {
+ uint32_t raw;
+ unsigned long timer;
+
+ debug_putstr("KASLR using RDTSC...\n");
+ raw = rdtsc();
+
+ /* Repeat the low bits of rdtsc. */
+ timer = raw & 0xffff;
+ timer |= (timer << 16);
+#ifdef CONFIG_X86_64
+ timer |= (timer << 32) | (timer << 48);
+#endif
+
+ return timer;
+ }
+
+ debug_putstr("KASLR found no entropy source...\n");
+ return 0;
+}
+
unsigned char *choose_kernel_location(unsigned char *hint, unsigned long size)
{
unsigned char *choice = hint;
- unsigned long random;
+ unsigned long random, mask;

if (cmdline_find_option_bool("noaslr")) {
debug_putstr("KASLR disabled...\n");
goto out;
}

- /* XXX: choose random location. */
+ random = get_random_long();
+
+ /* Clip off top of the range. */
+ mask = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - 1;
+ random &= mask;
+
+ /* XXX: Find an appropriate E820 hole, instead of adding hint. */
+ random += (unsigned long)hint;
+
+ /* XXX: Clip to E820 hole, instead of just using hint. */
+ mask = (unsigned long)hint + CONFIG_RANDOMIZE_BASE_MAX_OFFSET;
+ while (random + size > mask)
+ random >>= 1;
+
+ /* Clip off bottom of range (via alignment). */
+ mask = CONFIG_PHYSICAL_ALIGN - 1;
+ random &= ~mask;

+ choice = (unsigned char *)random;
out:
return choice;
}
--
1.7.9.5