Select a random location when CONFIG_RANDOMIZE_BASE is used, bounded
by CONFIG_RANDOMIZE_BASE_MAX_OFFSET. Sources of randomness currently
include RDRAND, RDTSC, or the i8254.
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
arch/x86/boot/compressed/aslr.c | 83 ++++++++++++++++++++++++++++++++++++++-
arch/x86/boot/compressed/misc.h | 2 +
2 files changed, 84 insertions(+), 1 deletion(-)
diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
index ed7e9f0..2f63f9f 100644
--- a/arch/x86/boot/compressed/aslr.c
+++ b/arch/x86/boot/compressed/aslr.c
@@ -1,6 +1,72 @@
#include "misc.h"
#ifdef CONFIG_RANDOMIZE_BASE
+#include <asm/msr.h>
+
+#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;
+}
+
+#define I8254_PORT_CONTROL 0x43
+#define I8254_PORT_COUNTER0 0x40
+#define I8254_CMD_READBACK 0xC0
+#define I8254_SELECT_COUNTER0 0x02
+#define I8254_STATUS_NOTREADY 0x40
+static inline u16 i8254(void)
+{
+ u16 status, timer;
+
+ do {
+ outb(I8254_PORT_CONTROL,
+ I8254_CMD_READBACK | I8254_SELECT_COUNTER0);
+ status = inb(I8254_PORT_COUNTER0);
+ timer = inb(I8254_PORT_COUNTER0);
+ timer |= inb(I8254_PORT_COUNTER0) << 8;
+ } while (status & I8254_STATUS_NOTREADY);
+
+ return timer;
+}
+
+static unsigned long get_random_long(void)
+{
+ unsigned long random;
+
+ if (has_cpuflag(X86_FEATURE_RDRAND)) {
+ debug_putstr("KASLR using RDRAND...\n");
+ if (rdrand(&random))
+ return random;
+ }
+
+ if (has_cpuflag(X86_FEATURE_TSC)) {
+ uint32_t raw;
+
+ debug_putstr("KASLR using RDTSC...\n");
+ rdtscl(raw);
+
+ /* Only use the low bits of rdtsc. */
+ random = raw & 0xffff;
+ } else {
+ debug_putstr("KASLR using i8254...\n");
+ random = i8254();
+ }
+
+ /* Extend timer bits poorly... */
+ random |= (random << 16);
+#ifdef CONFIG_X86_64
+ random |= (random << 32) | (random << 48);
+#endif
+ return random;
+}
static unsigned long find_minimum_location(unsigned long input,
unsigned long input_size,
@@ -55,6 +121,7 @@ unsigned char *choose_kernel_location(unsigned char *input,
unsigned long output_size)
{
unsigned long choice = (unsigned long)output;
+ unsigned long random;
if (cmdline_find_option_bool("nokaslr")) {
debug_putstr("KASLR disabled...\n");
@@ -64,8 +131,22 @@ unsigned char *choose_kernel_location(unsigned char *input,
choice = find_minimum_location((unsigned long)input, input_size,
(unsigned long)output, output_size);
- /* XXX: choose random location. */
+ /* XXX: Find an appropriate E820 hole, instead of adding offset. */
+ random = get_random_long();
+
+ /* Clip off top of the range. */
+ random &= (CONFIG_RANDOMIZE_BASE_MAX_OFFSET - 1);
+ while (random + output_size > CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
+ random >>= 1;
+
+ /* Clip off bottom of range. */
+ random &= ~(choice - 1);
+
+ /* Always enforce the minimum. */
+ if (random < choice)
+ goto out;
+ choice = random;
out:
return (unsigned char *)choice;
}
diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 42f71bb..24e3e56 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -60,6 +60,8 @@ unsigned char *choose_kernel_location(unsigned char *input,
unsigned long input_size,
unsigned char *output,
unsigned long output_size);
+/* cpuflags.c */
+bool has_cpuflag(int flag);
#else
static inline
unsigned char *choose_kernel_location(unsigned char *input,
--
1.7.9.5
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