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

[Kees Cook]

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@xxxxxxxxxxxx>
---
v2:
 - use rdtscl from msr.h, thanks to Mathias Krause.
---
 arch/x86/Kconfig                |   29 +++++++++++++++--
 arch/x86/boot/compressed/aslr.c |   67 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 92 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..4647e3f 100644
--- a/arch/x86/boot/compressed/aslr.c
+++ b/arch/x86/boot/compressed/aslr.c
@@ -1,19 +1,82 @@
 #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;
+}
+
+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");
+		rdtscl(raw);
+
+		/* 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

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