Re: [PATCH v3] x86, kaslr: mix entropy sources together as needed

[H. Peter Anvin]

You probably want to rotate by an odd number of bits... the point is to spread out any pattern in the bytes.

Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>Depending on availability, mix the RDRAND and RDTSC entropy together
>with
>XOR. Only when neither is available should the i8254 be used. Update
>the Kconfig documentation to reflect this. Additionally, since bits
>used for entropy is masked elsewhere, drop the needless masking in
>the get_random_long(). Similarly, use the entire TSC, not just the low
>32 bits.
>
>Finally, to improve the starting entropy, do a simple hashing of a
>build-time versions string and the boot-time boot_params structure for
>some additional level of unpredictability.
>
>Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>---
>v3:
> - do not limit ourself to the low 32 bits of TSC; Ingo Molnar.
>v2:
> - added build-time string to add to starting entropy; Ingo Molnar.
>---
> arch/x86/Kconfig                |   14 +++++---
>arch/x86/boot/compressed/aslr.c |   73
>++++++++++++++++++++++++++++++---------
> 2 files changed, 65 insertions(+), 22 deletions(-)
>
>diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>index ee3b38363063..119455802d57 100644
>--- a/arch/x86/Kconfig
>+++ b/arch/x86/Kconfig
>@@ -1736,13 +1736,17 @@ config RANDOMIZE_BASE
> 	   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.
>+	   Entropy is generated using the RDRAND instruction if it is
>+	   supported. If RDTSC is supported, it is used as well. If
>+	   neither RDRAND nor RDTSC are supported, then randomness is
>+	   read from the i8254 timer.
> 
> 	   The kernel will be offset by up to RANDOMIZE_BASE_MAX_OFFSET,
>-	   and aligned according to PHYSICAL_ALIGN.
>+	   and aligned according to PHYSICAL_ALIGN. Since the kernel is
>+	   built using 2GiB addressing, and PHYSICAL_ALGIN must be at a
>+	   minimum of 2MiB, only 10 bits of entropy is theoretically
>+	   possible. At best, due to page table layouts, 64-bit can use
>+	   9 bits of entropy and 32-bit uses 8 bits.
> 
> config RANDOMIZE_BASE_MAX_OFFSET
> 	hex "Maximum ASLR offset allowed"
>diff --git a/arch/x86/boot/compressed/aslr.c
>b/arch/x86/boot/compressed/aslr.c
>index 05957986d123..0981227bc3f0 100644
>--- a/arch/x86/boot/compressed/aslr.c
>+++ b/arch/x86/boot/compressed/aslr.c
>@@ -5,6 +5,17 @@
> #include <asm/archrandom.h>
> #include <asm/e820.h>
> 
>+#include <generated/compile.h>
>+#include <linux/module.h>
>+#include <linux/uts.h>
>+#include <linux/utsname.h>
>+#include <generated/utsrelease.h>
>+#include <linux/version.h>
>+
>+/* Simplified build-specific string for starting entropy. */
>+static const char *build_str = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
>+		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
>+
> #define I8254_PORT_CONTROL	0x43
> #define I8254_PORT_COUNTER0	0x40
> #define I8254_CMD_READBACK	0xC0
>@@ -25,34 +36,62 @@ static inline u16 i8254(void)
> 	return timer;
> }
> 
>+static unsigned long rotate_xor(unsigned long hash, const void *area,
>+				size_t size)
>+{
>+	size_t i;
>+	unsigned long *ptr = (unsigned long *)area;
>+
>+	for (i = 0; i < size / sizeof(hash); i++) {
>+		/* Rotate and XOR */
>+		hash = (hash << ((sizeof(hash) - 1) * 8)) | (hash >> 8);
>+		hash ^= ptr[i];
>+	}
>+
>+	return hash;
>+}
>+
>+/* Attempt to create a simple but unpredictable starting entropy. */
>+static unsigned long get_random_boot(void)
>+{
>+	unsigned long hash = 0;
>+
>+	hash = rotate_xor(hash, build_str, sizeof(build_str));
>+	hash = rotate_xor(hash, real_mode, sizeof(*real_mode));
>+
>+	return hash;
>+}
>+
> static unsigned long get_random_long(void)
> {
>-	unsigned long random;
>+	unsigned long raw, random = get_random_boot();
>+	bool use_i8254 = true;
>+
>+	debug_putstr("KASLR using");
> 
> 	if (has_cpuflag(X86_FEATURE_RDRAND)) {
>-		debug_putstr("KASLR using RDRAND...\n");
>-		if (rdrand_long(&random))
>-			return random;
>+		debug_putstr(" RDRAND");
>+		if (rdrand_long(&raw)) {
>+			random ^= raw;
>+			use_i8254 = false;
>+		}
> 	}
> 
> 	if (has_cpuflag(X86_FEATURE_TSC)) {
>-		uint32_t raw;
>+		debug_putstr(" RDTSC");
>+		rdtscll(raw);
> 
>-		debug_putstr("KASLR using RDTSC...\n");
>-		rdtscl(raw);
>+		random ^= raw;
>+		use_i8254 = false;
>+	}
> 
>-		/* Only use the low bits of rdtsc. */
>-		random = raw & 0xffff;
>-	} else {
>-		debug_putstr("KASLR using i8254...\n");
>-		random = i8254();
>+	if (use_i8254) {
>+		debug_putstr(" i8254");
>+		random ^= i8254();
> 	}
> 
>-	/* Extend timer bits poorly... */
>-	random |= (random << 16);
>-#ifdef CONFIG_X86_64
>-	random |= (random << 32);
>-#endif
>+	debug_putstr("...\n");
>+
> 	return random;
> }
> 

-- 
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