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

[Andy Lutomirski]

On 11/11/2013 02:10 PM, H. Peter Anvin wrote:
> You probably want to rotate by an odd number of bits... the point is to spread out any pattern in the bytes.

Would it make more sense to just use a real cryptographic hash function?
 I don't think that any of the real kernel code is available this early,
but something like siphash can be implemented in well under 100 lines of
code.

(Admittedly, unless an attacker knows something about the dependency
between different word in the input, then all that's needed is for
rotate_xor to be injective in one input word if the others are held
fixed.  Yours has that property.)

It would probably be better if rotate_xor didn't truncate extra bytes
off the end, though.

--Andy

> 
> 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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