diff options
| -rw-r--r-- | arch/x86/Kconfig | 14 | ||||
| -rw-r--r-- | 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 51f439953d2..596cd9edeb9 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -1735,13 +1735,17 @@ 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 05957986d12..8746487fa91 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 by odd number of bits and XOR. */ + hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7); + 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; } |