arch/x86/boot/compressed/aslr.c

   1 #include "misc.h"
   2
   3 #include <asm/msr.h>
   4 #include <asm/archrandom.h>
   5 #include <asm/e820.h>
   6
   7 #include <generated/compile.h>
   8 #include <linux/module.h>
   9 #include <linux/uts.h>
  10 #include <linux/utsname.h>
  11 #include <generated/utsrelease.h>
  12
  13 /* Simplified build-specific string for starting entropy. */
  14 static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
  15                 LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
  16
  17 struct kaslr_setup_data {
  18         __u64 next;
  19         __u32 type;
  20         __u32 len;
  21         __u8 data[1];
  22 } kaslr_setup_data;
  23
  24 #define I8254_PORT_CONTROL      0x43
  25 #define I8254_PORT_COUNTER0     0x40
  26 #define I8254_CMD_READBACK      0xC0
  27 #define I8254_SELECT_COUNTER0   0x02
  28 #define I8254_STATUS_NOTREADY   0x40
  29 static inline u16 i8254(void)
  30 {
  31         u16 status, timer;
  32
  33         do {
  34                 outb(I8254_PORT_CONTROL,
  35                      I8254_CMD_READBACK | I8254_SELECT_COUNTER0);
  36                 status = inb(I8254_PORT_COUNTER0);
  37                 timer  = inb(I8254_PORT_COUNTER0);
  38                 timer |= inb(I8254_PORT_COUNTER0) << 8;
  39         } while (status & I8254_STATUS_NOTREADY);
  40
  41         return timer;
  42 }
  43
  44 static unsigned long rotate_xor(unsigned long hash, const void *area,
  45                                 size_t size)
  46 {
  47         size_t i;
  48         unsigned long *ptr = (unsigned long *)area;
  49
  50         for (i = 0; i < size / sizeof(hash); i++) {
  51                 /* Rotate by odd number of bits and XOR. */
  52                 hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
  53                 hash ^= ptr[i];
  54         }
  55
  56         return hash;
  57 }
  58
  59 /* Attempt to create a simple but unpredictable starting entropy. */
  60 static unsigned long get_random_boot(void)
  61 {
  62         unsigned long hash = 0;
  63
  64         hash = rotate_xor(hash, build_str, sizeof(build_str));
  65         hash = rotate_xor(hash, real_mode, sizeof(*real_mode));
  66
  67         return hash;
  68 }
  69
  70 static unsigned long get_random_long(void)
  71 {
  72 #ifdef CONFIG_X86_64
  73         const unsigned long mix_const = 0x5d6008cbf3848dd3UL;
  74 #else
  75         const unsigned long mix_const = 0x3f39e593UL;
  76 #endif
  77         unsigned long raw, random = get_random_boot();
  78         bool use_i8254 = true;
  79
  80         debug_putstr("KASLR using");
  81
  82         if (has_cpuflag(X86_FEATURE_RDRAND)) {
  83                 debug_putstr(" RDRAND");
  84                 if (rdrand_long(&raw)) {
  85                         random ^= raw;
  86                         use_i8254 = false;
  87                 }
  88         }
  89
  90         if (has_cpuflag(X86_FEATURE_TSC)) {
  91                 debug_putstr(" RDTSC");
  92                 rdtscll(raw);
  93
  94                 random ^= raw;
  95                 use_i8254 = false;
  96         }
  97
  98         if (use_i8254) {
  99                 debug_putstr(" i8254");
 100                 random ^= i8254();
 101         }
 102
 103         /* Circular multiply for better bit diffusion */
 104         asm("mul %3"
 105             : "=a" (random), "=d" (raw)
 106             : "a" (random), "rm" (mix_const));
 107         random += raw;
 108
 109         debug_putstr("...\n");
 110
 111         return random;
 112 }
 113
 114 struct mem_vector {
 115         unsigned long start;
 116         unsigned long size;
 117 };
 118
 119 #define MEM_AVOID_MAX 5
 120 static struct mem_vector mem_avoid[MEM_AVOID_MAX];
 121
 122 static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
 123 {
 124         /* Item at least partially before region. */
 125         if (item->start < region->start)
 126                 return false;
 127         /* Item at least partially after region. */
 128         if (item->start + item->size > region->start + region->size)
 129                 return false;
 130         return true;
 131 }
 132
 133 static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
 134 {
 135         /* Item one is entirely before item two. */
 136         if (one->start + one->size <= two->start)
 137                 return false;
 138         /* Item one is entirely after item two. */
 139         if (one->start >= two->start + two->size)
 140                 return false;
 141         return true;
 142 }
 143
 144 static void mem_avoid_init(unsigned long input, unsigned long input_size,
 145                            unsigned long output, unsigned long output_size)
 146 {
 147         u64 initrd_start, initrd_size;
 148         u64 cmd_line, cmd_line_size;
 149         unsigned long unsafe, unsafe_len;
 150         char *ptr;
 151
 152         /*
 153          * Avoid the region that is unsafe to overlap during
 154          * decompression (see calculations at top of misc.c).
 155          */
 156         unsafe_len = (output_size >> 12) + 32768 + 18;
 157         unsafe = (unsigned long)input + input_size - unsafe_len;
 158         mem_avoid[0].start = unsafe;
 159         mem_avoid[0].size = unsafe_len;
 160
 161         /* Avoid initrd. */
 162         initrd_start  = (u64)real_mode->ext_ramdisk_image << 32;
 163         initrd_start |= real_mode->hdr.ramdisk_image;
 164         initrd_size  = (u64)real_mode->ext_ramdisk_size << 32;
 165         initrd_size |= real_mode->hdr.ramdisk_size;
 166         mem_avoid[1].start = initrd_start;
 167         mem_avoid[1].size = initrd_size;
 168
 169         /* Avoid kernel command line. */
 170         cmd_line  = (u64)real_mode->ext_cmd_line_ptr << 32;
 171         cmd_line |= real_mode->hdr.cmd_line_ptr;
 172         /* Calculate size of cmd_line. */
 173         ptr = (char *)(unsigned long)cmd_line;
 174         for (cmd_line_size = 0; ptr[cmd_line_size++]; )
 175                 ;
 176         mem_avoid[2].start = cmd_line;
 177         mem_avoid[2].size = cmd_line_size;
 178
 179         /* Avoid heap memory. */
 180         mem_avoid[3].start = (unsigned long)free_mem_ptr;
 181         mem_avoid[3].size = BOOT_HEAP_SIZE;
 182
 183         /* Avoid stack memory. */
 184         mem_avoid[4].start = (unsigned long)free_mem_end_ptr;
 185         mem_avoid[4].size = BOOT_STACK_SIZE;
 186 }
 187
 188 /* Does this memory vector overlap a known avoided area? */
 189 static bool mem_avoid_overlap(struct mem_vector *img)
 190 {
 191         int i;
 192         struct setup_data *ptr;
 193
 194         for (i = 0; i < MEM_AVOID_MAX; i++) {
 195                 if (mem_overlaps(img, &mem_avoid[i]))
 196                         return true;
 197         }
 198
 199         /* Avoid all entries in the setup_data linked list. */
 200         ptr = (struct setup_data *)(unsigned long)real_mode->hdr.setup_data;
 201         while (ptr) {
 202                 struct mem_vector avoid;
 203
 204                 avoid.start = (unsigned long)ptr;
 205                 avoid.size = sizeof(*ptr) + ptr->len;
 206
 207                 if (mem_overlaps(img, &avoid))
 208                         return true;
 209
 210                 ptr = (struct setup_data *)(unsigned long)ptr->next;
 211         }
 212
 213         return false;
 214 }
 215
 216 static unsigned long slots[CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
 217                            CONFIG_PHYSICAL_ALIGN];
 218 static unsigned long slot_max;
 219
 220 static void slots_append(unsigned long addr)
 221 {
 222         /* Overflowing the slots list should be impossible. */
 223         if (slot_max >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
 224                         CONFIG_PHYSICAL_ALIGN)
 225                 return;
 226
 227         slots[slot_max++] = addr;
 228 }
 229
 230 static unsigned long slots_fetch_random(void)
 231 {
 232         /* Handle case of no slots stored. */
 233         if (slot_max == 0)
 234                 return 0;
 235
 236         return slots[get_random_long() % slot_max];
 237 }
 238
 239 static void process_e820_entry(struct e820entry *entry,
 240                                unsigned long minimum,
 241                                unsigned long image_size)
 242 {
 243         struct mem_vector region, img;
 244
 245         /* Skip non-RAM entries. */
 246         if (entry->type != E820_RAM)
 247                 return;
 248
 249         /* Ignore entries entirely above our maximum. */
 250         if (entry->addr >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
 251                 return;
 252
 253         /* Ignore entries entirely below our minimum. */
 254         if (entry->addr + entry->size < minimum)
 255                 return;
 256
 257         region.start = entry->addr;
 258         region.size = entry->size;
 259
 260         /* Potentially raise address to minimum location. */
 261         if (region.start < minimum)
 262                 region.start = minimum;
 263
 264         /* Potentially raise address to meet alignment requirements. */
 265         region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
 266
 267         /* Did we raise the address above the bounds of this e820 region? */
 268         if (region.start > entry->addr + entry->size)
 269                 return;
 270
 271         /* Reduce size by any delta from the original address. */
 272         region.size -= region.start - entry->addr;
 273
 274         /* Reduce maximum size to fit end of image within maximum limit. */
 275         if (region.start + region.size > CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
 276                 region.size = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - region.start;
 277
 278         /* Walk each aligned slot and check for avoided areas. */
 279         for (img.start = region.start, img.size = image_size ;
 280              mem_contains(&region, &img) ;
 281              img.start += CONFIG_PHYSICAL_ALIGN) {
 282                 if (mem_avoid_overlap(&img))
 283                         continue;
 284                 slots_append(img.start);
 285         }
 286 }
 287
 288 static unsigned long find_random_addr(unsigned long minimum,
 289                                       unsigned long size)
 290 {
 291         int i;
 292         unsigned long addr;
 293
 294         /* Make sure minimum is aligned. */
 295         minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
 296
 297         /* Verify potential e820 positions, appending to slots list. */
 298         for (i = 0; i < real_mode->e820_entries; i++) {
 299                 process_e820_entry(&real_mode->e820_map[i], minimum, size);
 300         }
 301
 302         return slots_fetch_random();
 303 }
 304
 305 static void add_kaslr_setup_data(struct boot_params *params, __u8 enabled)
 306 {
 307         struct setup_data *data;
 308
 309         kaslr_setup_data.type = SETUP_KASLR;
 310         kaslr_setup_data.len = 1;
 311         kaslr_setup_data.next = 0;
 312         kaslr_setup_data.data[0] = enabled;
 313
 314         data = (struct setup_data *)(unsigned long)params->hdr.setup_data;
 315
 316         while (data && data->next)
 317                 data = (struct setup_data *)(unsigned long)data->next;
 318
 319         if (data)
 320                 data->next = (unsigned long)&kaslr_setup_data;
 321         else
 322                 params->hdr.setup_data = (unsigned long)&kaslr_setup_data;
 323
 324 }
 325
 326 unsigned char *choose_kernel_location(struct boot_params *params,
 327                                       unsigned char *input,
 328                                       unsigned long input_size,
 329                                       unsigned char *output,
 330                                       unsigned long output_size)
 331 {
 332         unsigned long choice = (unsigned long)output;
 333         unsigned long random;
 334
 335 #ifdef CONFIG_HIBERNATION
 336         if (!cmdline_find_option_bool("kaslr")) {
 337                 debug_putstr("KASLR disabled by default...\n");
 338                 add_kaslr_setup_data(params, 0);
 339                 goto out;
 340         }
 341 #else
 342         if (cmdline_find_option_bool("nokaslr")) {
 343                 debug_putstr("KASLR disabled by cmdline...\n");
 344                 add_kaslr_setup_data(params, 0);
 345                 goto out;
 346         }
 347 #endif
 348         add_kaslr_setup_data(params, 1);
 349
 350         /* Record the various known unsafe memory ranges. */
 351         mem_avoid_init((unsigned long)input, input_size,
 352                        (unsigned long)output, output_size);
 353
 354         /* Walk e820 and find a random address. */
 355         random = find_random_addr(choice, output_size);
 356         if (!random) {
 357                 debug_putstr("KASLR could not find suitable E820 region...\n");
 358                 goto out;
 359         }
 360
 361         /* Always enforce the minimum. */
 362         if (random < choice)
 363                 goto out;
 364
 365         choice = random;
 366 out:
 367         return (unsigned char *)choice;
 368 }