arch/x86/boot/compressed/aslr.c

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