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