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