arch/x86/boot/compressed/kaslr.c

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