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.
16 #include <asm/archrandom.h>
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>
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;
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)
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);
49 static unsigned long rotate_xor(unsigned long hash, const void *area,
53 unsigned long *ptr = (unsigned long *)area;
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);
64 /* Attempt to create a simple but unpredictable starting entropy. */
65 static unsigned long get_random_boot(void)
67 unsigned long hash = 0;
69 hash = rotate_xor(hash, build_str, sizeof(build_str));
70 hash = rotate_xor(hash, boot_params, sizeof(*boot_params));
75 static unsigned long get_random_long(const char *purpose)
78 const unsigned long mix_const = 0x5d6008cbf3848dd3UL;
80 const unsigned long mix_const = 0x3f39e593UL;
82 unsigned long raw, random = get_random_boot();
83 bool use_i8254 = true;
85 debug_putstr(purpose);
86 debug_putstr(" KASLR using");
88 if (has_cpuflag(X86_FEATURE_RDRAND)) {
89 debug_putstr(" RDRAND");
90 if (rdrand_long(&raw)) {
96 if (has_cpuflag(X86_FEATURE_TSC)) {
97 debug_putstr(" RDTSC");
105 debug_putstr(" i8254");
109 /* Circular multiply for better bit diffusion */
111 : "=a" (random), "=d" (raw)
112 : "a" (random), "rm" (mix_const));
115 debug_putstr("...\n");
125 enum mem_avoid_index {
126 MEM_AVOID_ZO_RANGE = 0,
129 MEM_AVOID_BOOTPARAMS,
133 static struct mem_vector mem_avoid[MEM_AVOID_MAX];
135 static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
137 /* Item at least partially before region. */
138 if (item->start < region->start)
140 /* Item at least partially after region. */
141 if (item->start + item->size > region->start + region->size)
146 static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
148 /* Item one is entirely before item two. */
149 if (one->start + one->size <= two->start)
151 /* Item one is entirely after item two. */
152 if (one->start >= two->start + two->size)
158 * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
159 * The mem_avoid array is used to store the ranges that need to be avoided
160 * when KASLR searches for an appropriate random address. We must avoid any
161 * regions that are unsafe to overlap with during decompression, and other
162 * things like the initrd, cmdline and boot_params. This comment seeks to
163 * explain mem_avoid as clearly as possible since incorrect mem_avoid
164 * memory ranges lead to really hard to debug boot failures.
166 * The initrd, cmdline, and boot_params are trivial to identify for
167 * avoiding. They are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
168 * MEM_AVOID_BOOTPARAMS respectively below.
170 * What is not obvious how to avoid is the range of memory that is used
171 * during decompression (MEM_AVOID_ZO_RANGE below). This range must cover
172 * the compressed kernel (ZO) and its run space, which is used to extract
173 * the uncompressed kernel (VO) and relocs.
175 * ZO's full run size sits against the end of the decompression buffer, so
176 * we can calculate where text, data, bss, etc of ZO are positioned more
179 * For additional background, the decompression calculations can be found
180 * in header.S, and the memory diagram is based on the one found in misc.c.
182 * The following conditions are already enforced by the image layouts and
184 * - input + input_size >= output + output_size
185 * - kernel_total_size <= init_size
186 * - kernel_total_size <= output_size (see Note below)
187 * - output + init_size >= output + output_size
189 * (Note that kernel_total_size and output_size have no fundamental
190 * relationship, but output_size is passed to choose_random_location
191 * as a maximum of the two. The diagram is showing a case where
192 * kernel_total_size is larger than output_size, but this case is
193 * handled by bumping output_size.)
195 * The above conditions can be illustrated by a diagram:
197 * 0 output input input+input_size output+init_size
200 * |-----|--------|--------|--------------|-----------|--|-------------|
203 * output+init_size-ZO_INIT_SIZE output+output_size output+kernel_total_size
205 * [output, output+init_size) is the entire memory range used for
206 * extracting the compressed image.
208 * [output, output+kernel_total_size) is the range needed for the
209 * uncompressed kernel (VO) and its run size (bss, brk, etc).
211 * [output, output+output_size) is VO plus relocs (i.e. the entire
212 * uncompressed payload contained by ZO). This is the area of the buffer
213 * written to during decompression.
215 * [output+init_size-ZO_INIT_SIZE, output+init_size) is the worst-case
216 * range of the copied ZO and decompression code. (i.e. the range
217 * covered backwards of size ZO_INIT_SIZE, starting from output+init_size.)
219 * [input, input+input_size) is the original copied compressed image (ZO)
220 * (i.e. it does not include its run size). This range must be avoided
221 * because it contains the data used for decompression.
223 * [input+input_size, output+init_size) is [_text, _end) for ZO. This
224 * range includes ZO's heap and stack, and must be avoided since it
225 * performs the decompression.
227 * Since the above two ranges need to be avoided and they are adjacent,
228 * they can be merged, resulting in: [input, output+init_size) which
229 * becomes the MEM_AVOID_ZO_RANGE below.
231 static void mem_avoid_init(unsigned long input, unsigned long input_size,
232 unsigned long output)
234 unsigned long init_size = boot_params->hdr.init_size;
235 u64 initrd_start, initrd_size;
236 u64 cmd_line, cmd_line_size;
240 * Avoid the region that is unsafe to overlap during
243 mem_avoid[MEM_AVOID_ZO_RANGE].start = input;
244 mem_avoid[MEM_AVOID_ZO_RANGE].size = (output + init_size) - input;
245 add_identity_map(mem_avoid[MEM_AVOID_ZO_RANGE].start,
246 mem_avoid[MEM_AVOID_ZO_RANGE].size);
249 initrd_start = (u64)boot_params->ext_ramdisk_image << 32;
250 initrd_start |= boot_params->hdr.ramdisk_image;
251 initrd_size = (u64)boot_params->ext_ramdisk_size << 32;
252 initrd_size |= boot_params->hdr.ramdisk_size;
253 mem_avoid[MEM_AVOID_INITRD].start = initrd_start;
254 mem_avoid[MEM_AVOID_INITRD].size = initrd_size;
255 /* No need to set mapping for initrd, it will be handled in VO. */
257 /* Avoid kernel command line. */
258 cmd_line = (u64)boot_params->ext_cmd_line_ptr << 32;
259 cmd_line |= boot_params->hdr.cmd_line_ptr;
260 /* Calculate size of cmd_line. */
261 ptr = (char *)(unsigned long)cmd_line;
262 for (cmd_line_size = 0; ptr[cmd_line_size++]; )
264 mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
265 mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
266 add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start,
267 mem_avoid[MEM_AVOID_CMDLINE].size);
269 /* Avoid boot parameters. */
270 mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params;
271 mem_avoid[MEM_AVOID_BOOTPARAMS].size = sizeof(*boot_params);
272 add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start,
273 mem_avoid[MEM_AVOID_BOOTPARAMS].size);
275 /* We don't need to set a mapping for setup_data. */
277 #ifdef CONFIG_X86_VERBOSE_BOOTUP
278 /* Make sure video RAM can be used. */
279 add_identity_map(0, PMD_SIZE);
284 * Does this memory vector overlap a known avoided area? If so, record the
285 * overlap region with the lowest address.
287 static bool mem_avoid_overlap(struct mem_vector *img,
288 struct mem_vector *overlap)
291 struct setup_data *ptr;
292 unsigned long earliest = img->start + img->size;
293 bool is_overlapping = false;
295 for (i = 0; i < MEM_AVOID_MAX; i++) {
296 if (mem_overlaps(img, &mem_avoid[i]) &&
297 mem_avoid[i].start < earliest) {
298 *overlap = mem_avoid[i];
299 is_overlapping = true;
303 /* Avoid all entries in the setup_data linked list. */
304 ptr = (struct setup_data *)(unsigned long)boot_params->hdr.setup_data;
306 struct mem_vector avoid;
308 avoid.start = (unsigned long)ptr;
309 avoid.size = sizeof(*ptr) + ptr->len;
311 if (mem_overlaps(img, &avoid) && (avoid.start < earliest)) {
313 is_overlapping = true;
316 ptr = (struct setup_data *)(unsigned long)ptr->next;
319 return is_overlapping;
322 static unsigned long slots[KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN];
329 #define MAX_SLOT_AREA 100
331 static struct slot_area slot_areas[MAX_SLOT_AREA];
333 static unsigned long slot_max;
335 static unsigned long slot_area_index;
337 static void store_slot_info(struct mem_vector *region, unsigned long image_size)
339 struct slot_area slot_area;
341 if (slot_area_index == MAX_SLOT_AREA)
344 slot_area.addr = region->start;
345 slot_area.num = (region->size - image_size) /
346 CONFIG_PHYSICAL_ALIGN + 1;
348 if (slot_area.num > 0) {
349 slot_areas[slot_area_index++] = slot_area;
350 slot_max += slot_area.num;
354 static void slots_append(unsigned long addr)
356 /* Overflowing the slots list should be impossible. */
357 if (slot_max >= KERNEL_IMAGE_SIZE / CONFIG_PHYSICAL_ALIGN)
360 slots[slot_max++] = addr;
363 static unsigned long slots_fetch_random(void)
365 /* Handle case of no slots stored. */
369 return slots[get_random_long("Physical") % slot_max];
372 static void process_e820_entry(struct e820entry *entry,
373 unsigned long minimum,
374 unsigned long image_size)
376 struct mem_vector region, img, overlap;
378 /* Skip non-RAM entries. */
379 if (entry->type != E820_RAM)
382 /* Ignore entries entirely above our maximum. */
383 if (entry->addr >= KERNEL_IMAGE_SIZE)
386 /* Ignore entries entirely below our minimum. */
387 if (entry->addr + entry->size < minimum)
390 region.start = entry->addr;
391 region.size = entry->size;
393 /* Potentially raise address to minimum location. */
394 if (region.start < minimum)
395 region.start = minimum;
397 /* Potentially raise address to meet alignment requirements. */
398 region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
400 /* Did we raise the address above the bounds of this e820 region? */
401 if (region.start > entry->addr + entry->size)
404 /* Reduce size by any delta from the original address. */
405 region.size -= region.start - entry->addr;
407 /* Reduce maximum size to fit end of image within maximum limit. */
408 if (region.start + region.size > KERNEL_IMAGE_SIZE)
409 region.size = KERNEL_IMAGE_SIZE - region.start;
411 /* Walk each aligned slot and check for avoided areas. */
412 for (img.start = region.start, img.size = image_size ;
413 mem_contains(®ion, &img) ;
414 img.start += CONFIG_PHYSICAL_ALIGN) {
415 if (mem_avoid_overlap(&img, &overlap))
417 slots_append(img.start);
421 static unsigned long find_random_phys_addr(unsigned long minimum,
422 unsigned long image_size)
427 /* Make sure minimum is aligned. */
428 minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
430 /* Verify potential e820 positions, appending to slots list. */
431 for (i = 0; i < boot_params->e820_entries; i++) {
432 process_e820_entry(&boot_params->e820_map[i], minimum,
436 return slots_fetch_random();
439 static unsigned long find_random_virt_addr(unsigned long minimum,
440 unsigned long image_size)
442 unsigned long slots, random_addr;
444 /* Make sure minimum is aligned. */
445 minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
446 /* Align image_size for easy slot calculations. */
447 image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);
450 * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
451 * that can hold image_size within the range of minimum to
454 slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
455 CONFIG_PHYSICAL_ALIGN + 1;
457 random_addr = get_random_long("Virtual") % slots;
459 return random_addr * CONFIG_PHYSICAL_ALIGN + minimum;
463 * Since this function examines addresses much more numerically,
464 * it takes the input and output pointers as 'unsigned long'.
466 unsigned char *choose_random_location(unsigned long input,
467 unsigned long input_size,
468 unsigned long output,
469 unsigned long output_size)
471 unsigned long choice = output;
472 unsigned long random_addr;
474 #ifdef CONFIG_HIBERNATION
475 if (!cmdline_find_option_bool("kaslr")) {
476 warn("KASLR disabled: 'kaslr' not on cmdline (hibernation selected).");
480 if (cmdline_find_option_bool("nokaslr")) {
481 warn("KASLR disabled: 'nokaslr' on cmdline.");
486 boot_params->hdr.loadflags |= KASLR_FLAG;
488 /* Record the various known unsafe memory ranges. */
489 mem_avoid_init(input, input_size, output);
491 /* Walk e820 and find a random address. */
492 random_addr = find_random_phys_addr(output, output_size);
494 warn("KASLR disabled: could not find suitable E820 region!");
498 /* Always enforce the minimum. */
499 if (random_addr < choice)
502 choice = random_addr;
504 add_identity_map(choice, output_size);
506 /* This actually loads the identity pagetable on x86_64. */
507 finalize_identity_maps();
509 return (unsigned char *)choice;