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Linux/arch/x86/boot/compressed/aslr.c

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  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                 rdtscll(raw);
 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 }
340 

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