summaryrefslogtreecommitdiff
path: root/arch/x86/boot/compressed/aslr.c
blob: bb1376381985edb9f96e49c0a1b0269e56bd0f9e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
#include "misc.h"

#include <asm/msr.h>
#include <asm/archrandom.h>
#include <asm/e820.h>

#include <generated/compile.h>
#include <linux/module.h>
#include <linux/uts.h>
#include <linux/utsname.h>
#include <generated/utsrelease.h>

/* Simplified build-specific string for starting entropy. */
static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;

#define I8254_PORT_CONTROL	0x43
#define I8254_PORT_COUNTER0	0x40
#define I8254_CMD_READBACK	0xC0
#define I8254_SELECT_COUNTER0	0x02
#define I8254_STATUS_NOTREADY	0x40
static inline u16 i8254(void)
{
	u16 status, timer;

	do {
		outb(I8254_PORT_CONTROL,
		     I8254_CMD_READBACK | I8254_SELECT_COUNTER0);
		status = inb(I8254_PORT_COUNTER0);
		timer  = inb(I8254_PORT_COUNTER0);
		timer |= inb(I8254_PORT_COUNTER0) << 8;
	} while (status & I8254_STATUS_NOTREADY);

	return timer;
}

static unsigned long rotate_xor(unsigned long hash, const void *area,
				size_t size)
{
	size_t i;
	unsigned long *ptr = (unsigned long *)area;

	for (i = 0; i < size / sizeof(hash); i++) {
		/* Rotate by odd number of bits and XOR. */
		hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
		hash ^= ptr[i];
	}

	return hash;
}

/* Attempt to create a simple but unpredictable starting entropy. */
static unsigned long get_random_boot(void)
{
	unsigned long hash = 0;

	hash = rotate_xor(hash, build_str, sizeof(build_str));
	hash = rotate_xor(hash, real_mode, sizeof(*real_mode));

	return hash;
}

static unsigned long get_random_long(void)
{
#ifdef CONFIG_X86_64
	const unsigned long mix_const = 0x5d6008cbf3848dd3UL;
#else
	const unsigned long mix_const = 0x3f39e593UL;
#endif
	unsigned long raw, random = get_random_boot();
	bool use_i8254 = true;

	debug_putstr("KASLR using");

	if (has_cpuflag(X86_FEATURE_RDRAND)) {
		debug_putstr(" RDRAND");
		if (rdrand_long(&raw)) {
			random ^= raw;
			use_i8254 = false;
		}
	}

	if (has_cpuflag(X86_FEATURE_TSC)) {
		debug_putstr(" RDTSC");
		rdtscll(raw);

		random ^= raw;
		use_i8254 = false;
	}

	if (use_i8254) {
		debug_putstr(" i8254");
		random ^= i8254();
	}

	/* Circular multiply for better bit diffusion */
	asm("mul %3"
	    : "=a" (random), "=d" (raw)
	    : "a" (random), "rm" (mix_const));
	random += raw;

	debug_putstr("...\n");

	return random;
}

struct mem_vector {
	unsigned long start;
	unsigned long size;
};

#define MEM_AVOID_MAX 5
static struct mem_vector mem_avoid[MEM_AVOID_MAX];

static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
{
	/* Item at least partially before region. */
	if (item->start < region->start)
		return false;
	/* Item at least partially after region. */
	if (item->start + item->size > region->start + region->size)
		return false;
	return true;
}

static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
{
	/* Item one is entirely before item two. */
	if (one->start + one->size <= two->start)
		return false;
	/* Item one is entirely after item two. */
	if (one->start >= two->start + two->size)
		return false;
	return true;
}

static void mem_avoid_init(unsigned long input, unsigned long input_size,
			   unsigned long output, unsigned long output_size)
{
	u64 initrd_start, initrd_size;
	u64 cmd_line, cmd_line_size;
	unsigned long unsafe, unsafe_len;
	char *ptr;

	/*
	 * Avoid the region that is unsafe to overlap during
	 * decompression (see calculations at top of misc.c).
	 */
	unsafe_len = (output_size >> 12) + 32768 + 18;
	unsafe = (unsigned long)input + input_size - unsafe_len;
	mem_avoid[0].start = unsafe;
	mem_avoid[0].size = unsafe_len;

	/* Avoid initrd. */
	initrd_start  = (u64)real_mode->ext_ramdisk_image << 32;
	initrd_start |= real_mode->hdr.ramdisk_image;
	initrd_size  = (u64)real_mode->ext_ramdisk_size << 32;
	initrd_size |= real_mode->hdr.ramdisk_size;
	mem_avoid[1].start = initrd_start;
	mem_avoid[1].size = initrd_size;

	/* Avoid kernel command line. */
	cmd_line  = (u64)real_mode->ext_cmd_line_ptr << 32;
	cmd_line |= real_mode->hdr.cmd_line_ptr;
	/* Calculate size of cmd_line. */
	ptr = (char *)(unsigned long)cmd_line;
	for (cmd_line_size = 0; ptr[cmd_line_size++]; )
		;
	mem_avoid[2].start = cmd_line;
	mem_avoid[2].size = cmd_line_size;

	/* Avoid heap memory. */
	mem_avoid[3].start = (unsigned long)free_mem_ptr;
	mem_avoid[3].size = BOOT_HEAP_SIZE;

	/* Avoid stack memory. */
	mem_avoid[4].start = (unsigned long)free_mem_end_ptr;
	mem_avoid[4].size = BOOT_STACK_SIZE;
}

/* Does this memory vector overlap a known avoided area? */
static bool mem_avoid_overlap(struct mem_vector *img)
{
	int i;
	struct setup_data *ptr;

	for (i = 0; i < MEM_AVOID_MAX; i++) {
		if (mem_overlaps(img, &mem_avoid[i]))
			return true;
	}

	/* Avoid all entries in the setup_data linked list. */
	ptr = (struct setup_data *)(unsigned long)real_mode->hdr.setup_data;
	while (ptr) {
		struct mem_vector avoid;

		avoid.start = (unsigned long)ptr;
		avoid.size = sizeof(*ptr) + ptr->len;

		if (mem_overlaps(img, &avoid))
			return true;

		ptr = (struct setup_data *)(unsigned long)ptr->next;
	}

	return false;
}

static unsigned long slots[CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
			   CONFIG_PHYSICAL_ALIGN];
static unsigned long slot_max;

static void slots_append(unsigned long addr)
{
	/* Overflowing the slots list should be impossible. */
	if (slot_max >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET /
			CONFIG_PHYSICAL_ALIGN)
		return;

	slots[slot_max++] = addr;
}

static unsigned long slots_fetch_random(void)
{
	/* Handle case of no slots stored. */
	if (slot_max == 0)
		return 0;

	return slots[get_random_long() % slot_max];
}

static void process_e820_entry(struct e820entry *entry,
			       unsigned long minimum,
			       unsigned long image_size)
{
	struct mem_vector region, img;

	/* Skip non-RAM entries. */
	if (entry->type != E820_RAM)
		return;

	/* Ignore entries entirely above our maximum. */
	if (entry->addr >= CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
		return;

	/* Ignore entries entirely below our minimum. */
	if (entry->addr + entry->size < minimum)
		return;

	region.start = entry->addr;
	region.size = entry->size;

	/* Potentially raise address to minimum location. */
	if (region.start < minimum)
		region.start = minimum;

	/* Potentially raise address to meet alignment requirements. */
	region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);

	/* Did we raise the address above the bounds of this e820 region? */
	if (region.start > entry->addr + entry->size)
		return;

	/* Reduce size by any delta from the original address. */
	region.size -= region.start - entry->addr;

	/* Reduce maximum size to fit end of image within maximum limit. */
	if (region.start + region.size > CONFIG_RANDOMIZE_BASE_MAX_OFFSET)
		region.size = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - region.start;

	/* Walk each aligned slot and check for avoided areas. */
	for (img.start = region.start, img.size = image_size ;
	     mem_contains(&region, &img) ;
	     img.start += CONFIG_PHYSICAL_ALIGN) {
		if (mem_avoid_overlap(&img))
			continue;
		slots_append(img.start);
	}
}

static unsigned long find_random_addr(unsigned long minimum,
				      unsigned long size)
{
	int i;
	unsigned long addr;

	/* Make sure minimum is aligned. */
	minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);

	/* Verify potential e820 positions, appending to slots list. */
	for (i = 0; i < real_mode->e820_entries; i++) {
		process_e820_entry(&real_mode->e820_map[i], minimum, size);
	}

	return slots_fetch_random();
}

unsigned char *choose_kernel_location(unsigned char *input,
				      unsigned long input_size,
				      unsigned char *output,
				      unsigned long output_size)
{
	unsigned long choice = (unsigned long)output;
	unsigned long random;

#ifdef CONFIG_HIBERNATION
	if (!cmdline_find_option_bool("kaslr")) {
		debug_putstr("KASLR disabled by default...\n");
		goto out;
	}
#else
	if (cmdline_find_option_bool("nokaslr")) {
		debug_putstr("KASLR disabled by cmdline...\n");
		goto out;
	}
#endif

	/* Record the various known unsafe memory ranges. */
	mem_avoid_init((unsigned long)input, input_size,
		       (unsigned long)output, output_size);

	/* Walk e820 and find a random address. */
	random = find_random_addr(choice, output_size);
	if (!random) {
		debug_putstr("KASLR could not find suitable E820 region...\n");
		goto out;
	}

	/* Always enforce the minimum. */
	if (random < choice)
		goto out;

	choice = random;
out:
	return (unsigned char *)choice;
}