aboutsummaryrefslogtreecommitdiff
path: root/arch/x86/mm/kaslr.c
blob: aedebd2ebf1ead0ff2b1b17816ff80d9fce88b8f (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
// SPDX-License-Identifier: GPL-2.0
/*
 * This file implements KASLR memory randomization for x86_64. It randomizes
 * the virtual address space of kernel memory regions (physical memory
 * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates
 * exploits relying on predictable kernel addresses.
 *
 * Entropy is generated using the KASLR early boot functions now shared in
 * the lib directory (originally written by Kees Cook). Randomization is
 * done on PGD & P4D/PUD page table levels to increase possible addresses.
 * The physical memory mapping code was adapted to support P4D/PUD level
 * virtual addresses. This implementation on the best configuration provides
 * 30,000 possible virtual addresses in average for each memory region.
 * An additional low memory page is used to ensure each CPU can start with
 * a PGD aligned virtual address (for realmode).
 *
 * The order of each memory region is not changed. The feature looks at
 * the available space for the regions based on different configuration
 * options and randomizes the base and space between each. The size of the
 * physical memory mapping is the available physical memory.
 */

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/random.h>

#include <asm/pgalloc.h>
#include <asm/pgtable.h>
#include <asm/setup.h>
#include <asm/kaslr.h>

#include "mm_internal.h"

#define TB_SHIFT 40

/*
 * Virtual address start and end range for randomization.
 *
 * The end address could depend on more configuration options to make the
 * highest amount of space for randomization available, but that's too hard
 * to keep straight and caused issues already.
 */
static const unsigned long vaddr_start = __PAGE_OFFSET_BASE;
static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;

/* Default values */
unsigned long page_offset_base = __PAGE_OFFSET_BASE;
EXPORT_SYMBOL(page_offset_base);
unsigned long vmalloc_base = __VMALLOC_BASE;
EXPORT_SYMBOL(vmalloc_base);
unsigned long vmemmap_base = __VMEMMAP_BASE;
EXPORT_SYMBOL(vmemmap_base);

/*
 * Memory regions randomized by KASLR (except modules that use a separate logic
 * earlier during boot). The list is ordered based on virtual addresses. This
 * order is kept after randomization.
 */
static __initdata struct kaslr_memory_region {
	unsigned long *base;
	unsigned long size_tb;
} kaslr_regions[] = {
	{ &page_offset_base, 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT) /* Maximum */ },
	{ &vmalloc_base, VMALLOC_SIZE_TB },
	{ &vmemmap_base, 1 },
};

/* Get size in bytes used by the memory region */
static inline unsigned long get_padding(struct kaslr_memory_region *region)
{
	return (region->size_tb << TB_SHIFT);
}

/*
 * Apply no randomization if KASLR was disabled at boot or if KASAN
 * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
 */
static inline bool kaslr_memory_enabled(void)
{
	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
}

/* Initialize base and padding for each memory region randomized with KASLR */
void __init kernel_randomize_memory(void)
{
	size_t i;
	unsigned long vaddr = vaddr_start;
	unsigned long rand, memory_tb;
	struct rnd_state rand_state;
	unsigned long remain_entropy;

	/*
	 * These BUILD_BUG_ON checks ensure the memory layout is consistent
	 * with the vaddr_start/vaddr_end variables. These checks are very
	 * limited....
	 */
	BUILD_BUG_ON(vaddr_start >= vaddr_end);
	BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);

	if (!kaslr_memory_enabled())
		return;

	/*
	 * Update Physical memory mapping to available and
	 * add padding if needed (especially for memory hotplug support).
	 */
	BUG_ON(kaslr_regions[0].base != &page_offset_base);
	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;

	/* Adapt phyiscal memory region size based on available memory */
	if (memory_tb < kaslr_regions[0].size_tb)
		kaslr_regions[0].size_tb = memory_tb;

	/* Calculate entropy available between regions */
	remain_entropy = vaddr_end - vaddr_start;
	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
		remain_entropy -= get_padding(&kaslr_regions[i]);

	prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));

	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
		unsigned long entropy;

		/*
		 * Select a random virtual address using the extra entropy
		 * available.
		 */
		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
		if (IS_ENABLED(CONFIG_X86_5LEVEL))
			entropy = (rand % (entropy + 1)) & P4D_MASK;
		else
			entropy = (rand % (entropy + 1)) & PUD_MASK;
		vaddr += entropy;
		*kaslr_regions[i].base = vaddr;

		/*
		 * Jump the region and add a minimum padding based on
		 * randomization alignment.
		 */
		vaddr += get_padding(&kaslr_regions[i]);
		if (IS_ENABLED(CONFIG_X86_5LEVEL))
			vaddr = round_up(vaddr + 1, P4D_SIZE);
		else
			vaddr = round_up(vaddr + 1, PUD_SIZE);
		remain_entropy -= entropy;
	}
}

static void __meminit init_trampoline_pud(void)
{
	unsigned long paddr, paddr_next;
	pgd_t *pgd;
	pud_t *pud_page, *pud_page_tramp;
	int i;

	pud_page_tramp = alloc_low_page();

	paddr = 0;
	pgd = pgd_offset_k((unsigned long)__va(paddr));
	pud_page = (pud_t *) pgd_page_vaddr(*pgd);

	for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
		pud_t *pud, *pud_tramp;
		unsigned long vaddr = (unsigned long)__va(paddr);

		pud_tramp = pud_page_tramp + pud_index(paddr);
		pud = pud_page + pud_index(vaddr);
		paddr_next = (paddr & PUD_MASK) + PUD_SIZE;

		*pud_tramp = *pud;
	}

	set_pgd(&trampoline_pgd_entry,
		__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
}

static void __meminit init_trampoline_p4d(void)
{
	unsigned long paddr, paddr_next;
	pgd_t *pgd;
	p4d_t *p4d_page, *p4d_page_tramp;
	int i;

	p4d_page_tramp = alloc_low_page();

	paddr = 0;
	pgd = pgd_offset_k((unsigned long)__va(paddr));
	p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);

	for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
		p4d_t *p4d, *p4d_tramp;
		unsigned long vaddr = (unsigned long)__va(paddr);

		p4d_tramp = p4d_page_tramp + p4d_index(paddr);
		p4d = p4d_page + p4d_index(vaddr);
		paddr_next = (paddr & P4D_MASK) + P4D_SIZE;

		*p4d_tramp = *p4d;
	}

	set_pgd(&trampoline_pgd_entry,
		__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
}

/*
 * Create PGD aligned trampoline table to allow real mode initialization
 * of additional CPUs. Consume only 1 low memory page.
 */
void __meminit init_trampoline(void)
{

	if (!kaslr_memory_enabled()) {
		init_trampoline_default();
		return;
	}

	if (IS_ENABLED(CONFIG_X86_5LEVEL))
		init_trampoline_p4d();
	else
		init_trampoline_pud();
}