xref: /openbmc/linux/arch/x86/mm/kaslr.c (revision bdeeed09)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * This file implements KASLR memory randomization for x86_64. It randomizes
4  * the virtual address space of kernel memory regions (physical memory
5  * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates
6  * exploits relying on predictable kernel addresses.
7  *
8  * Entropy is generated using the KASLR early boot functions now shared in
9  * the lib directory (originally written by Kees Cook). Randomization is
10  * done on PGD & P4D/PUD page table levels to increase possible addresses.
11  * The physical memory mapping code was adapted to support P4D/PUD level
12  * virtual addresses. This implementation on the best configuration provides
13  * 30,000 possible virtual addresses in average for each memory region.
14  * An additional low memory page is used to ensure each CPU can start with
15  * a PGD aligned virtual address (for realmode).
16  *
17  * The order of each memory region is not changed. The feature looks at
18  * the available space for the regions based on different configuration
19  * options and randomizes the base and space between each. The size of the
20  * physical memory mapping is the available physical memory.
21  */
22 
23 #include <linux/kernel.h>
24 #include <linux/init.h>
25 #include <linux/random.h>
26 
27 #include <asm/pgalloc.h>
28 #include <asm/pgtable.h>
29 #include <asm/setup.h>
30 #include <asm/kaslr.h>
31 
32 #include "mm_internal.h"
33 
34 #define TB_SHIFT 40
35 
36 /*
37  * The end address could depend on more configuration options to make the
38  * highest amount of space for randomization available, but that's too hard
39  * to keep straight and caused issues already.
40  */
41 static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
42 
43 /*
44  * Memory regions randomized by KASLR (except modules that use a separate logic
45  * earlier during boot). The list is ordered based on virtual addresses. This
46  * order is kept after randomization.
47  */
48 static __initdata struct kaslr_memory_region {
49 	unsigned long *base;
50 	unsigned long size_tb;
51 } kaslr_regions[] = {
52 	{ &page_offset_base, 0 },
53 	{ &vmalloc_base, 0 },
54 	{ &vmemmap_base, 1 },
55 };
56 
57 /* Get size in bytes used by the memory region */
58 static inline unsigned long get_padding(struct kaslr_memory_region *region)
59 {
60 	return (region->size_tb << TB_SHIFT);
61 }
62 
63 /*
64  * Apply no randomization if KASLR was disabled at boot or if KASAN
65  * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
66  */
67 static inline bool kaslr_memory_enabled(void)
68 {
69 	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
70 }
71 
72 /* Initialize base and padding for each memory region randomized with KASLR */
73 void __init kernel_randomize_memory(void)
74 {
75 	size_t i;
76 	unsigned long vaddr_start, vaddr;
77 	unsigned long rand, memory_tb;
78 	struct rnd_state rand_state;
79 	unsigned long remain_entropy;
80 
81 	vaddr_start = pgtable_l5_enabled ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
82 	vaddr = vaddr_start;
83 
84 	/*
85 	 * These BUILD_BUG_ON checks ensure the memory layout is consistent
86 	 * with the vaddr_start/vaddr_end variables. These checks are very
87 	 * limited....
88 	 */
89 	BUILD_BUG_ON(vaddr_start >= vaddr_end);
90 	BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
91 	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
92 
93 	if (!kaslr_memory_enabled())
94 		return;
95 
96 	kaslr_regions[0].size_tb = 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT);
97 	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
98 
99 	/*
100 	 * Update Physical memory mapping to available and
101 	 * add padding if needed (especially for memory hotplug support).
102 	 */
103 	BUG_ON(kaslr_regions[0].base != &page_offset_base);
104 	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
105 		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
106 
107 	/* Adapt phyiscal memory region size based on available memory */
108 	if (memory_tb < kaslr_regions[0].size_tb)
109 		kaslr_regions[0].size_tb = memory_tb;
110 
111 	/* Calculate entropy available between regions */
112 	remain_entropy = vaddr_end - vaddr_start;
113 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
114 		remain_entropy -= get_padding(&kaslr_regions[i]);
115 
116 	prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
117 
118 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
119 		unsigned long entropy;
120 
121 		/*
122 		 * Select a random virtual address using the extra entropy
123 		 * available.
124 		 */
125 		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
126 		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
127 		if (pgtable_l5_enabled)
128 			entropy = (rand % (entropy + 1)) & P4D_MASK;
129 		else
130 			entropy = (rand % (entropy + 1)) & PUD_MASK;
131 		vaddr += entropy;
132 		*kaslr_regions[i].base = vaddr;
133 
134 		/*
135 		 * Jump the region and add a minimum padding based on
136 		 * randomization alignment.
137 		 */
138 		vaddr += get_padding(&kaslr_regions[i]);
139 		if (pgtable_l5_enabled)
140 			vaddr = round_up(vaddr + 1, P4D_SIZE);
141 		else
142 			vaddr = round_up(vaddr + 1, PUD_SIZE);
143 		remain_entropy -= entropy;
144 	}
145 }
146 
147 static void __meminit init_trampoline_pud(void)
148 {
149 	unsigned long paddr, paddr_next;
150 	pgd_t *pgd;
151 	pud_t *pud_page, *pud_page_tramp;
152 	int i;
153 
154 	pud_page_tramp = alloc_low_page();
155 
156 	paddr = 0;
157 	pgd = pgd_offset_k((unsigned long)__va(paddr));
158 	pud_page = (pud_t *) pgd_page_vaddr(*pgd);
159 
160 	for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
161 		pud_t *pud, *pud_tramp;
162 		unsigned long vaddr = (unsigned long)__va(paddr);
163 
164 		pud_tramp = pud_page_tramp + pud_index(paddr);
165 		pud = pud_page + pud_index(vaddr);
166 		paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
167 
168 		*pud_tramp = *pud;
169 	}
170 
171 	set_pgd(&trampoline_pgd_entry,
172 		__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
173 }
174 
175 static void __meminit init_trampoline_p4d(void)
176 {
177 	unsigned long paddr, paddr_next;
178 	pgd_t *pgd;
179 	p4d_t *p4d_page, *p4d_page_tramp;
180 	int i;
181 
182 	p4d_page_tramp = alloc_low_page();
183 
184 	paddr = 0;
185 	pgd = pgd_offset_k((unsigned long)__va(paddr));
186 	p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);
187 
188 	for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
189 		p4d_t *p4d, *p4d_tramp;
190 		unsigned long vaddr = (unsigned long)__va(paddr);
191 
192 		p4d_tramp = p4d_page_tramp + p4d_index(paddr);
193 		p4d = p4d_page + p4d_index(vaddr);
194 		paddr_next = (paddr & P4D_MASK) + P4D_SIZE;
195 
196 		*p4d_tramp = *p4d;
197 	}
198 
199 	set_pgd(&trampoline_pgd_entry,
200 		__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
201 }
202 
203 /*
204  * Create PGD aligned trampoline table to allow real mode initialization
205  * of additional CPUs. Consume only 1 low memory page.
206  */
207 void __meminit init_trampoline(void)
208 {
209 
210 	if (!kaslr_memory_enabled()) {
211 		init_trampoline_default();
212 		return;
213 	}
214 
215 	if (pgtable_l5_enabled)
216 		init_trampoline_p4d();
217 	else
218 		init_trampoline_pud();
219 }
220