1 // SPDX-License-Identifier: GPL-2.0
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.
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).
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.
23 #include <linux/kernel.h>
24 #include <linux/init.h>
25 #include <linux/random.h>
27 #include <asm/pgalloc.h>
28 #include <asm/pgtable.h>
29 #include <asm/setup.h>
30 #include <asm/kaslr.h>
32 #include "mm_internal.h"
37 * Virtual address start and end range for randomization. The end changes base
38 * on configuration to have the highest amount of space for randomization.
39 * It increases the possible random position for each randomized region.
41 * You need to add an if/def entry if you introduce a new memory region
42 * compatible with KASLR. Your entry must be in logical order with memory
43 * layout. For example, ESPFIX is before EFI because its virtual address is
44 * before. You also need to add a BUILD_BUG_ON() in kernel_randomize_memory() to
45 * ensure that this order is correct and won't be changed.
47 static const unsigned long vaddr_start = __PAGE_OFFSET_BASE;
49 #if defined(CONFIG_X86_ESPFIX64)
50 static const unsigned long vaddr_end = ESPFIX_BASE_ADDR;
51 #elif defined(CONFIG_EFI)
52 static const unsigned long vaddr_end = EFI_VA_END;
54 static const unsigned long vaddr_end = __START_KERNEL_map;
58 unsigned long page_offset_base = __PAGE_OFFSET_BASE;
59 EXPORT_SYMBOL(page_offset_base);
60 unsigned long vmalloc_base = __VMALLOC_BASE;
61 EXPORT_SYMBOL(vmalloc_base);
62 unsigned long vmemmap_base = __VMEMMAP_BASE;
63 EXPORT_SYMBOL(vmemmap_base);
66 * Memory regions randomized by KASLR (except modules that use a separate logic
67 * earlier during boot). The list is ordered based on virtual addresses. This
68 * order is kept after randomization.
70 static __initdata struct kaslr_memory_region {
72 unsigned long size_tb;
74 { &page_offset_base, 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT) /* Maximum */ },
75 { &vmalloc_base, VMALLOC_SIZE_TB },
79 /* Get size in bytes used by the memory region */
80 static inline unsigned long get_padding(struct kaslr_memory_region *region)
82 return (region->size_tb << TB_SHIFT);
86 * Apply no randomization if KASLR was disabled at boot or if KASAN
87 * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
89 static inline bool kaslr_memory_enabled(void)
91 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
94 /* Initialize base and padding for each memory region randomized with KASLR */
95 void __init kernel_randomize_memory(void)
98 unsigned long vaddr = vaddr_start;
99 unsigned long rand, memory_tb;
100 struct rnd_state rand_state;
101 unsigned long remain_entropy;
104 * All these BUILD_BUG_ON checks ensures the memory layout is
105 * consistent with the vaddr_start/vaddr_end variables.
107 BUILD_BUG_ON(vaddr_start >= vaddr_end);
108 BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_ESPFIX64) &&
109 vaddr_end >= EFI_VA_END);
110 BUILD_BUG_ON((IS_ENABLED(CONFIG_X86_ESPFIX64) ||
111 IS_ENABLED(CONFIG_EFI)) &&
112 vaddr_end >= __START_KERNEL_map);
113 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
115 if (!kaslr_memory_enabled())
119 * Update Physical memory mapping to available and
120 * add padding if needed (especially for memory hotplug support).
122 BUG_ON(kaslr_regions[0].base != &page_offset_base);
123 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
124 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
126 /* Adapt phyiscal memory region size based on available memory */
127 if (memory_tb < kaslr_regions[0].size_tb)
128 kaslr_regions[0].size_tb = memory_tb;
130 /* Calculate entropy available between regions */
131 remain_entropy = vaddr_end - vaddr_start;
132 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
133 remain_entropy -= get_padding(&kaslr_regions[i]);
135 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
137 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
138 unsigned long entropy;
141 * Select a random virtual address using the extra entropy
144 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
145 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
146 if (IS_ENABLED(CONFIG_X86_5LEVEL))
147 entropy = (rand % (entropy + 1)) & P4D_MASK;
149 entropy = (rand % (entropy + 1)) & PUD_MASK;
151 *kaslr_regions[i].base = vaddr;
154 * Jump the region and add a minimum padding based on
155 * randomization alignment.
157 vaddr += get_padding(&kaslr_regions[i]);
158 if (IS_ENABLED(CONFIG_X86_5LEVEL))
159 vaddr = round_up(vaddr + 1, P4D_SIZE);
161 vaddr = round_up(vaddr + 1, PUD_SIZE);
162 remain_entropy -= entropy;
166 static void __meminit init_trampoline_pud(void)
168 unsigned long paddr, paddr_next;
170 pud_t *pud_page, *pud_page_tramp;
173 pud_page_tramp = alloc_low_page();
176 pgd = pgd_offset_k((unsigned long)__va(paddr));
177 pud_page = (pud_t *) pgd_page_vaddr(*pgd);
179 for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
180 pud_t *pud, *pud_tramp;
181 unsigned long vaddr = (unsigned long)__va(paddr);
183 pud_tramp = pud_page_tramp + pud_index(paddr);
184 pud = pud_page + pud_index(vaddr);
185 paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
190 set_pgd(&trampoline_pgd_entry,
191 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
194 static void __meminit init_trampoline_p4d(void)
196 unsigned long paddr, paddr_next;
198 p4d_t *p4d_page, *p4d_page_tramp;
201 p4d_page_tramp = alloc_low_page();
204 pgd = pgd_offset_k((unsigned long)__va(paddr));
205 p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);
207 for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
208 p4d_t *p4d, *p4d_tramp;
209 unsigned long vaddr = (unsigned long)__va(paddr);
211 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
212 p4d = p4d_page + p4d_index(vaddr);
213 paddr_next = (paddr & P4D_MASK) + P4D_SIZE;
218 set_pgd(&trampoline_pgd_entry,
219 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
223 * Create PGD aligned trampoline table to allow real mode initialization
224 * of additional CPUs. Consume only 1 low memory page.
226 void __meminit init_trampoline(void)
229 if (!kaslr_memory_enabled()) {
230 init_trampoline_default();
234 if (IS_ENABLED(CONFIG_X86_5LEVEL))
235 init_trampoline_p4d();
237 init_trampoline_pud();