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 * 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. 40 * 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. 46 */ 47 static const unsigned long vaddr_start = __PAGE_OFFSET_BASE; 48 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; 53 #else 54 static const unsigned long vaddr_end = __START_KERNEL_map; 55 #endif 56 57 /* Default values */ 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); 64 65 /* 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. 69 */ 70 static __initdata struct kaslr_memory_region { 71 unsigned long *base; 72 unsigned long size_tb; 73 } kaslr_regions[] = { 74 { &page_offset_base, 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT) /* Maximum */ }, 75 { &vmalloc_base, VMALLOC_SIZE_TB }, 76 { &vmemmap_base, 1 }, 77 }; 78 79 /* Get size in bytes used by the memory region */ 80 static inline unsigned long get_padding(struct kaslr_memory_region *region) 81 { 82 return (region->size_tb << TB_SHIFT); 83 } 84 85 /* 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. 88 */ 89 static inline bool kaslr_memory_enabled(void) 90 { 91 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN); 92 } 93 94 /* Initialize base and padding for each memory region randomized with KASLR */ 95 void __init kernel_randomize_memory(void) 96 { 97 size_t i; 98 unsigned long vaddr = vaddr_start; 99 unsigned long rand, memory_tb; 100 struct rnd_state rand_state; 101 unsigned long remain_entropy; 102 103 /* 104 * All these BUILD_BUG_ON checks ensures the memory layout is 105 * consistent with the vaddr_start/vaddr_end variables. 106 */ 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); 114 115 if (!kaslr_memory_enabled()) 116 return; 117 118 /* 119 * Update Physical memory mapping to available and 120 * add padding if needed (especially for memory hotplug support). 121 */ 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; 125 126 /* 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; 129 130 /* 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]); 134 135 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory")); 136 137 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { 138 unsigned long entropy; 139 140 /* 141 * Select a random virtual address using the extra entropy 142 * available. 143 */ 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; 148 else 149 entropy = (rand % (entropy + 1)) & PUD_MASK; 150 vaddr += entropy; 151 *kaslr_regions[i].base = vaddr; 152 153 /* 154 * Jump the region and add a minimum padding based on 155 * randomization alignment. 156 */ 157 vaddr += get_padding(&kaslr_regions[i]); 158 if (IS_ENABLED(CONFIG_X86_5LEVEL)) 159 vaddr = round_up(vaddr + 1, P4D_SIZE); 160 else 161 vaddr = round_up(vaddr + 1, PUD_SIZE); 162 remain_entropy -= entropy; 163 } 164 } 165 166 static void __meminit init_trampoline_pud(void) 167 { 168 unsigned long paddr, paddr_next; 169 pgd_t *pgd; 170 pud_t *pud_page, *pud_page_tramp; 171 int i; 172 173 pud_page_tramp = alloc_low_page(); 174 175 paddr = 0; 176 pgd = pgd_offset_k((unsigned long)__va(paddr)); 177 pud_page = (pud_t *) pgd_page_vaddr(*pgd); 178 179 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); 182 183 pud_tramp = pud_page_tramp + pud_index(paddr); 184 pud = pud_page + pud_index(vaddr); 185 paddr_next = (paddr & PUD_MASK) + PUD_SIZE; 186 187 *pud_tramp = *pud; 188 } 189 190 set_pgd(&trampoline_pgd_entry, 191 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); 192 } 193 194 static void __meminit init_trampoline_p4d(void) 195 { 196 unsigned long paddr, paddr_next; 197 pgd_t *pgd; 198 p4d_t *p4d_page, *p4d_page_tramp; 199 int i; 200 201 p4d_page_tramp = alloc_low_page(); 202 203 paddr = 0; 204 pgd = pgd_offset_k((unsigned long)__va(paddr)); 205 p4d_page = (p4d_t *) pgd_page_vaddr(*pgd); 206 207 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); 210 211 p4d_tramp = p4d_page_tramp + p4d_index(paddr); 212 p4d = p4d_page + p4d_index(vaddr); 213 paddr_next = (paddr & P4D_MASK) + P4D_SIZE; 214 215 *p4d_tramp = *p4d; 216 } 217 218 set_pgd(&trampoline_pgd_entry, 219 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp))); 220 } 221 222 /* 223 * Create PGD aligned trampoline table to allow real mode initialization 224 * of additional CPUs. Consume only 1 low memory page. 225 */ 226 void __meminit init_trampoline(void) 227 { 228 229 if (!kaslr_memory_enabled()) { 230 init_trampoline_default(); 231 return; 232 } 233 234 if (IS_ENABLED(CONFIG_X86_5LEVEL)) 235 init_trampoline_p4d(); 236 else 237 init_trampoline_pud(); 238 } 239