1 /* 2 * This file implements KASLR memory randomization for x86_64. It randomizes 3 * the virtual address space of kernel memory regions (physical memory 4 * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates 5 * exploits relying on predictable kernel addresses. 6 * 7 * Entropy is generated using the KASLR early boot functions now shared in 8 * the lib directory (originally written by Kees Cook). Randomization is 9 * done on PGD & P4D/PUD page table levels to increase possible addresses. 10 * The physical memory mapping code was adapted to support P4D/PUD level 11 * virtual addresses. This implementation on the best configuration provides 12 * 30,000 possible virtual addresses in average for each memory region. 13 * An additional low memory page is used to ensure each CPU can start with 14 * a PGD aligned virtual address (for realmode). 15 * 16 * The order of each memory region is not changed. The feature looks at 17 * the available space for the regions based on different configuration 18 * options and randomizes the base and space between each. The size of the 19 * physical memory mapping is the available physical memory. 20 */ 21 22 #include <linux/kernel.h> 23 #include <linux/init.h> 24 #include <linux/random.h> 25 26 #include <asm/pgalloc.h> 27 #include <asm/pgtable.h> 28 #include <asm/setup.h> 29 #include <asm/kaslr.h> 30 31 #include "mm_internal.h" 32 33 #define TB_SHIFT 40 34 35 /* 36 * Virtual address start and end range for randomization. The end changes base 37 * on configuration to have the highest amount of space for randomization. 38 * It increases the possible random position for each randomized region. 39 * 40 * You need to add an if/def entry if you introduce a new memory region 41 * compatible with KASLR. Your entry must be in logical order with memory 42 * layout. For example, ESPFIX is before EFI because its virtual address is 43 * before. You also need to add a BUILD_BUG_ON() in kernel_randomize_memory() to 44 * ensure that this order is correct and won't be changed. 45 */ 46 static const unsigned long vaddr_start = __PAGE_OFFSET_BASE; 47 48 #if defined(CONFIG_X86_ESPFIX64) 49 static const unsigned long vaddr_end = ESPFIX_BASE_ADDR; 50 #elif defined(CONFIG_EFI) 51 static const unsigned long vaddr_end = EFI_VA_END; 52 #else 53 static const unsigned long vaddr_end = __START_KERNEL_map; 54 #endif 55 56 /* Default values */ 57 unsigned long page_offset_base = __PAGE_OFFSET_BASE; 58 EXPORT_SYMBOL(page_offset_base); 59 unsigned long vmalloc_base = __VMALLOC_BASE; 60 EXPORT_SYMBOL(vmalloc_base); 61 unsigned long vmemmap_base = __VMEMMAP_BASE; 62 EXPORT_SYMBOL(vmemmap_base); 63 64 /* 65 * Memory regions randomized by KASLR (except modules that use a separate logic 66 * earlier during boot). The list is ordered based on virtual addresses. This 67 * order is kept after randomization. 68 */ 69 static __initdata struct kaslr_memory_region { 70 unsigned long *base; 71 unsigned long size_tb; 72 } kaslr_regions[] = { 73 { &page_offset_base, 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT) /* Maximum */ }, 74 { &vmalloc_base, VMALLOC_SIZE_TB }, 75 { &vmemmap_base, 1 }, 76 }; 77 78 /* Get size in bytes used by the memory region */ 79 static inline unsigned long get_padding(struct kaslr_memory_region *region) 80 { 81 return (region->size_tb << TB_SHIFT); 82 } 83 84 /* 85 * Apply no randomization if KASLR was disabled at boot or if KASAN 86 * is enabled. KASAN shadow mappings rely on regions being PGD aligned. 87 */ 88 static inline bool kaslr_memory_enabled(void) 89 { 90 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN); 91 } 92 93 /* Initialize base and padding for each memory region randomized with KASLR */ 94 void __init kernel_randomize_memory(void) 95 { 96 size_t i; 97 unsigned long vaddr = vaddr_start; 98 unsigned long rand, memory_tb; 99 struct rnd_state rand_state; 100 unsigned long remain_entropy; 101 102 /* 103 * All these BUILD_BUG_ON checks ensures the memory layout is 104 * consistent with the vaddr_start/vaddr_end variables. 105 */ 106 BUILD_BUG_ON(vaddr_start >= vaddr_end); 107 BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_ESPFIX64) && 108 vaddr_end >= EFI_VA_END); 109 BUILD_BUG_ON((IS_ENABLED(CONFIG_X86_ESPFIX64) || 110 IS_ENABLED(CONFIG_EFI)) && 111 vaddr_end >= __START_KERNEL_map); 112 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map); 113 114 if (!kaslr_memory_enabled()) 115 return; 116 117 /* 118 * Update Physical memory mapping to available and 119 * add padding if needed (especially for memory hotplug support). 120 */ 121 BUG_ON(kaslr_regions[0].base != &page_offset_base); 122 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + 123 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; 124 125 /* Adapt phyiscal memory region size based on available memory */ 126 if (memory_tb < kaslr_regions[0].size_tb) 127 kaslr_regions[0].size_tb = memory_tb; 128 129 /* Calculate entropy available between regions */ 130 remain_entropy = vaddr_end - vaddr_start; 131 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) 132 remain_entropy -= get_padding(&kaslr_regions[i]); 133 134 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory")); 135 136 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { 137 unsigned long entropy; 138 139 /* 140 * Select a random virtual address using the extra entropy 141 * available. 142 */ 143 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i); 144 prandom_bytes_state(&rand_state, &rand, sizeof(rand)); 145 if (IS_ENABLED(CONFIG_X86_5LEVEL)) 146 entropy = (rand % (entropy + 1)) & P4D_MASK; 147 else 148 entropy = (rand % (entropy + 1)) & PUD_MASK; 149 vaddr += entropy; 150 *kaslr_regions[i].base = vaddr; 151 152 /* 153 * Jump the region and add a minimum padding based on 154 * randomization alignment. 155 */ 156 vaddr += get_padding(&kaslr_regions[i]); 157 if (IS_ENABLED(CONFIG_X86_5LEVEL)) 158 vaddr = round_up(vaddr + 1, P4D_SIZE); 159 else 160 vaddr = round_up(vaddr + 1, PUD_SIZE); 161 remain_entropy -= entropy; 162 } 163 } 164 165 static void __meminit init_trampoline_pud(void) 166 { 167 unsigned long paddr, paddr_next; 168 pgd_t *pgd; 169 pud_t *pud_page, *pud_page_tramp; 170 int i; 171 172 pud_page_tramp = alloc_low_page(); 173 174 paddr = 0; 175 pgd = pgd_offset_k((unsigned long)__va(paddr)); 176 pud_page = (pud_t *) pgd_page_vaddr(*pgd); 177 178 for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) { 179 pud_t *pud, *pud_tramp; 180 unsigned long vaddr = (unsigned long)__va(paddr); 181 182 pud_tramp = pud_page_tramp + pud_index(paddr); 183 pud = pud_page + pud_index(vaddr); 184 paddr_next = (paddr & PUD_MASK) + PUD_SIZE; 185 186 *pud_tramp = *pud; 187 } 188 189 set_pgd(&trampoline_pgd_entry, 190 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); 191 } 192 193 static void __meminit init_trampoline_p4d(void) 194 { 195 unsigned long paddr, paddr_next; 196 pgd_t *pgd; 197 p4d_t *p4d_page, *p4d_page_tramp; 198 int i; 199 200 p4d_page_tramp = alloc_low_page(); 201 202 paddr = 0; 203 pgd = pgd_offset_k((unsigned long)__va(paddr)); 204 p4d_page = (p4d_t *) pgd_page_vaddr(*pgd); 205 206 for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) { 207 p4d_t *p4d, *p4d_tramp; 208 unsigned long vaddr = (unsigned long)__va(paddr); 209 210 p4d_tramp = p4d_page_tramp + p4d_index(paddr); 211 p4d = p4d_page + p4d_index(vaddr); 212 paddr_next = (paddr & P4D_MASK) + P4D_SIZE; 213 214 *p4d_tramp = *p4d; 215 } 216 217 set_pgd(&trampoline_pgd_entry, 218 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp))); 219 } 220 221 /* 222 * Create PGD aligned trampoline table to allow real mode initialization 223 * of additional CPUs. Consume only 1 low memory page. 224 */ 225 void __meminit init_trampoline(void) 226 { 227 228 if (!kaslr_memory_enabled()) { 229 init_trampoline_default(); 230 return; 231 } 232 233 if (IS_ENABLED(CONFIG_X86_5LEVEL)) 234 init_trampoline_p4d(); 235 else 236 init_trampoline_pud(); 237 } 238