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