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 #include <linux/memblock.h> 27 28 #include <asm/pgalloc.h> 29 #include <asm/pgtable.h> 30 #include <asm/setup.h> 31 #include <asm/kaslr.h> 32 33 #include "mm_internal.h" 34 35 #define TB_SHIFT 40 36 37 /* 38 * The end address could depend on more configuration options to make the 39 * highest amount of space for randomization available, but that's too hard 40 * to keep straight and caused issues already. 41 */ 42 static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE; 43 44 /* 45 * Memory regions randomized by KASLR (except modules that use a separate logic 46 * earlier during boot). The list is ordered based on virtual addresses. This 47 * order is kept after randomization. 48 */ 49 static __initdata struct kaslr_memory_region { 50 unsigned long *base; 51 unsigned long size_tb; 52 } kaslr_regions[] = { 53 { &page_offset_base, 0 }, 54 { &vmalloc_base, 0 }, 55 { &vmemmap_base, 1 }, 56 }; 57 58 /* Get size in bytes used by the memory region */ 59 static inline unsigned long get_padding(struct kaslr_memory_region *region) 60 { 61 return (region->size_tb << TB_SHIFT); 62 } 63 64 /* 65 * Apply no randomization if KASLR was disabled at boot or if KASAN 66 * is enabled. KASAN shadow mappings rely on regions being PGD aligned. 67 */ 68 static inline bool kaslr_memory_enabled(void) 69 { 70 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN); 71 } 72 73 /* Initialize base and padding for each memory region randomized with KASLR */ 74 void __init kernel_randomize_memory(void) 75 { 76 size_t i; 77 unsigned long vaddr_start, vaddr; 78 unsigned long rand, memory_tb; 79 struct rnd_state rand_state; 80 unsigned long remain_entropy; 81 82 vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4; 83 vaddr = vaddr_start; 84 85 /* 86 * These BUILD_BUG_ON checks ensure the memory layout is consistent 87 * with the vaddr_start/vaddr_end variables. These checks are very 88 * limited.... 89 */ 90 BUILD_BUG_ON(vaddr_start >= vaddr_end); 91 BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE); 92 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map); 93 94 if (!kaslr_memory_enabled()) 95 return; 96 97 kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT); 98 kaslr_regions[1].size_tb = VMALLOC_SIZE_TB; 99 100 /* 101 * Update Physical memory mapping to available and 102 * add padding if needed (especially for memory hotplug support). 103 */ 104 BUG_ON(kaslr_regions[0].base != &page_offset_base); 105 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + 106 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; 107 108 /* Adapt phyiscal memory region size based on available memory */ 109 if (memory_tb < kaslr_regions[0].size_tb) 110 kaslr_regions[0].size_tb = memory_tb; 111 112 /* Calculate entropy available between regions */ 113 remain_entropy = vaddr_end - vaddr_start; 114 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) 115 remain_entropy -= get_padding(&kaslr_regions[i]); 116 117 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory")); 118 119 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { 120 unsigned long entropy; 121 122 /* 123 * Select a random virtual address using the extra entropy 124 * available. 125 */ 126 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i); 127 prandom_bytes_state(&rand_state, &rand, sizeof(rand)); 128 entropy = (rand % (entropy + 1)) & PUD_MASK; 129 vaddr += entropy; 130 *kaslr_regions[i].base = vaddr; 131 132 /* 133 * Jump the region and add a minimum padding based on 134 * randomization alignment. 135 */ 136 vaddr += get_padding(&kaslr_regions[i]); 137 vaddr = round_up(vaddr + 1, PUD_SIZE); 138 remain_entropy -= entropy; 139 } 140 } 141 142 static void __meminit init_trampoline_pud(void) 143 { 144 pud_t *pud_page_tramp, *pud, *pud_tramp; 145 p4d_t *p4d_page_tramp, *p4d, *p4d_tramp; 146 unsigned long paddr, vaddr; 147 pgd_t *pgd; 148 149 pud_page_tramp = alloc_low_page(); 150 151 /* 152 * There are two mappings for the low 1MB area, the direct mapping 153 * and the 1:1 mapping for the real mode trampoline: 154 * 155 * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET 156 * 1:1 mapping: virt_addr = phys_addr 157 */ 158 paddr = 0; 159 vaddr = (unsigned long)__va(paddr); 160 pgd = pgd_offset_k(vaddr); 161 162 p4d = p4d_offset(pgd, vaddr); 163 pud = pud_offset(p4d, vaddr); 164 165 pud_tramp = pud_page_tramp + pud_index(paddr); 166 *pud_tramp = *pud; 167 168 if (pgtable_l5_enabled()) { 169 p4d_page_tramp = alloc_low_page(); 170 171 p4d_tramp = p4d_page_tramp + p4d_index(paddr); 172 173 set_p4d(p4d_tramp, 174 __p4d(_KERNPG_TABLE | __pa(pud_page_tramp))); 175 176 set_pgd(&trampoline_pgd_entry, 177 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp))); 178 } else { 179 set_pgd(&trampoline_pgd_entry, 180 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); 181 } 182 } 183 184 /* 185 * The real mode trampoline, which is required for bootstrapping CPUs 186 * occupies only a small area under the low 1MB. See reserve_real_mode() 187 * for details. 188 * 189 * If KASLR is disabled the first PGD entry of the direct mapping is copied 190 * to map the real mode trampoline. 191 * 192 * If KASLR is enabled, copy only the PUD which covers the low 1MB 193 * area. This limits the randomization granularity to 1GB for both 4-level 194 * and 5-level paging. 195 */ 196 void __meminit init_trampoline(void) 197 { 198 if (!kaslr_memory_enabled()) { 199 init_trampoline_default(); 200 return; 201 } 202 203 init_trampoline_pud(); 204 } 205