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/prandom.h> 26 #include <linux/memblock.h> 27 #include <linux/pgtable.h> 28 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 *end; 51 unsigned long size_tb; 52 } kaslr_regions[] = { 53 { 54 .base = &page_offset_base, 55 .end = &direct_map_physmem_end, 56 }, 57 { 58 .base = &vmalloc_base, 59 }, 60 { 61 .base = &vmemmap_base, 62 }, 63 }; 64 65 /* 66 * The end of the physical address space that can be mapped directly by the 67 * kernel. This starts out at (1<<MAX_PHYSMEM_BITS) - 1), but KASLR may reduce 68 * that in order to increase the available entropy for mapping other regions. 69 */ 70 unsigned long direct_map_physmem_end __ro_after_init; 71 72 /* Get size in bytes used by the memory region */ 73 static inline unsigned long get_padding(struct kaslr_memory_region *region) 74 { 75 return (region->size_tb << TB_SHIFT); 76 } 77 78 /* Initialize base and padding for each memory region randomized with KASLR */ 79 void __init kernel_randomize_memory(void) 80 { 81 size_t i; 82 unsigned long vaddr_start, vaddr; 83 unsigned long rand, memory_tb; 84 struct rnd_state rand_state; 85 unsigned long remain_entropy; 86 unsigned long vmemmap_size; 87 88 vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4; 89 vaddr = vaddr_start; 90 91 /* 92 * These BUILD_BUG_ON checks ensure the memory layout is consistent 93 * with the vaddr_start/vaddr_end variables. These checks are very 94 * limited.... 95 */ 96 BUILD_BUG_ON(vaddr_start >= vaddr_end); 97 BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE); 98 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map); 99 100 /* Preset the end of the possible address space for physical memory */ 101 direct_map_physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1); 102 if (!kaslr_memory_enabled()) 103 return; 104 105 kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT); 106 kaslr_regions[1].size_tb = VMALLOC_SIZE_TB; 107 108 /* 109 * Update Physical memory mapping to available and 110 * add padding if needed (especially for memory hotplug support). 111 */ 112 BUG_ON(kaslr_regions[0].base != &page_offset_base); 113 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + 114 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; 115 116 /* 117 * Adapt physical memory region size based on available memory, 118 * except when CONFIG_PCI_P2PDMA is enabled. P2PDMA exposes the 119 * device BAR space assuming the direct map space is large enough 120 * for creating a ZONE_DEVICE mapping in the direct map corresponding 121 * to the physical BAR address. 122 */ 123 if (!IS_ENABLED(CONFIG_PCI_P2PDMA) && (memory_tb < kaslr_regions[0].size_tb)) 124 kaslr_regions[0].size_tb = memory_tb; 125 126 /* 127 * Calculate the vmemmap region size in TBs, aligned to a TB 128 * boundary. 129 */ 130 vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) * 131 sizeof(struct page); 132 kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT); 133 134 /* Calculate entropy available between regions */ 135 remain_entropy = vaddr_end - vaddr_start; 136 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) 137 remain_entropy -= get_padding(&kaslr_regions[i]); 138 139 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory")); 140 141 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { 142 unsigned long entropy; 143 144 /* 145 * Select a random virtual address using the extra entropy 146 * available. 147 */ 148 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i); 149 prandom_bytes_state(&rand_state, &rand, sizeof(rand)); 150 entropy = (rand % (entropy + 1)) & PUD_MASK; 151 vaddr += entropy; 152 *kaslr_regions[i].base = vaddr; 153 154 /* Calculate the end of the region */ 155 vaddr += get_padding(&kaslr_regions[i]); 156 /* 157 * KASLR trims the maximum possible size of the 158 * direct-map. Update the direct_map_physmem_end boundary. 159 * No rounding required as the region starts 160 * PUD aligned and size is in units of TB. 161 */ 162 if (kaslr_regions[i].end) 163 *kaslr_regions[i].end = __pa_nodebug(vaddr - 1); 164 165 /* Add a minimum padding based on randomization alignment. */ 166 vaddr = round_up(vaddr + 1, PUD_SIZE); 167 remain_entropy -= entropy; 168 } 169 } 170 171 void __meminit init_trampoline_kaslr(void) 172 { 173 pud_t *pud_page_tramp, *pud, *pud_tramp; 174 p4d_t *p4d_page_tramp, *p4d, *p4d_tramp; 175 unsigned long paddr, vaddr; 176 pgd_t *pgd; 177 178 pud_page_tramp = alloc_low_page(); 179 180 /* 181 * There are two mappings for the low 1MB area, the direct mapping 182 * and the 1:1 mapping for the real mode trampoline: 183 * 184 * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET 185 * 1:1 mapping: virt_addr = phys_addr 186 */ 187 paddr = 0; 188 vaddr = (unsigned long)__va(paddr); 189 pgd = pgd_offset_k(vaddr); 190 191 p4d = p4d_offset(pgd, vaddr); 192 pud = pud_offset(p4d, vaddr); 193 194 pud_tramp = pud_page_tramp + pud_index(paddr); 195 *pud_tramp = *pud; 196 197 if (pgtable_l5_enabled()) { 198 p4d_page_tramp = alloc_low_page(); 199 200 p4d_tramp = p4d_page_tramp + p4d_index(paddr); 201 202 set_p4d(p4d_tramp, 203 __p4d(_KERNPG_TABLE | __pa(pud_page_tramp))); 204 205 trampoline_pgd_entry = 206 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)); 207 } else { 208 trampoline_pgd_entry = 209 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)); 210 } 211 } 212