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 #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 size_tb; 51 } kaslr_regions[] = { 52 { &page_offset_base, 0 }, 53 { &vmalloc_base, 0 }, 54 { &vmemmap_base, 0 }, 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 /* Initialize base and padding for each memory region randomized with KASLR */ 64 void __init kernel_randomize_memory(void) 65 { 66 size_t i; 67 unsigned long vaddr_start, vaddr; 68 unsigned long rand, memory_tb; 69 struct rnd_state rand_state; 70 unsigned long remain_entropy; 71 unsigned long vmemmap_size; 72 73 vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4; 74 vaddr = vaddr_start; 75 76 /* 77 * These BUILD_BUG_ON checks ensure the memory layout is consistent 78 * with the vaddr_start/vaddr_end variables. These checks are very 79 * limited.... 80 */ 81 BUILD_BUG_ON(vaddr_start >= vaddr_end); 82 BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE); 83 BUILD_BUG_ON(vaddr_end > __START_KERNEL_map); 84 85 if (!kaslr_memory_enabled()) 86 return; 87 88 kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT); 89 kaslr_regions[1].size_tb = VMALLOC_SIZE_TB; 90 91 /* 92 * Update Physical memory mapping to available and 93 * add padding if needed (especially for memory hotplug support). 94 */ 95 BUG_ON(kaslr_regions[0].base != &page_offset_base); 96 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + 97 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; 98 99 /* Adapt physical memory region size based on available memory */ 100 if (memory_tb < kaslr_regions[0].size_tb) 101 kaslr_regions[0].size_tb = memory_tb; 102 103 /* 104 * Calculate the vmemmap region size in TBs, aligned to a TB 105 * boundary. 106 */ 107 vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) * 108 sizeof(struct page); 109 kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT); 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 entropy = (rand % (entropy + 1)) & PUD_MASK; 128 vaddr += entropy; 129 *kaslr_regions[i].base = vaddr; 130 131 /* 132 * Jump the region and add a minimum padding based on 133 * randomization alignment. 134 */ 135 vaddr += get_padding(&kaslr_regions[i]); 136 vaddr = round_up(vaddr + 1, PUD_SIZE); 137 remain_entropy -= entropy; 138 } 139 } 140 141 void __meminit init_trampoline_kaslr(void) 142 { 143 pud_t *pud_page_tramp, *pud, *pud_tramp; 144 p4d_t *p4d_page_tramp, *p4d, *p4d_tramp; 145 unsigned long paddr, vaddr; 146 pgd_t *pgd; 147 148 pud_page_tramp = alloc_low_page(); 149 150 /* 151 * There are two mappings for the low 1MB area, the direct mapping 152 * and the 1:1 mapping for the real mode trampoline: 153 * 154 * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET 155 * 1:1 mapping: virt_addr = phys_addr 156 */ 157 paddr = 0; 158 vaddr = (unsigned long)__va(paddr); 159 pgd = pgd_offset_k(vaddr); 160 161 p4d = p4d_offset(pgd, vaddr); 162 pud = pud_offset(p4d, vaddr); 163 164 pud_tramp = pud_page_tramp + pud_index(paddr); 165 *pud_tramp = *pud; 166 167 if (pgtable_l5_enabled()) { 168 p4d_page_tramp = alloc_low_page(); 169 170 p4d_tramp = p4d_page_tramp + p4d_index(paddr); 171 172 set_p4d(p4d_tramp, 173 __p4d(_KERNPG_TABLE | __pa(pud_page_tramp))); 174 175 set_pgd(&trampoline_pgd_entry, 176 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp))); 177 } else { 178 set_pgd(&trampoline_pgd_entry, 179 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); 180 } 181 } 182