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 & PUD page table levels to increase possible addresses. The 10 * physical memory mapping code was adapted to support PUD level virtual 11 * addresses. This implementation on the best configuration provides 30,000 12 * possible virtual addresses in average for each memory region. An additional 13 * low memory page is used to ensure each CPU can start with a PGD aligned 14 * 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 static const unsigned long vaddr_end = VMEMMAP_START; 48 49 /* Default values */ 50 unsigned long page_offset_base = __PAGE_OFFSET_BASE; 51 EXPORT_SYMBOL(page_offset_base); 52 unsigned long vmalloc_base = __VMALLOC_BASE; 53 EXPORT_SYMBOL(vmalloc_base); 54 55 /* 56 * Memory regions randomized by KASLR (except modules that use a separate logic 57 * earlier during boot). The list is ordered based on virtual addresses. This 58 * order is kept after randomization. 59 */ 60 static __initdata struct kaslr_memory_region { 61 unsigned long *base; 62 unsigned long size_tb; 63 } kaslr_regions[] = { 64 { &page_offset_base, 64/* Maximum */ }, 65 { &vmalloc_base, VMALLOC_SIZE_TB }, 66 }; 67 68 /* Get size in bytes used by the memory region */ 69 static inline unsigned long get_padding(struct kaslr_memory_region *region) 70 { 71 return (region->size_tb << TB_SHIFT); 72 } 73 74 /* 75 * Apply no randomization if KASLR was disabled at boot or if KASAN 76 * is enabled. KASAN shadow mappings rely on regions being PGD aligned. 77 */ 78 static inline bool kaslr_memory_enabled(void) 79 { 80 return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN); 81 } 82 83 /* Initialize base and padding for each memory region randomized with KASLR */ 84 void __init kernel_randomize_memory(void) 85 { 86 size_t i; 87 unsigned long vaddr = vaddr_start; 88 unsigned long rand, memory_tb; 89 struct rnd_state rand_state; 90 unsigned long remain_entropy; 91 92 if (!kaslr_memory_enabled()) 93 return; 94 95 /* 96 * Update Physical memory mapping to available and 97 * add padding if needed (especially for memory hotplug support). 98 */ 99 BUG_ON(kaslr_regions[0].base != &page_offset_base); 100 memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) + 101 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING; 102 103 /* Adapt phyiscal memory region size based on available memory */ 104 if (memory_tb < kaslr_regions[0].size_tb) 105 kaslr_regions[0].size_tb = memory_tb; 106 107 /* Calculate entropy available between regions */ 108 remain_entropy = vaddr_end - vaddr_start; 109 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) 110 remain_entropy -= get_padding(&kaslr_regions[i]); 111 112 prandom_seed_state(&rand_state, kaslr_get_random_long("Memory")); 113 114 for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) { 115 unsigned long entropy; 116 117 /* 118 * Select a random virtual address using the extra entropy 119 * available. 120 */ 121 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i); 122 prandom_bytes_state(&rand_state, &rand, sizeof(rand)); 123 entropy = (rand % (entropy + 1)) & PUD_MASK; 124 vaddr += entropy; 125 *kaslr_regions[i].base = vaddr; 126 127 /* 128 * Jump the region and add a minimum padding based on 129 * randomization alignment. 130 */ 131 vaddr += get_padding(&kaslr_regions[i]); 132 vaddr = round_up(vaddr + 1, PUD_SIZE); 133 remain_entropy -= entropy; 134 } 135 } 136 137 /* 138 * Create PGD aligned trampoline table to allow real mode initialization 139 * of additional CPUs. Consume only 1 low memory page. 140 */ 141 void __meminit init_trampoline(void) 142 { 143 unsigned long paddr, paddr_next; 144 pgd_t *pgd; 145 pud_t *pud_page, *pud_page_tramp; 146 int i; 147 148 if (!kaslr_memory_enabled()) { 149 init_trampoline_default(); 150 return; 151 } 152 153 pud_page_tramp = alloc_low_page(); 154 155 paddr = 0; 156 pgd = pgd_offset_k((unsigned long)__va(paddr)); 157 pud_page = (pud_t *) pgd_page_vaddr(*pgd); 158 159 for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) { 160 pud_t *pud, *pud_tramp; 161 unsigned long vaddr = (unsigned long)__va(paddr); 162 163 pud_tramp = pud_page_tramp + pud_index(paddr); 164 pud = pud_page + pud_index(vaddr); 165 paddr_next = (paddr & PUD_MASK) + PUD_SIZE; 166 167 *pud_tramp = *pud; 168 } 169 170 set_pgd(&trampoline_pgd_entry, 171 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp))); 172 } 173