arch/x86/mm/kaslr.c

   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
  48 #if defined(CONFIG_X86_ESPFIX64)
  49 static const unsigned long vaddr_end = ESPFIX_BASE_ADDR;
  50 #elif defined(CONFIG_EFI)
  51 static const unsigned long vaddr_end = EFI_VA_END;
  52 #else
  53 static const unsigned long vaddr_end = __START_KERNEL_map;
  54 #endif
  55
  56 /* Default values */
  57 unsigned long page_offset_base = __PAGE_OFFSET_BASE;
  58 EXPORT_SYMBOL(page_offset_base);
  59 unsigned long vmalloc_base = __VMALLOC_BASE;
  60 EXPORT_SYMBOL(vmalloc_base);
  61 unsigned long vmemmap_base = __VMEMMAP_BASE;
  62 EXPORT_SYMBOL(vmemmap_base);
  63
  64 /*
  65  * Memory regions randomized by KASLR (except modules that use a separate logic
  66  * earlier during boot). The list is ordered based on virtual addresses. This
  67  * order is kept after randomization.
  68  */
  69 static __initdata struct kaslr_memory_region {
  70         unsigned long *base;
  71         unsigned long size_tb;
  72 } kaslr_regions[] = {
  73         { &page_offset_base, 64/* Maximum */ },
  74         { &vmalloc_base, VMALLOC_SIZE_TB },
  75         { &vmemmap_base, 1 },
  76 };
  77
  78 /* Get size in bytes used by the memory region */
  79 static inline unsigned long get_padding(struct kaslr_memory_region *region)
  80 {
  81         return (region->size_tb << TB_SHIFT);
  82 }
  83
  84 /*
  85  * Apply no randomization if KASLR was disabled at boot or if KASAN
  86  * is enabled. KASAN shadow mappings rely on regions being PGD aligned.
  87  */
  88 static inline bool kaslr_memory_enabled(void)
  89 {
  90         return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
  91 }
  92
  93 /* Initialize base and padding for each memory region randomized with KASLR */
  94 void __init kernel_randomize_memory(void)
  95 {
  96         size_t i;
  97         unsigned long vaddr = vaddr_start;
  98         unsigned long rand, memory_tb;
  99         struct rnd_state rand_state;
 100         unsigned long remain_entropy;
 101
 102         /*
 103          * All these BUILD_BUG_ON checks ensures the memory layout is
 104          * consistent with the vaddr_start/vaddr_end variables.
 105          */
 106         BUILD_BUG_ON(vaddr_start >= vaddr_end);
 107         BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_ESPFIX64) &&
 108                      vaddr_end >= EFI_VA_END);
 109         BUILD_BUG_ON((IS_ENABLED(CONFIG_X86_ESPFIX64) ||
 110                       IS_ENABLED(CONFIG_EFI)) &&
 111                      vaddr_end >= __START_KERNEL_map);
 112         BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
 113
 114         if (!kaslr_memory_enabled())
 115                 return;
 116
 117         /*
 118          * Update Physical memory mapping to available and
 119          * add padding if needed (especially for memory hotplug support).
 120          */
 121         BUG_ON(kaslr_regions[0].base != &page_offset_base);
 122         memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
 123                 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
 124
 125         /* Adapt phyiscal memory region size based on available memory */
 126         if (memory_tb < kaslr_regions[0].size_tb)
 127                 kaslr_regions[0].size_tb = memory_tb;
 128
 129         /* Calculate entropy available between regions */
 130         remain_entropy = vaddr_end - vaddr_start;
 131         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 132                 remain_entropy -= get_padding(&kaslr_regions[i]);
 133
 134         prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
 135
 136         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
 137                 unsigned long entropy;
 138
 139                 /*
 140                  * Select a random virtual address using the extra entropy
 141                  * available.
 142                  */
 143                 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
 144                 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
 145                 entropy = (rand % (entropy + 1)) & PUD_MASK;
 146                 vaddr += entropy;
 147                 *kaslr_regions[i].base = vaddr;
 148
 149                 /*
 150                  * Jump the region and add a minimum padding based on
 151                  * randomization alignment.
 152                  */
 153                 vaddr += get_padding(&kaslr_regions[i]);
 154                 vaddr = round_up(vaddr + 1, PUD_SIZE);
 155                 remain_entropy -= entropy;
 156         }
 157 }
 158
 159 /*
 160  * Create PGD aligned trampoline table to allow real mode initialization
 161  * of additional CPUs. Consume only 1 low memory page.
 162  */
 163 void __meminit init_trampoline(void)
 164 {
 165         unsigned long paddr, paddr_next;
 166         pgd_t *pgd;
 167         pud_t *pud_page, *pud_page_tramp;
 168         int i;
 169
 170         if (!kaslr_memory_enabled()) {
 171                 init_trampoline_default();
 172                 return;
 173         }
 174
 175         pud_page_tramp = alloc_low_page();
 176
 177         paddr = 0;
 178         pgd = pgd_offset_k((unsigned long)__va(paddr));
 179         pud_page = (pud_t *) pgd_page_vaddr(*pgd);
 180
 181         for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
 182                 pud_t *pud, *pud_tramp;
 183                 unsigned long vaddr = (unsigned long)__va(paddr);
 184
 185                 pud_tramp = pud_page_tramp + pud_index(paddr);
 186                 pud = pud_page + pud_index(vaddr);
 187                 paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
 188
 189                 *pud_tramp = *pud;
 190         }
 191
 192         set_pgd(&trampoline_pgd_entry,
 193                 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
 194 }