arch/x86/mm/kaslr.c

   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 << (__PHYSICAL_MASK_SHIFT - 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                 if (pgtable_l5_enabled())
 129                         entropy = (rand % (entropy + 1)) & P4D_MASK;
 130                 else
 131                         entropy = (rand % (entropy + 1)) & PUD_MASK;
 132                 vaddr += entropy;
 133                 *kaslr_regions[i].base = vaddr;
 134
 135                 /*
 136                  * Jump the region and add a minimum padding based on
 137                  * randomization alignment.
 138                  */
 139                 vaddr += get_padding(&kaslr_regions[i]);
 140                 if (pgtable_l5_enabled())
 141                         vaddr = round_up(vaddr + 1, P4D_SIZE);
 142                 else
 143                         vaddr = round_up(vaddr + 1, PUD_SIZE);
 144                 remain_entropy -= entropy;
 145         }
 146 }
 147
 148 static void __meminit init_trampoline_pud(void)
 149 {
 150         unsigned long paddr, paddr_next;
 151         pgd_t *pgd;
 152         pud_t *pud_page, *pud_page_tramp;
 153         int i;
 154
 155         pud_page_tramp = alloc_low_page();
 156
 157         paddr = 0;
 158         pgd = pgd_offset_k((unsigned long)__va(paddr));
 159         pud_page = (pud_t *) pgd_page_vaddr(*pgd);
 160
 161         for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
 162                 pud_t *pud, *pud_tramp;
 163                 unsigned long vaddr = (unsigned long)__va(paddr);
 164
 165                 pud_tramp = pud_page_tramp + pud_index(paddr);
 166                 pud = pud_page + pud_index(vaddr);
 167                 paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
 168
 169                 *pud_tramp = *pud;
 170         }
 171
 172         set_pgd(&trampoline_pgd_entry,
 173                 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
 174 }
 175
 176 static void __meminit init_trampoline_p4d(void)
 177 {
 178         unsigned long paddr, paddr_next;
 179         pgd_t *pgd;
 180         p4d_t *p4d_page, *p4d_page_tramp;
 181         int i;
 182
 183         p4d_page_tramp = alloc_low_page();
 184
 185         paddr = 0;
 186         pgd = pgd_offset_k((unsigned long)__va(paddr));
 187         p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);
 188
 189         for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
 190                 p4d_t *p4d, *p4d_tramp;
 191                 unsigned long vaddr = (unsigned long)__va(paddr);
 192
 193                 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
 194                 p4d = p4d_page + p4d_index(vaddr);
 195                 paddr_next = (paddr & P4D_MASK) + P4D_SIZE;
 196
 197                 *p4d_tramp = *p4d;
 198         }
 199
 200         set_pgd(&trampoline_pgd_entry,
 201                 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
 202 }
 203
 204 /*
 205  * Create PGD aligned trampoline table to allow real mode initialization
 206  * of additional CPUs. Consume only 1 low memory page.
 207  */
 208 void __meminit init_trampoline(void)
 209 {
 210
 211         if (!kaslr_memory_enabled()) {
 212                 init_trampoline_default();
 213                 return;
 214         }
 215
 216         if (pgtable_l5_enabled())
 217                 init_trampoline_p4d();
 218         else
 219                 init_trampoline_pud();
 220 }