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
  27 #include <asm/pgalloc.h>
  28 #include <asm/pgtable.h>
  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  * Virtual address start and end range for randomization.
  38  *
  39  * The end address could depend on more configuration options to make the
  40  * highest amount of space for randomization available, but that's too hard
  41  * to keep straight and caused issues already.
  42  */
  43 static const unsigned long vaddr_start = __PAGE_OFFSET_BASE;
  44 static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
  45
  46 /* Default values */
  47 unsigned long page_offset_base = __PAGE_OFFSET_BASE;
  48 EXPORT_SYMBOL(page_offset_base);
  49 unsigned long vmalloc_base = __VMALLOC_BASE;
  50 EXPORT_SYMBOL(vmalloc_base);
  51 unsigned long vmemmap_base = __VMEMMAP_BASE;
  52 EXPORT_SYMBOL(vmemmap_base);
  53
  54 /*
  55  * Memory regions randomized by KASLR (except modules that use a separate logic
  56  * earlier during boot). The list is ordered based on virtual addresses. This
  57  * order is kept after randomization.
  58  */
  59 static __initdata struct kaslr_memory_region {
  60         unsigned long *base;
  61         unsigned long size_tb;
  62 } kaslr_regions[] = {
  63         { &page_offset_base, 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT) /* Maximum */ },
  64         { &vmalloc_base, VMALLOC_SIZE_TB },
  65         { &vmemmap_base, 1 },
  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         /*
  93          * These BUILD_BUG_ON checks ensure the memory layout is consistent
  94          * with the vaddr_start/vaddr_end variables. These checks are very
  95          * limited....
  96          */
  97         BUILD_BUG_ON(vaddr_start >= vaddr_end);
  98         BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
  99         BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
 100
 101         if (!kaslr_memory_enabled())
 102                 return;
 103
 104         /*
 105          * Update Physical memory mapping to available and
 106          * add padding if needed (especially for memory hotplug support).
 107          */
 108         BUG_ON(kaslr_regions[0].base != &page_offset_base);
 109         memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
 110                 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
 111
 112         /* Adapt phyiscal memory region size based on available memory */
 113         if (memory_tb < kaslr_regions[0].size_tb)
 114                 kaslr_regions[0].size_tb = memory_tb;
 115
 116         /* Calculate entropy available between regions */
 117         remain_entropy = vaddr_end - vaddr_start;
 118         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 119                 remain_entropy -= get_padding(&kaslr_regions[i]);
 120
 121         prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
 122
 123         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
 124                 unsigned long entropy;
 125
 126                 /*
 127                  * Select a random virtual address using the extra entropy
 128                  * available.
 129                  */
 130                 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
 131                 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
 132                 if (IS_ENABLED(CONFIG_X86_5LEVEL))
 133                         entropy = (rand % (entropy + 1)) & P4D_MASK;
 134                 else
 135                         entropy = (rand % (entropy + 1)) & PUD_MASK;
 136                 vaddr += entropy;
 137                 *kaslr_regions[i].base = vaddr;
 138
 139                 /*
 140                  * Jump the region and add a minimum padding based on
 141                  * randomization alignment.
 142                  */
 143                 vaddr += get_padding(&kaslr_regions[i]);
 144                 if (IS_ENABLED(CONFIG_X86_5LEVEL))
 145                         vaddr = round_up(vaddr + 1, P4D_SIZE);
 146                 else
 147                         vaddr = round_up(vaddr + 1, PUD_SIZE);
 148                 remain_entropy -= entropy;
 149         }
 150 }
 151
 152 static void __meminit init_trampoline_pud(void)
 153 {
 154         unsigned long paddr, paddr_next;
 155         pgd_t *pgd;
 156         pud_t *pud_page, *pud_page_tramp;
 157         int i;
 158
 159         pud_page_tramp = alloc_low_page();
 160
 161         paddr = 0;
 162         pgd = pgd_offset_k((unsigned long)__va(paddr));
 163         pud_page = (pud_t *) pgd_page_vaddr(*pgd);
 164
 165         for (i = pud_index(paddr); i < PTRS_PER_PUD; i++, paddr = paddr_next) {
 166                 pud_t *pud, *pud_tramp;
 167                 unsigned long vaddr = (unsigned long)__va(paddr);
 168
 169                 pud_tramp = pud_page_tramp + pud_index(paddr);
 170                 pud = pud_page + pud_index(vaddr);
 171                 paddr_next = (paddr & PUD_MASK) + PUD_SIZE;
 172
 173                 *pud_tramp = *pud;
 174         }
 175
 176         set_pgd(&trampoline_pgd_entry,
 177                 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
 178 }
 179
 180 static void __meminit init_trampoline_p4d(void)
 181 {
 182         unsigned long paddr, paddr_next;
 183         pgd_t *pgd;
 184         p4d_t *p4d_page, *p4d_page_tramp;
 185         int i;
 186
 187         p4d_page_tramp = alloc_low_page();
 188
 189         paddr = 0;
 190         pgd = pgd_offset_k((unsigned long)__va(paddr));
 191         p4d_page = (p4d_t *) pgd_page_vaddr(*pgd);
 192
 193         for (i = p4d_index(paddr); i < PTRS_PER_P4D; i++, paddr = paddr_next) {
 194                 p4d_t *p4d, *p4d_tramp;
 195                 unsigned long vaddr = (unsigned long)__va(paddr);
 196
 197                 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
 198                 p4d = p4d_page + p4d_index(vaddr);
 199                 paddr_next = (paddr & P4D_MASK) + P4D_SIZE;
 200
 201                 *p4d_tramp = *p4d;
 202         }
 203
 204         set_pgd(&trampoline_pgd_entry,
 205                 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
 206 }
 207
 208 /*
 209  * Create PGD aligned trampoline table to allow real mode initialization
 210  * of additional CPUs. Consume only 1 low memory page.
 211  */
 212 void __meminit init_trampoline(void)
 213 {
 214
 215         if (!kaslr_memory_enabled()) {
 216                 init_trampoline_default();
 217                 return;
 218         }
 219
 220         if (IS_ENABLED(CONFIG_X86_5LEVEL))
 221                 init_trampoline_p4d();
 222         else
 223                 init_trampoline_pud();
 224 }