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/prandom.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 *end;
  51         unsigned long size_tb;
  52 } kaslr_regions[] = {
  53         {
  54                 .base   = &page_offset_base,
  55                 .end    = &direct_map_physmem_end,
  56         },
  57         {
  58                 .base   = &vmalloc_base,
  59         },
  60         {
  61                 .base   = &vmemmap_base,
  62         },
  63 };
  64
  65 /*
  66  * The end of the physical address space that can be mapped directly by the
  67  * kernel. This starts out at (1<<MAX_PHYSMEM_BITS) - 1), but KASLR may reduce
  68  * that in order to increase the available entropy for mapping other regions.
  69  */
  70 unsigned long direct_map_physmem_end __ro_after_init;
  71
  72 /* Get size in bytes used by the memory region */
  73 static inline unsigned long get_padding(struct kaslr_memory_region *region)
  74 {
  75         return (region->size_tb << TB_SHIFT);
  76 }
  77
  78 /* Initialize base and padding for each memory region randomized with KASLR */
  79 void __init kernel_randomize_memory(void)
  80 {
  81         size_t i;
  82         unsigned long vaddr_start, vaddr;
  83         unsigned long rand, memory_tb;
  84         struct rnd_state rand_state;
  85         unsigned long remain_entropy;
  86         unsigned long vmemmap_size;
  87
  88         vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
  89         vaddr = vaddr_start;
  90
  91         /*
  92          * These BUILD_BUG_ON checks ensure the memory layout is consistent
  93          * with the vaddr_start/vaddr_end variables. These checks are very
  94          * limited....
  95          */
  96         BUILD_BUG_ON(vaddr_start >= vaddr_end);
  97         BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
  98         BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
  99
 100         /* Preset the end of the possible address space for physical memory */
 101         direct_map_physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
 102         if (!kaslr_memory_enabled())
 103                 return;
 104
 105         kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
 106         kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
 107
 108         /*
 109          * Update Physical memory mapping to available and
 110          * add padding if needed (especially for memory hotplug support).
 111          */
 112         BUG_ON(kaslr_regions[0].base != &page_offset_base);
 113         memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
 114                 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
 115
 116         /* Adapt physical memory region size based on available memory */
 117         if (memory_tb < kaslr_regions[0].size_tb)
 118                 kaslr_regions[0].size_tb = memory_tb;
 119
 120         /*
 121          * Calculate the vmemmap region size in TBs, aligned to a TB
 122          * boundary.
 123          */
 124         vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
 125                         sizeof(struct page);
 126         kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
 127
 128         /* Calculate entropy available between regions */
 129         remain_entropy = vaddr_end - vaddr_start;
 130         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 131                 remain_entropy -= get_padding(&kaslr_regions[i]);
 132
 133         prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
 134
 135         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
 136                 unsigned long entropy;
 137
 138                 /*
 139                  * Select a random virtual address using the extra entropy
 140                  * available.
 141                  */
 142                 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
 143                 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
 144                 entropy = (rand % (entropy + 1)) & PUD_MASK;
 145                 vaddr += entropy;
 146                 *kaslr_regions[i].base = vaddr;
 147
 148                 /* Calculate the end of the region */
 149                 vaddr += get_padding(&kaslr_regions[i]);
 150                 /*
 151                  * KASLR trims the maximum possible size of the
 152                  * direct-map. Update the direct_map_physmem_end boundary.
 153                  * No rounding required as the region starts
 154                  * PUD aligned and size is in units of TB.
 155                  */
 156                 if (kaslr_regions[i].end)
 157                         *kaslr_regions[i].end = __pa_nodebug(vaddr - 1);
 158
 159                 /* Add a minimum padding based on randomization alignment. */
 160                 vaddr = round_up(vaddr + 1, PUD_SIZE);
 161                 remain_entropy -= entropy;
 162         }
 163 }
 164
 165 void __meminit init_trampoline_kaslr(void)
 166 {
 167         pud_t *pud_page_tramp, *pud, *pud_tramp;
 168         p4d_t *p4d_page_tramp, *p4d, *p4d_tramp;
 169         unsigned long paddr, vaddr;
 170         pgd_t *pgd;
 171
 172         pud_page_tramp = alloc_low_page();
 173
 174         /*
 175          * There are two mappings for the low 1MB area, the direct mapping
 176          * and the 1:1 mapping for the real mode trampoline:
 177          *
 178          * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET
 179          * 1:1 mapping:    virt_addr = phys_addr
 180          */
 181         paddr = 0;
 182         vaddr = (unsigned long)__va(paddr);
 183         pgd = pgd_offset_k(vaddr);
 184
 185         p4d = p4d_offset(pgd, vaddr);
 186         pud = pud_offset(p4d, vaddr);
 187
 188         pud_tramp = pud_page_tramp + pud_index(paddr);
 189         *pud_tramp = *pud;
 190
 191         if (pgtable_l5_enabled()) {
 192                 p4d_page_tramp = alloc_low_page();
 193
 194                 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
 195
 196                 set_p4d(p4d_tramp,
 197                         __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
 198
 199                 trampoline_pgd_entry =
 200                         __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
 201         } else {
 202                 trampoline_pgd_entry =
 203                         __pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
 204         }
 205 }