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 #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 size_tb;
  51 } kaslr_regions[] = {
  52         { &page_offset_base, 0 },
  53         { &vmalloc_base, 0 },
  54         { &vmemmap_base, 0 },
  55 };
  56
  57 /* Get size in bytes used by the memory region */
  58 static inline unsigned long get_padding(struct kaslr_memory_region *region)
  59 {
  60         return (region->size_tb << TB_SHIFT);
  61 }
  62
  63 /* Initialize base and padding for each memory region randomized with KASLR */
  64 void __init kernel_randomize_memory(void)
  65 {
  66         size_t i;
  67         unsigned long vaddr_start, vaddr;
  68         unsigned long rand, memory_tb;
  69         struct rnd_state rand_state;
  70         unsigned long remain_entropy;
  71         unsigned long vmemmap_size;
  72
  73         vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
  74         vaddr = vaddr_start;
  75
  76         /*
  77          * These BUILD_BUG_ON checks ensure the memory layout is consistent
  78          * with the vaddr_start/vaddr_end variables. These checks are very
  79          * limited....
  80          */
  81         BUILD_BUG_ON(vaddr_start >= vaddr_end);
  82         BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
  83         BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
  84
  85         if (!kaslr_memory_enabled())
  86                 return;
  87
  88         kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
  89         kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
  90
  91         /*
  92          * Update Physical memory mapping to available and
  93          * add padding if needed (especially for memory hotplug support).
  94          */
  95         BUG_ON(kaslr_regions[0].base != &page_offset_base);
  96         memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
  97                 CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
  98
  99         /* Adapt phyiscal memory region size based on available memory */
 100         if (memory_tb < kaslr_regions[0].size_tb)
 101                 kaslr_regions[0].size_tb = memory_tb;
 102
 103         /*
 104          * Calculate the vmemmap region size in TBs, aligned to a TB
 105          * boundary.
 106          */
 107         vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
 108                         sizeof(struct page);
 109         kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
 110
 111         /* Calculate entropy available between regions */
 112         remain_entropy = vaddr_end - vaddr_start;
 113         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 114                 remain_entropy -= get_padding(&kaslr_regions[i]);
 115
 116         prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
 117
 118         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
 119                 unsigned long entropy;
 120
 121                 /*
 122                  * Select a random virtual address using the extra entropy
 123                  * available.
 124                  */
 125                 entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
 126                 prandom_bytes_state(&rand_state, &rand, sizeof(rand));
 127                 entropy = (rand % (entropy + 1)) & PUD_MASK;
 128                 vaddr += entropy;
 129                 *kaslr_regions[i].base = vaddr;
 130
 131                 /*
 132                  * Jump the region and add a minimum padding based on
 133                  * randomization alignment.
 134                  */
 135                 vaddr += get_padding(&kaslr_regions[i]);
 136                 vaddr = round_up(vaddr + 1, PUD_SIZE);
 137                 remain_entropy -= entropy;
 138         }
 139 }
 140
 141 void __meminit init_trampoline_kaslr(void)
 142 {
 143         pud_t *pud_page_tramp, *pud, *pud_tramp;
 144         p4d_t *p4d_page_tramp, *p4d, *p4d_tramp;
 145         unsigned long paddr, vaddr;
 146         pgd_t *pgd;
 147
 148         pud_page_tramp = alloc_low_page();
 149
 150         /*
 151          * There are two mappings for the low 1MB area, the direct mapping
 152          * and the 1:1 mapping for the real mode trampoline:
 153          *
 154          * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET
 155          * 1:1 mapping:    virt_addr = phys_addr
 156          */
 157         paddr = 0;
 158         vaddr = (unsigned long)__va(paddr);
 159         pgd = pgd_offset_k(vaddr);
 160
 161         p4d = p4d_offset(pgd, vaddr);
 162         pud = pud_offset(p4d, vaddr);
 163
 164         pud_tramp = pud_page_tramp + pud_index(paddr);
 165         *pud_tramp = *pud;
 166
 167         if (pgtable_l5_enabled()) {
 168                 p4d_page_tramp = alloc_low_page();
 169
 170                 p4d_tramp = p4d_page_tramp + p4d_index(paddr);
 171
 172                 set_p4d(p4d_tramp,
 173                         __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
 174
 175                 set_pgd(&trampoline_pgd_entry,
 176                         __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
 177         } else {
 178                 set_pgd(&trampoline_pgd_entry,
 179                         __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
 180         }
 181 }