mm/page_table_check.c

   1 // SPDX-License-Identifier: GPL-2.0
   2
   3 /*
   4  * Copyright (c) 2021, Google LLC.
   5  * Pasha Tatashin <pasha.tatashin@soleen.com>
   6  */
   7 #include <linux/kstrtox.h>
   8 #include <linux/mm.h>
   9 #include <linux/page_table_check.h>
  10
  11 #undef pr_fmt
  12 #define pr_fmt(fmt)     "page_table_check: " fmt
  13
  14 struct page_table_check {
  15         atomic_t anon_map_count;
  16         atomic_t file_map_count;
  17 };
  18
  19 static bool __page_table_check_enabled __initdata =
  20                                 IS_ENABLED(CONFIG_PAGE_TABLE_CHECK_ENFORCED);
  21
  22 DEFINE_STATIC_KEY_TRUE(page_table_check_disabled);
  23 EXPORT_SYMBOL(page_table_check_disabled);
  24
  25 static int __init early_page_table_check_param(char *buf)
  26 {
  27         return kstrtobool(buf, &__page_table_check_enabled);
  28 }
  29
  30 early_param("page_table_check", early_page_table_check_param);
  31
  32 static bool __init need_page_table_check(void)
  33 {
  34         return __page_table_check_enabled;
  35 }
  36
  37 static void __init init_page_table_check(void)
  38 {
  39         if (!__page_table_check_enabled)
  40                 return;
  41         static_branch_disable(&page_table_check_disabled);
  42 }
  43
  44 struct page_ext_operations page_table_check_ops = {
  45         .size = sizeof(struct page_table_check),
  46         .need = need_page_table_check,
  47         .init = init_page_table_check,
  48         .need_shared_flags = false,
  49 };
  50
  51 static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
  52 {
  53         BUG_ON(!page_ext);
  54         return (void *)(page_ext) + page_table_check_ops.offset;
  55 }
  56
  57 /*
  58  * An entry is removed from the page table, decrement the counters for that page
  59  * verify that it is of correct type and counters do not become negative.
  60  */
  61 static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
  62                                    unsigned long pfn, unsigned long pgcnt)
  63 {
  64         struct page_ext *page_ext;
  65         struct page *page;
  66         unsigned long i;
  67         bool anon;
  68
  69         if (!pfn_valid(pfn))
  70                 return;
  71
  72         page = pfn_to_page(pfn);
  73         page_ext = page_ext_get(page);
  74         anon = PageAnon(page);
  75
  76         for (i = 0; i < pgcnt; i++) {
  77                 struct page_table_check *ptc = get_page_table_check(page_ext);
  78
  79                 if (anon) {
  80                         BUG_ON(atomic_read(&ptc->file_map_count));
  81                         BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
  82                 } else {
  83                         BUG_ON(atomic_read(&ptc->anon_map_count));
  84                         BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
  85                 }
  86                 page_ext = page_ext_next(page_ext);
  87         }
  88         page_ext_put(page_ext);
  89 }
  90
  91 /*
  92  * A new entry is added to the page table, increment the counters for that page
  93  * verify that it is of correct type and is not being mapped with a different
  94  * type to a different process.
  95  */
  96 static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
  97                                  unsigned long pfn, unsigned long pgcnt,
  98                                  bool rw)
  99 {
 100         struct page_ext *page_ext;
 101         struct page *page;
 102         unsigned long i;
 103         bool anon;
 104
 105         if (!pfn_valid(pfn))
 106                 return;
 107
 108         page = pfn_to_page(pfn);
 109         page_ext = page_ext_get(page);
 110         anon = PageAnon(page);
 111
 112         for (i = 0; i < pgcnt; i++) {
 113                 struct page_table_check *ptc = get_page_table_check(page_ext);
 114
 115                 if (anon) {
 116                         BUG_ON(atomic_read(&ptc->file_map_count));
 117                         BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
 118                 } else {
 119                         BUG_ON(atomic_read(&ptc->anon_map_count));
 120                         BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
 121                 }
 122                 page_ext = page_ext_next(page_ext);
 123         }
 124         page_ext_put(page_ext);
 125 }
 126
 127 /*
 128  * page is on free list, or is being allocated, verify that counters are zeroes
 129  * crash if they are not.
 130  */
 131 void __page_table_check_zero(struct page *page, unsigned int order)
 132 {
 133         struct page_ext *page_ext;
 134         unsigned long i;
 135
 136         page_ext = page_ext_get(page);
 137         BUG_ON(!page_ext);
 138         for (i = 0; i < (1ul << order); i++) {
 139                 struct page_table_check *ptc = get_page_table_check(page_ext);
 140
 141                 BUG_ON(atomic_read(&ptc->anon_map_count));
 142                 BUG_ON(atomic_read(&ptc->file_map_count));
 143                 page_ext = page_ext_next(page_ext);
 144         }
 145         page_ext_put(page_ext);
 146 }
 147
 148 void __page_table_check_pte_clear(struct mm_struct *mm, unsigned long addr,
 149                                   pte_t pte)
 150 {
 151         if (&init_mm == mm)
 152                 return;
 153
 154         if (pte_user_accessible_page(pte)) {
 155                 page_table_check_clear(mm, addr, pte_pfn(pte),
 156                                        PAGE_SIZE >> PAGE_SHIFT);
 157         }
 158 }
 159 EXPORT_SYMBOL(__page_table_check_pte_clear);
 160
 161 void __page_table_check_pmd_clear(struct mm_struct *mm, unsigned long addr,
 162                                   pmd_t pmd)
 163 {
 164         if (&init_mm == mm)
 165                 return;
 166
 167         if (pmd_user_accessible_page(pmd)) {
 168                 page_table_check_clear(mm, addr, pmd_pfn(pmd),
 169                                        PMD_SIZE >> PAGE_SHIFT);
 170         }
 171 }
 172 EXPORT_SYMBOL(__page_table_check_pmd_clear);
 173
 174 void __page_table_check_pud_clear(struct mm_struct *mm, unsigned long addr,
 175                                   pud_t pud)
 176 {
 177         if (&init_mm == mm)
 178                 return;
 179
 180         if (pud_user_accessible_page(pud)) {
 181                 page_table_check_clear(mm, addr, pud_pfn(pud),
 182                                        PUD_SIZE >> PAGE_SHIFT);
 183         }
 184 }
 185 EXPORT_SYMBOL(__page_table_check_pud_clear);
 186
 187 void __page_table_check_pte_set(struct mm_struct *mm, unsigned long addr,
 188                                 pte_t *ptep, pte_t pte)
 189 {
 190         if (&init_mm == mm)
 191                 return;
 192
 193         __page_table_check_pte_clear(mm, addr, *ptep);
 194         if (pte_user_accessible_page(pte)) {
 195                 page_table_check_set(mm, addr, pte_pfn(pte),
 196                                      PAGE_SIZE >> PAGE_SHIFT,
 197                                      pte_write(pte));
 198         }
 199 }
 200 EXPORT_SYMBOL(__page_table_check_pte_set);
 201
 202 void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
 203                                 pmd_t *pmdp, pmd_t pmd)
 204 {
 205         if (&init_mm == mm)
 206                 return;
 207
 208         __page_table_check_pmd_clear(mm, addr, *pmdp);
 209         if (pmd_user_accessible_page(pmd)) {
 210                 page_table_check_set(mm, addr, pmd_pfn(pmd),
 211                                      PMD_SIZE >> PAGE_SHIFT,
 212                                      pmd_write(pmd));
 213         }
 214 }
 215 EXPORT_SYMBOL(__page_table_check_pmd_set);
 216
 217 void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
 218                                 pud_t *pudp, pud_t pud)
 219 {
 220         if (&init_mm == mm)
 221                 return;
 222
 223         __page_table_check_pud_clear(mm, addr, *pudp);
 224         if (pud_user_accessible_page(pud)) {
 225                 page_table_check_set(mm, addr, pud_pfn(pud),
 226                                      PUD_SIZE >> PAGE_SHIFT,
 227                                      pud_write(pud));
 228         }
 229 }
 230 EXPORT_SYMBOL(__page_table_check_pud_set);
 231
 232 void __page_table_check_pte_clear_range(struct mm_struct *mm,
 233                                         unsigned long addr,
 234                                         pmd_t pmd)
 235 {
 236         if (&init_mm == mm)
 237                 return;
 238
 239         if (!pmd_bad(pmd) && !pmd_leaf(pmd)) {
 240                 pte_t *ptep = pte_offset_map(&pmd, addr);
 241                 unsigned long i;
 242
 243                 for (i = 0; i < PTRS_PER_PTE; i++) {
 244                         __page_table_check_pte_clear(mm, addr, *ptep);
 245                         addr += PAGE_SIZE;
 246                         ptep++;
 247                 }
 248                 pte_unmap(ptep - PTRS_PER_PTE);
 249         }
 250 }