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