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         if (!buf)
  27                 return -EINVAL;
  28
  29         if (strcmp(buf, "on") == 0)
  30                 __page_table_check_enabled = true;
  31         else if (strcmp(buf, "off") == 0)
  32                 __page_table_check_enabled = false;
  33
  34         return 0;
  35 }
  36
  37 early_param("page_table_check", early_page_table_check_param);
  38
  39 static bool __init need_page_table_check(void)
  40 {
  41         return __page_table_check_enabled;
  42 }
  43
  44 static void __init init_page_table_check(void)
  45 {
  46         if (!__page_table_check_enabled)
  47                 return;
  48         static_branch_disable(&page_table_check_disabled);
  49 }
  50
  51 struct page_ext_operations page_table_check_ops = {
  52         .size = sizeof(struct page_table_check),
  53         .need = need_page_table_check,
  54         .init = init_page_table_check,
  55 };
  56
  57 static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
  58 {
  59         BUG_ON(!page_ext);
  60         return (void *)(page_ext) + page_table_check_ops.offset;
  61 }
  62
  63 static inline bool pte_user_accessible_page(pte_t pte)
  64 {
  65         return (pte_val(pte) & _PAGE_PRESENT) && (pte_val(pte) & _PAGE_USER);
  66 }
  67
  68 static inline bool pmd_user_accessible_page(pmd_t pmd)
  69 {
  70         return pmd_leaf(pmd) && (pmd_val(pmd) & _PAGE_PRESENT) &&
  71                 (pmd_val(pmd) & _PAGE_USER);
  72 }
  73
  74 static inline bool pud_user_accessible_page(pud_t pud)
  75 {
  76         return pud_leaf(pud) && (pud_val(pud) & _PAGE_PRESENT) &&
  77                 (pud_val(pud) & _PAGE_USER);
  78 }
  79
  80 /*
  81  * An enty is removed from the page table, decrement the counters for that page
  82  * verify that it is of correct type and counters do not become negative.
  83  */
  84 static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
  85                                    unsigned long pfn, unsigned long pgcnt)
  86 {
  87         struct page_ext *page_ext;
  88         struct page *page;
  89         bool anon;
  90         int i;
  91
  92         if (!pfn_valid(pfn))
  93                 return;
  94
  95         page = pfn_to_page(pfn);
  96         page_ext = lookup_page_ext(page);
  97         anon = PageAnon(page);
  98
  99         for (i = 0; i < pgcnt; i++) {
 100                 struct page_table_check *ptc = get_page_table_check(page_ext);
 101
 102                 if (anon) {
 103                         BUG_ON(atomic_read(&ptc->file_map_count));
 104                         BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
 105                 } else {
 106                         BUG_ON(atomic_read(&ptc->anon_map_count));
 107                         BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
 108                 }
 109                 page_ext = page_ext_next(page_ext);
 110         }
 111 }
 112
 113 /*
 114  * A new enty is added to the page table, increment the counters for that page
 115  * verify that it is of correct type and is not being mapped with a different
 116  * type to a different process.
 117  */
 118 static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
 119                                  unsigned long pfn, unsigned long pgcnt,
 120                                  bool rw)
 121 {
 122         struct page_ext *page_ext;
 123         struct page *page;
 124         bool anon;
 125         int i;
 126
 127         if (!pfn_valid(pfn))
 128                 return;
 129
 130         page = pfn_to_page(pfn);
 131         page_ext = lookup_page_ext(page);
 132         anon = PageAnon(page);
 133
 134         for (i = 0; i < pgcnt; i++) {
 135                 struct page_table_check *ptc = get_page_table_check(page_ext);
 136
 137                 if (anon) {
 138                         BUG_ON(atomic_read(&ptc->file_map_count));
 139                         BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
 140                 } else {
 141                         BUG_ON(atomic_read(&ptc->anon_map_count));
 142                         BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
 143                 }
 144                 page_ext = page_ext_next(page_ext);
 145         }
 146 }
 147
 148 /*
 149  * page is on free list, or is being allocated, verify that counters are zeroes
 150  * crash if they are not.
 151  */
 152 void __page_table_check_zero(struct page *page, unsigned int order)
 153 {
 154         struct page_ext *page_ext = lookup_page_ext(page);
 155         int i;
 156
 157         BUG_ON(!page_ext);
 158         for (i = 0; i < (1 << order); i++) {
 159                 struct page_table_check *ptc = get_page_table_check(page_ext);
 160
 161                 BUG_ON(atomic_read(&ptc->anon_map_count));
 162                 BUG_ON(atomic_read(&ptc->file_map_count));
 163                 page_ext = page_ext_next(page_ext);
 164         }
 165 }
 166
 167 void __page_table_check_pte_clear(struct mm_struct *mm, unsigned long addr,
 168                                   pte_t pte)
 169 {
 170         if (&init_mm == mm)
 171                 return;
 172
 173         if (pte_user_accessible_page(pte)) {
 174                 page_table_check_clear(mm, addr, pte_pfn(pte),
 175                                        PAGE_SIZE >> PAGE_SHIFT);
 176         }
 177 }
 178 EXPORT_SYMBOL(__page_table_check_pte_clear);
 179
 180 void __page_table_check_pmd_clear(struct mm_struct *mm, unsigned long addr,
 181                                   pmd_t pmd)
 182 {
 183         if (&init_mm == mm)
 184                 return;
 185
 186         if (pmd_user_accessible_page(pmd)) {
 187                 page_table_check_clear(mm, addr, pmd_pfn(pmd),
 188                                        PMD_PAGE_SIZE >> PAGE_SHIFT);
 189         }
 190 }
 191 EXPORT_SYMBOL(__page_table_check_pmd_clear);
 192
 193 void __page_table_check_pud_clear(struct mm_struct *mm, unsigned long addr,
 194                                   pud_t pud)
 195 {
 196         if (&init_mm == mm)
 197                 return;
 198
 199         if (pud_user_accessible_page(pud)) {
 200                 page_table_check_clear(mm, addr, pud_pfn(pud),
 201                                        PUD_PAGE_SIZE >> PAGE_SHIFT);
 202         }
 203 }
 204 EXPORT_SYMBOL(__page_table_check_pud_clear);
 205
 206 void __page_table_check_pte_set(struct mm_struct *mm, unsigned long addr,
 207                                 pte_t *ptep, pte_t pte)
 208 {
 209         pte_t old_pte;
 210
 211         if (&init_mm == mm)
 212                 return;
 213
 214         old_pte = *ptep;
 215         if (pte_user_accessible_page(old_pte)) {
 216                 page_table_check_clear(mm, addr, pte_pfn(old_pte),
 217                                        PAGE_SIZE >> PAGE_SHIFT);
 218         }
 219
 220         if (pte_user_accessible_page(pte)) {
 221                 page_table_check_set(mm, addr, pte_pfn(pte),
 222                                      PAGE_SIZE >> PAGE_SHIFT,
 223                                      pte_write(pte));
 224         }
 225 }
 226 EXPORT_SYMBOL(__page_table_check_pte_set);
 227
 228 void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
 229                                 pmd_t *pmdp, pmd_t pmd)
 230 {
 231         pmd_t old_pmd;
 232
 233         if (&init_mm == mm)
 234                 return;
 235
 236         old_pmd = *pmdp;
 237         if (pmd_user_accessible_page(old_pmd)) {
 238                 page_table_check_clear(mm, addr, pmd_pfn(old_pmd),
 239                                        PMD_PAGE_SIZE >> PAGE_SHIFT);
 240         }
 241
 242         if (pmd_user_accessible_page(pmd)) {
 243                 page_table_check_set(mm, addr, pmd_pfn(pmd),
 244                                      PMD_PAGE_SIZE >> PAGE_SHIFT,
 245                                      pmd_write(pmd));
 246         }
 247 }
 248 EXPORT_SYMBOL(__page_table_check_pmd_set);
 249
 250 void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
 251                                 pud_t *pudp, pud_t pud)
 252 {
 253         pud_t old_pud;
 254
 255         if (&init_mm == mm)
 256                 return;
 257
 258         old_pud = *pudp;
 259         if (pud_user_accessible_page(old_pud)) {
 260                 page_table_check_clear(mm, addr, pud_pfn(old_pud),
 261                                        PUD_PAGE_SIZE >> PAGE_SHIFT);
 262         }
 263
 264         if (pud_user_accessible_page(pud)) {
 265                 page_table_check_set(mm, addr, pud_pfn(pud),
 266                                      PUD_PAGE_SIZE >> PAGE_SHIFT,
 267                                      pud_write(pud));
 268         }
 269 }
 270 EXPORT_SYMBOL(__page_table_check_pud_set);