lib/buildid.c

   1 // SPDX-License-Identifier: GPL-2.0
   2
   3 #include <linux/buildid.h>
   4 #include <linux/cache.h>
   5 #include <linux/elf.h>
   6 #include <linux/kernel.h>
   7 #include <linux/pagemap.h>
   8 #include <linux/secretmem.h>
   9
  10 #define BUILD_ID 3
  11
  12 #define MAX_PHDR_CNT 256
  13
  14 struct freader {
  15         void *buf;
  16         u32 buf_sz;
  17         int err;
  18         union {
  19                 struct {
  20                         struct file *file;
  21                         struct folio *folio;
  22                         void *addr;
  23                         loff_t folio_off;
  24                         bool may_fault;
  25                 };
  26                 struct {
  27                         const char *data;
  28                         u64 data_sz;
  29                 };
  30         };
  31 };
  32
  33 static void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
  34                                    struct file *file, bool may_fault)
  35 {
  36         memset(r, 0, sizeof(*r));
  37         r->buf = buf;
  38         r->buf_sz = buf_sz;
  39         r->file = file;
  40         r->may_fault = may_fault;
  41 }
  42
  43 static void freader_init_from_mem(struct freader *r, const char *data, u64 data_sz)
  44 {
  45         memset(r, 0, sizeof(*r));
  46         r->data = data;
  47         r->data_sz = data_sz;
  48 }
  49
  50 static void freader_put_folio(struct freader *r)
  51 {
  52         if (!r->folio)
  53                 return;
  54         kunmap_local(r->addr);
  55         folio_put(r->folio);
  56         r->folio = NULL;
  57 }
  58
  59 static int freader_get_folio(struct freader *r, loff_t file_off)
  60 {
  61         /* check if we can just reuse current folio */
  62         if (r->folio && file_off >= r->folio_off &&
  63             file_off < r->folio_off + folio_size(r->folio))
  64                 return 0;
  65
  66         freader_put_folio(r);
  67
  68         /* reject secretmem folios created with memfd_secret() */
  69         if (secretmem_mapping(r->file->f_mapping))
  70                 return -EFAULT;
  71
  72         r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);
  73
  74         /* if sleeping is allowed, wait for the page, if necessary */
  75         if (r->may_fault && (IS_ERR(r->folio) || !folio_test_uptodate(r->folio))) {
  76                 filemap_invalidate_lock_shared(r->file->f_mapping);
  77                 r->folio = read_cache_folio(r->file->f_mapping, file_off >> PAGE_SHIFT,
  78                                             NULL, r->file);
  79                 filemap_invalidate_unlock_shared(r->file->f_mapping);
  80         }
  81
  82         if (IS_ERR(r->folio) || !folio_test_uptodate(r->folio)) {
  83                 if (!IS_ERR(r->folio))
  84                         folio_put(r->folio);
  85                 r->folio = NULL;
  86                 return -EFAULT;
  87         }
  88
  89         r->folio_off = folio_pos(r->folio);
  90         r->addr = kmap_local_folio(r->folio, 0);
  91
  92         return 0;
  93 }
  94
  95 static const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
  96 {
  97         size_t folio_sz;
  98
  99         /* provided internal temporary buffer should be sized correctly */
 100         if (WARN_ON(r->buf && sz > r->buf_sz)) {
 101                 r->err = -E2BIG;
 102                 return NULL;
 103         }
 104
 105         if (unlikely(file_off + sz < file_off)) {
 106                 r->err = -EOVERFLOW;
 107                 return NULL;
 108         }
 109
 110         /* working with memory buffer is much more straightforward */
 111         if (!r->buf) {
 112                 if (file_off + sz > r->data_sz) {
 113                         r->err = -ERANGE;
 114                         return NULL;
 115                 }
 116                 return r->data + file_off;
 117         }
 118
 119         /* fetch or reuse folio for given file offset */
 120         r->err = freader_get_folio(r, file_off);
 121         if (r->err)
 122                 return NULL;
 123
 124         /* if requested data is crossing folio boundaries, we have to copy
 125          * everything into our local buffer to keep a simple linear memory
 126          * access interface
 127          */
 128         folio_sz = folio_size(r->folio);
 129         if (file_off + sz > r->folio_off + folio_sz) {
 130                 int part_sz = r->folio_off + folio_sz - file_off;
 131
 132                 /* copy the part that resides in the current folio */
 133                 memcpy(r->buf, r->addr + (file_off - r->folio_off), part_sz);
 134
 135                 /* fetch next folio */
 136                 r->err = freader_get_folio(r, r->folio_off + folio_sz);
 137                 if (r->err)
 138                         return NULL;
 139
 140                 /* copy the rest of requested data */
 141                 memcpy(r->buf + part_sz, r->addr, sz - part_sz);
 142
 143                 return r->buf;
 144         }
 145
 146         /* if data fits in a single folio, just return direct pointer */
 147         return r->addr + (file_off - r->folio_off);
 148 }
 149
 150 static void freader_cleanup(struct freader *r)
 151 {
 152         if (!r->buf)
 153                 return; /* non-file-backed mode */
 154
 155         freader_put_folio(r);
 156 }
 157
 158 /*
 159  * Parse build id from the note segment. This logic can be shared between
 160  * 32-bit and 64-bit system, because Elf32_Nhdr and Elf64_Nhdr are
 161  * identical.
 162  */
 163 static int parse_build_id(struct freader *r, unsigned char *build_id, __u32 *size,
 164                           loff_t note_off, Elf32_Word note_size)
 165 {
 166         const char note_name[] = "GNU";
 167         const size_t note_name_sz = sizeof(note_name);
 168         u32 build_id_off, new_off, note_end, name_sz, desc_sz;
 169         const Elf32_Nhdr *nhdr;
 170         const char *data;
 171
 172         if (check_add_overflow(note_off, note_size, &note_end))
 173                 return -EINVAL;
 174
 175         while (note_end - note_off > sizeof(Elf32_Nhdr) + note_name_sz) {
 176                 nhdr = freader_fetch(r, note_off, sizeof(Elf32_Nhdr) + note_name_sz);
 177                 if (!nhdr)
 178                         return r->err;
 179
 180                 name_sz = READ_ONCE(nhdr->n_namesz);
 181                 desc_sz = READ_ONCE(nhdr->n_descsz);
 182
 183                 new_off = note_off + sizeof(Elf32_Nhdr);
 184                 if (check_add_overflow(new_off, ALIGN(name_sz, 4), &new_off) ||
 185                     check_add_overflow(new_off, ALIGN(desc_sz, 4), &new_off) ||
 186                     new_off > note_end)
 187                         break;
 188
 189                 if (nhdr->n_type == BUILD_ID &&
 190                     name_sz == note_name_sz &&
 191                     memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
 192                     desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
 193                         build_id_off = note_off + sizeof(Elf32_Nhdr) + ALIGN(note_name_sz, 4);
 194
 195                         /* freader_fetch() will invalidate nhdr pointer */
 196                         data = freader_fetch(r, build_id_off, desc_sz);
 197                         if (!data)
 198                                 return r->err;
 199
 200                         memcpy(build_id, data, desc_sz);
 201                         memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
 202                         if (size)
 203                                 *size = desc_sz;
 204                         return 0;
 205                 }
 206
 207                 note_off = new_off;
 208         }
 209
 210         return -EINVAL;
 211 }
 212
 213 /* Parse build ID from 32-bit ELF */
 214 static int get_build_id_32(struct freader *r, unsigned char *build_id, __u32 *size)
 215 {
 216         const Elf32_Ehdr *ehdr;
 217         const Elf32_Phdr *phdr;
 218         __u32 phnum, phoff, i;
 219
 220         ehdr = freader_fetch(r, 0, sizeof(Elf32_Ehdr));
 221         if (!ehdr)
 222                 return r->err;
 223
 224         /* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 225         phnum = READ_ONCE(ehdr->e_phnum);
 226         phoff = READ_ONCE(ehdr->e_phoff);
 227
 228         /* set upper bound on amount of segments (phdrs) we iterate */
 229         if (phnum > MAX_PHDR_CNT)
 230                 phnum = MAX_PHDR_CNT;
 231
 232         /* check that phoff is not large enough to cause an overflow */
 233         if (phoff + phnum * sizeof(Elf32_Phdr) < phoff)
 234                 return -EINVAL;
 235
 236         for (i = 0; i < phnum; ++i) {
 237                 phdr = freader_fetch(r, phoff + i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
 238                 if (!phdr)
 239                         return r->err;
 240
 241                 if (phdr->p_type == PT_NOTE &&
 242                     !parse_build_id(r, build_id, size, READ_ONCE(phdr->p_offset),
 243                                     READ_ONCE(phdr->p_filesz)))
 244                         return 0;
 245         }
 246         return -EINVAL;
 247 }
 248
 249 /* Parse build ID from 64-bit ELF */
 250 static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *size)
 251 {
 252         const Elf64_Ehdr *ehdr;
 253         const Elf64_Phdr *phdr;
 254         __u32 phnum, i;
 255         __u64 phoff;
 256
 257         ehdr = freader_fetch(r, 0, sizeof(Elf64_Ehdr));
 258         if (!ehdr)
 259                 return r->err;
 260
 261         /* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 262         phnum = READ_ONCE(ehdr->e_phnum);
 263         phoff = READ_ONCE(ehdr->e_phoff);
 264
 265         /* set upper bound on amount of segments (phdrs) we iterate */
 266         if (phnum > MAX_PHDR_CNT)
 267                 phnum = MAX_PHDR_CNT;
 268
 269         /* check that phoff is not large enough to cause an overflow */
 270         if (phoff + phnum * sizeof(Elf64_Phdr) < phoff)
 271                 return -EINVAL;
 272
 273         for (i = 0; i < phnum; ++i) {
 274                 phdr = freader_fetch(r, phoff + i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
 275                 if (!phdr)
 276                         return r->err;
 277
 278                 if (phdr->p_type == PT_NOTE &&
 279                     !parse_build_id(r, build_id, size, READ_ONCE(phdr->p_offset),
 280                                     READ_ONCE(phdr->p_filesz)))
 281                         return 0;
 282         }
 283
 284         return -EINVAL;
 285 }
 286
 287 /* enough for Elf64_Ehdr, Elf64_Phdr, and all the smaller requests */
 288 #define MAX_FREADER_BUF_SZ 64
 289
 290 static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
 291                             __u32 *size, bool may_fault)
 292 {
 293         const Elf32_Ehdr *ehdr;
 294         struct freader r;
 295         char buf[MAX_FREADER_BUF_SZ];
 296         int ret;
 297
 298         /* only works for page backed storage  */
 299         if (!vma->vm_file)
 300                 return -EINVAL;
 301
 302         freader_init_from_file(&r, buf, sizeof(buf), vma->vm_file, may_fault);
 303
 304         /* fetch first 18 bytes of ELF header for checks */
 305         ehdr = freader_fetch(&r, 0, offsetofend(Elf32_Ehdr, e_type));
 306         if (!ehdr) {
 307                 ret = r.err;
 308                 goto out;
 309         }
 310
 311         ret = -EINVAL;
 312
 313         /* compare magic x7f "ELF" */
 314         if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
 315                 goto out;
 316
 317         /* only support executable file and shared object file */
 318         if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
 319                 goto out;
 320
 321         if (ehdr->e_ident[EI_CLASS] == ELFCLASS32)
 322                 ret = get_build_id_32(&r, build_id, size);
 323         else if (ehdr->e_ident[EI_CLASS] == ELFCLASS64)
 324                 ret = get_build_id_64(&r, build_id, size);
 325 out:
 326         freader_cleanup(&r);
 327         return ret;
 328 }
 329
 330 /*
 331  * Parse build ID of ELF file mapped to vma
 332  * @vma:      vma object
 333  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
 334  * @size:     returns actual build id size in case of success
 335  *
 336  * Assumes no page fault can be taken, so if relevant portions of ELF file are
 337  * not already paged in, fetching of build ID fails.
 338  *
 339  * Return: 0 on success; negative error, otherwise
 340  */
 341 int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 342 {
 343         return __build_id_parse(vma, build_id, size, false /* !may_fault */);
 344 }
 345
 346 /*
 347  * Parse build ID of ELF file mapped to VMA
 348  * @vma:      vma object
 349  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
 350  * @size:     returns actual build id size in case of success
 351  *
 352  * Assumes faultable context and can cause page faults to bring in file data
 353  * into page cache.
 354  *
 355  * Return: 0 on success; negative error, otherwise
 356  */
 357 int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 358 {
 359         return __build_id_parse(vma, build_id, size, true /* may_fault */);
 360 }
 361
 362 /**
 363  * build_id_parse_buf - Get build ID from a buffer
 364  * @buf:      ELF note section(s) to parse
 365  * @buf_size: Size of @buf in bytes
 366  * @build_id: Build ID parsed from @buf, at least BUILD_ID_SIZE_MAX long
 367  *
 368  * Return: 0 on success, -EINVAL otherwise
 369  */
 370 int build_id_parse_buf(const void *buf, unsigned char *build_id, u32 buf_size)
 371 {
 372         struct freader r;
 373         int err;
 374
 375         freader_init_from_mem(&r, buf, buf_size);
 376
 377         err = parse_build_id(&r, build_id, NULL, 0, buf_size);
 378
 379         freader_cleanup(&r);
 380         return err;
 381 }
 382
 383 #if IS_ENABLED(CONFIG_STACKTRACE_BUILD_ID) || IS_ENABLED(CONFIG_VMCORE_INFO)
 384 unsigned char vmlinux_build_id[BUILD_ID_SIZE_MAX] __ro_after_init;
 385
 386 /**
 387  * init_vmlinux_build_id - Compute and stash the running kernel's build ID
 388  */
 389 void __init init_vmlinux_build_id(void)
 390 {
 391         extern const void __start_notes;
 392         extern const void __stop_notes;
 393         unsigned int size = &__stop_notes - &__start_notes;
 394
 395         build_id_parse_buf(&__start_notes, vmlinux_build_id, size);
 396 }
 397 #endif