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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * memfd_create system call and file sealing support
4 *
5 * Code was originally included in shmem.c, and broken out to facilitate
6 * use by hugetlbfs as well as tmpfs.
7 */
8
9#include <linux/fs.h>
10#include <linux/vfs.h>
11#include <linux/pagemap.h>
12#include <linux/file.h>
13#include <linux/mm.h>
14#include <linux/sched/signal.h>
15#include <linux/khugepaged.h>
16#include <linux/syscalls.h>
17#include <linux/hugetlb.h>
18#include <linux/shmem_fs.h>
19#include <linux/memfd.h>
20#include <linux/pid_namespace.h>
21#include <uapi/linux/memfd.h>
22#include "internal.h"
23#include "swap.h"
24
25/*
26 * We need a tag: a new tag would expand every xa_node by 8 bytes,
27 * so reuse a tag which we firmly believe is never set or cleared on tmpfs
28 * or hugetlbfs because they are memory only filesystems.
29 */
30#define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
31#define LAST_SCAN 4 /* about 150ms max */
32
33static bool memfd_folio_has_extra_refs(struct folio *folio)
34{
35 return folio_ref_count(folio) != folio_expected_ref_count(folio);
36}
37
38static void memfd_tag_pins(struct xa_state *xas)
39{
40 struct folio *folio;
41 int latency = 0;
42
43 lru_add_drain();
44
45 xas_lock_irq(xas);
46 xas_for_each(xas, folio, ULONG_MAX) {
47 if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
48 xas_set_mark(xas, MEMFD_TAG_PINNED);
49
50 if (++latency < XA_CHECK_SCHED)
51 continue;
52 latency = 0;
53
54 xas_pause(xas);
55 xas_unlock_irq(xas);
56 cond_resched();
57 xas_lock_irq(xas);
58 }
59 xas_unlock_irq(xas);
60}
61
62/*
63 * This is a helper function used by memfd_pin_user_pages() in GUP (gup.c).
64 * It is mainly called to allocate a folio in a memfd when the caller
65 * (memfd_pin_folios()) cannot find a folio in the page cache at a given
66 * index in the mapping.
67 */
68struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
69{
70#ifdef CONFIG_HUGETLB_PAGE
71 struct folio *folio;
72 gfp_t gfp_mask;
73
74 if (is_file_hugepages(memfd)) {
75 /*
76 * The folio would most likely be accessed by a DMA driver,
77 * therefore, we have zone memory constraints where we can
78 * alloc from. Also, the folio will be pinned for an indefinite
79 * amount of time, so it is not expected to be migrated away.
80 */
81 struct inode *inode = file_inode(memfd);
82 struct hstate *h = hstate_file(memfd);
83 int err = -ENOMEM;
84 long nr_resv;
85
86 gfp_mask = htlb_alloc_mask(h);
87 gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
88 idx >>= huge_page_order(h);
89
90 nr_resv = hugetlb_reserve_pages(inode, idx, idx + 1, NULL, EMPTY_VMA_FLAGS);
91 if (nr_resv < 0)
92 return ERR_PTR(nr_resv);
93
94 folio = alloc_hugetlb_folio_reserve(h,
95 numa_node_id(),
96 NULL,
97 gfp_mask);
98 if (folio) {
99 u32 hash;
100
101 /*
102 * Zero the folio to prevent information leaks to userspace.
103 * Use folio_zero_user() which is optimized for huge/gigantic
104 * pages. Pass 0 as addr_hint since this is not a faulting path
105 * and we don't have a user virtual address yet.
106 */
107 folio_zero_user(folio, 0);
108
109 /*
110 * Mark the folio uptodate before adding to page cache,
111 * as required by filemap.c and other hugetlb paths.
112 */
113 __folio_mark_uptodate(folio);
114
115 /*
116 * Serialize hugepage allocation and instantiation to prevent
117 * races with concurrent allocations, as required by all other
118 * callers of hugetlb_add_to_page_cache().
119 */
120 hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
121 mutex_lock(&hugetlb_fault_mutex_table[hash]);
122
123 err = hugetlb_add_to_page_cache(folio,
124 memfd->f_mapping,
125 idx);
126
127 mutex_unlock(&hugetlb_fault_mutex_table[hash]);
128
129 if (err) {
130 folio_put(folio);
131 goto err_unresv;
132 }
133
134 hugetlb_set_folio_subpool(folio, subpool_inode(inode));
135 folio_unlock(folio);
136 return folio;
137 }
138err_unresv:
139 if (nr_resv > 0)
140 hugetlb_unreserve_pages(inode, idx, idx + 1, 0);
141 return ERR_PTR(err);
142 }
143#endif
144 return shmem_read_folio(memfd->f_mapping, idx);
145}
146
147/*
148 * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
149 * via get_user_pages(), drivers might have some pending I/O without any active
150 * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
151 * and see whether it has an elevated ref-count. If so, we tag them and wait for
152 * them to be dropped.
153 * The caller must guarantee that no new user will acquire writable references
154 * to those folios to avoid races.
155 */
156static int memfd_wait_for_pins(struct address_space *mapping)
157{
158 XA_STATE(xas, &mapping->i_pages, 0);
159 struct folio *folio;
160 int error, scan;
161
162 memfd_tag_pins(&xas);
163
164 error = 0;
165 for (scan = 0; scan <= LAST_SCAN; scan++) {
166 int latency = 0;
167
168 if (!xas_marked(&xas, MEMFD_TAG_PINNED))
169 break;
170
171 if (!scan)
172 lru_add_drain_all();
173 else if (schedule_timeout_killable((HZ << scan) / 200))
174 scan = LAST_SCAN;
175
176 xas_set(&xas, 0);
177 xas_lock_irq(&xas);
178 xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
179 bool clear = true;
180
181 if (!xa_is_value(folio) &&
182 memfd_folio_has_extra_refs(folio)) {
183 /*
184 * On the last scan, we clean up all those tags
185 * we inserted; but make a note that we still
186 * found folios pinned.
187 */
188 if (scan == LAST_SCAN)
189 error = -EBUSY;
190 else
191 clear = false;
192 }
193 if (clear)
194 xas_clear_mark(&xas, MEMFD_TAG_PINNED);
195
196 if (++latency < XA_CHECK_SCHED)
197 continue;
198 latency = 0;
199
200 xas_pause(&xas);
201 xas_unlock_irq(&xas);
202 cond_resched();
203 xas_lock_irq(&xas);
204 }
205 xas_unlock_irq(&xas);
206 }
207
208 return error;
209}
210
211static unsigned int *memfd_file_seals_ptr(struct file *file)
212{
213 if (shmem_file(file))
214 return &SHMEM_I(file_inode(file))->seals;
215
216#ifdef CONFIG_HUGETLBFS
217 if (is_file_hugepages(file))
218 return &HUGETLBFS_I(file_inode(file))->seals;
219#endif
220
221 return NULL;
222}
223
224#define F_ALL_SEALS (F_SEAL_SEAL | \
225 F_SEAL_EXEC | \
226 F_SEAL_SHRINK | \
227 F_SEAL_GROW | \
228 F_SEAL_WRITE | \
229 F_SEAL_FUTURE_WRITE)
230
231int memfd_add_seals(struct file *file, unsigned int seals)
232{
233 struct inode *inode = file_inode(file);
234 unsigned int *file_seals;
235 int error;
236
237 /*
238 * SEALING
239 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
240 * but restrict access to a specific subset of file operations. Seals
241 * can only be added, but never removed. This way, mutually untrusted
242 * parties can share common memory regions with a well-defined policy.
243 * A malicious peer can thus never perform unwanted operations on a
244 * shared object.
245 *
246 * Seals are only supported on special tmpfs or hugetlbfs files and
247 * always affect the whole underlying inode. Once a seal is set, it
248 * may prevent some kinds of access to the file. Currently, the
249 * following seals are defined:
250 * SEAL_SEAL: Prevent further seals from being set on this file
251 * SEAL_SHRINK: Prevent the file from shrinking
252 * SEAL_GROW: Prevent the file from growing
253 * SEAL_WRITE: Prevent write access to the file
254 * SEAL_EXEC: Prevent modification of the exec bits in the file mode
255 *
256 * As we don't require any trust relationship between two parties, we
257 * must prevent seals from being removed. Therefore, sealing a file
258 * only adds a given set of seals to the file, it never touches
259 * existing seals. Furthermore, the "setting seals"-operation can be
260 * sealed itself, which basically prevents any further seal from being
261 * added.
262 *
263 * Semantics of sealing are only defined on volatile files. Only
264 * anonymous tmpfs and hugetlbfs files support sealing. More
265 * importantly, seals are never written to disk. Therefore, there's
266 * no plan to support it on other file types.
267 */
268
269 if (!(file->f_mode & FMODE_WRITE))
270 return -EPERM;
271 if (seals & ~(unsigned int)F_ALL_SEALS)
272 return -EINVAL;
273
274 inode_lock(inode);
275
276 file_seals = memfd_file_seals_ptr(file);
277 if (!file_seals) {
278 error = -EINVAL;
279 goto unlock;
280 }
281
282 if (*file_seals & F_SEAL_SEAL) {
283 error = -EPERM;
284 goto unlock;
285 }
286
287 /*
288 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
289 */
290 if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
291 seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
292
293 if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
294 error = mapping_deny_writable(file->f_mapping);
295 if (error)
296 goto unlock;
297
298 error = memfd_wait_for_pins(file->f_mapping);
299 if (error) {
300 mapping_allow_writable(file->f_mapping);
301 goto unlock;
302 }
303 }
304
305 *file_seals |= seals;
306 error = 0;
307
308unlock:
309 inode_unlock(inode);
310 return error;
311}
312
313int memfd_get_seals(struct file *file)
314{
315 unsigned int *seals = memfd_file_seals_ptr(file);
316
317 return seals ? *seals : -EINVAL;
318}
319
320long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
321{
322 long error;
323
324 switch (cmd) {
325 case F_ADD_SEALS:
326 error = memfd_add_seals(file, arg);
327 break;
328 case F_GET_SEALS:
329 error = memfd_get_seals(file);
330 break;
331 default:
332 error = -EINVAL;
333 break;
334 }
335
336 return error;
337}
338
339#define MFD_NAME_PREFIX "memfd:"
340#define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
341#define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
342
343#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)
344
345static int check_sysctl_memfd_noexec(unsigned int *flags)
346{
347#ifdef CONFIG_SYSCTL
348 struct pid_namespace *ns = task_active_pid_ns(current);
349 int sysctl = pidns_memfd_noexec_scope(ns);
350
351 if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
352 if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL)
353 *flags |= MFD_NOEXEC_SEAL;
354 else
355 *flags |= MFD_EXEC;
356 }
357
358 if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) {
359 pr_err_ratelimited(
360 "%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n",
361 current->comm, task_pid_nr(current), sysctl);
362 return -EACCES;
363 }
364#endif
365 return 0;
366}
367
368static inline bool is_write_sealed(unsigned int seals)
369{
370 return seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE);
371}
372
373static int check_write_seal(vma_flags_t *vma_flags_ptr)
374{
375 /* If a private mapping then writability is irrelevant. */
376 if (!vma_flags_test(vma_flags_ptr, VMA_SHARED_BIT))
377 return 0;
378
379 /*
380 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
381 * write seals are active.
382 */
383 if (vma_flags_test(vma_flags_ptr, VMA_WRITE_BIT))
384 return -EPERM;
385
386 /*
387 * This is a read-only mapping, disallow mprotect() from making a
388 * write-sealed mapping writable in future.
389 */
390 vma_flags_clear(vma_flags_ptr, VMA_MAYWRITE_BIT);
391
392 return 0;
393}
394
395int memfd_check_seals_mmap(struct file *file, vma_flags_t *vma_flags_ptr)
396{
397 int err = 0;
398 unsigned int *seals_ptr = memfd_file_seals_ptr(file);
399 unsigned int seals = seals_ptr ? *seals_ptr : 0;
400
401 if (is_write_sealed(seals))
402 err = check_write_seal(vma_flags_ptr);
403
404 return err;
405}
406
407static int sanitize_flags(unsigned int *flags_ptr)
408{
409 unsigned int flags = *flags_ptr;
410
411 if (!(flags & MFD_HUGETLB)) {
412 if (flags & ~MFD_ALL_FLAGS)
413 return -EINVAL;
414 } else {
415 /* Allow huge page size encoding in flags. */
416 if (flags & ~(MFD_ALL_FLAGS |
417 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
418 return -EINVAL;
419 }
420
421 /* Invalid if both EXEC and NOEXEC_SEAL are set.*/
422 if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
423 return -EINVAL;
424
425 return check_sysctl_memfd_noexec(flags_ptr);
426}
427
428static char *alloc_name(const char __user *uname)
429{
430 int error;
431 char *name;
432 long len;
433
434 name = kmalloc(NAME_MAX + 1, GFP_KERNEL);
435 if (!name)
436 return ERR_PTR(-ENOMEM);
437
438 memcpy(name, MFD_NAME_PREFIX, MFD_NAME_PREFIX_LEN);
439 /* returned length does not include terminating zero */
440 len = strncpy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, MFD_NAME_MAX_LEN + 1);
441 if (len < 0) {
442 error = -EFAULT;
443 goto err_name;
444 } else if (len > MFD_NAME_MAX_LEN) {
445 error = -EINVAL;
446 goto err_name;
447 }
448
449 return name;
450
451err_name:
452 kfree(name);
453 return ERR_PTR(error);
454}
455
456struct file *memfd_alloc_file(const char *name, unsigned int flags)
457{
458 unsigned int *file_seals;
459 struct file *file;
460 struct inode *inode;
461 int err = 0;
462
463 if (flags & MFD_HUGETLB) {
464 file = hugetlb_file_setup(name, 0, mk_vma_flags(VMA_NORESERVE_BIT),
465 HUGETLB_ANONHUGE_INODE,
466 (flags >> MFD_HUGE_SHIFT) &
467 MFD_HUGE_MASK);
468 } else {
469 file = shmem_file_setup(name, 0, mk_vma_flags(VMA_NORESERVE_BIT));
470 }
471 if (IS_ERR(file))
472 return file;
473
474 inode = file_inode(file);
475 err = security_inode_init_security_anon(inode,
476 &QSTR(MEMFD_ANON_NAME), NULL);
477 if (err) {
478 fput(file);
479 file = ERR_PTR(err);
480 return file;
481 }
482
483 file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
484 file->f_flags |= O_LARGEFILE;
485
486 if (flags & MFD_NOEXEC_SEAL) {
487 inode->i_mode &= ~0111;
488 file_seals = memfd_file_seals_ptr(file);
489 if (file_seals) {
490 *file_seals &= ~F_SEAL_SEAL;
491 *file_seals |= F_SEAL_EXEC;
492 }
493 } else if (flags & MFD_ALLOW_SEALING) {
494 /* MFD_EXEC and MFD_ALLOW_SEALING are set */
495 file_seals = memfd_file_seals_ptr(file);
496 if (file_seals)
497 *file_seals &= ~F_SEAL_SEAL;
498 }
499
500 return file;
501}
502
503SYSCALL_DEFINE2(memfd_create,
504 const char __user *, uname,
505 unsigned int, flags)
506{
507 char *name __free(kfree) = NULL;
508 unsigned int fd_flags;
509 int error;
510
511 error = sanitize_flags(&flags);
512 if (error < 0)
513 return error;
514
515 name = alloc_name(uname);
516 if (IS_ERR(name))
517 return PTR_ERR(name);
518
519 fd_flags = (flags & MFD_CLOEXEC) ? O_CLOEXEC : 0;
520 return FD_ADD(fd_flags, memfd_alloc_file(name, flags));
521}