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Linux/mm/memfd.c

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  1 /*
  2  * memfd_create system call and file sealing support
  3  *
  4  * Code was originally included in shmem.c, and broken out to facilitate
  5  * use by hugetlbfs as well as tmpfs.
  6  *
  7  * This file is released under the GPL.
  8  */
  9 
 10 #include <linux/fs.h>
 11 #include <linux/vfs.h>
 12 #include <linux/pagemap.h>
 13 #include <linux/file.h>
 14 #include <linux/mm.h>
 15 #include <linux/sched/signal.h>
 16 #include <linux/khugepaged.h>
 17 #include <linux/syscalls.h>
 18 #include <linux/hugetlb.h>
 19 #include <linux/shmem_fs.h>
 20 #include <linux/memfd.h>
 21 #include <uapi/linux/memfd.h>
 22 
 23 /*
 24  * We need a tag: a new tag would expand every xa_node by 8 bytes,
 25  * so reuse a tag which we firmly believe is never set or cleared on tmpfs
 26  * or hugetlbfs because they are memory only filesystems.
 27  */
 28 #define MEMFD_TAG_PINNED        PAGECACHE_TAG_TOWRITE
 29 #define LAST_SCAN               4       /* about 150ms max */
 30 
 31 static void memfd_tag_pins(struct xa_state *xas)
 32 {
 33         struct page *page;
 34         unsigned int tagged = 0;
 35 
 36         lru_add_drain();
 37 
 38         xas_lock_irq(xas);
 39         xas_for_each(xas, page, ULONG_MAX) {
 40                 if (xa_is_value(page))
 41                         continue;
 42                 page = find_subpage(page, xas->xa_index);
 43                 if (page_count(page) - page_mapcount(page) > 1)
 44                         xas_set_mark(xas, MEMFD_TAG_PINNED);
 45 
 46                 if (++tagged % XA_CHECK_SCHED)
 47                         continue;
 48 
 49                 xas_pause(xas);
 50                 xas_unlock_irq(xas);
 51                 cond_resched();
 52                 xas_lock_irq(xas);
 53         }
 54         xas_unlock_irq(xas);
 55 }
 56 
 57 /*
 58  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
 59  * via get_user_pages(), drivers might have some pending I/O without any active
 60  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
 61  * and see whether it has an elevated ref-count. If so, we tag them and wait for
 62  * them to be dropped.
 63  * The caller must guarantee that no new user will acquire writable references
 64  * to those pages to avoid races.
 65  */
 66 static int memfd_wait_for_pins(struct address_space *mapping)
 67 {
 68         XA_STATE(xas, &mapping->i_pages, 0);
 69         struct page *page;
 70         int error, scan;
 71 
 72         memfd_tag_pins(&xas);
 73 
 74         error = 0;
 75         for (scan = 0; scan <= LAST_SCAN; scan++) {
 76                 unsigned int tagged = 0;
 77 
 78                 if (!xas_marked(&xas, MEMFD_TAG_PINNED))
 79                         break;
 80 
 81                 if (!scan)
 82                         lru_add_drain_all();
 83                 else if (schedule_timeout_killable((HZ << scan) / 200))
 84                         scan = LAST_SCAN;
 85 
 86                 xas_set(&xas, 0);
 87                 xas_lock_irq(&xas);
 88                 xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
 89                         bool clear = true;
 90                         if (xa_is_value(page))
 91                                 continue;
 92                         page = find_subpage(page, xas.xa_index);
 93                         if (page_count(page) - page_mapcount(page) != 1) {
 94                                 /*
 95                                  * On the last scan, we clean up all those tags
 96                                  * we inserted; but make a note that we still
 97                                  * found pages pinned.
 98                                  */
 99                                 if (scan == LAST_SCAN)
100                                         error = -EBUSY;
101                                 else
102                                         clear = false;
103                         }
104                         if (clear)
105                                 xas_clear_mark(&xas, MEMFD_TAG_PINNED);
106                         if (++tagged % XA_CHECK_SCHED)
107                                 continue;
108 
109                         xas_pause(&xas);
110                         xas_unlock_irq(&xas);
111                         cond_resched();
112                         xas_lock_irq(&xas);
113                 }
114                 xas_unlock_irq(&xas);
115         }
116 
117         return error;
118 }
119 
120 static unsigned int *memfd_file_seals_ptr(struct file *file)
121 {
122         if (shmem_file(file))
123                 return &SHMEM_I(file_inode(file))->seals;
124 
125 #ifdef CONFIG_HUGETLBFS
126         if (is_file_hugepages(file))
127                 return &HUGETLBFS_I(file_inode(file))->seals;
128 #endif
129 
130         return NULL;
131 }
132 
133 #define F_ALL_SEALS (F_SEAL_SEAL | \
134                      F_SEAL_SHRINK | \
135                      F_SEAL_GROW | \
136                      F_SEAL_WRITE | \
137                      F_SEAL_FUTURE_WRITE)
138 
139 static int memfd_add_seals(struct file *file, unsigned int seals)
140 {
141         struct inode *inode = file_inode(file);
142         unsigned int *file_seals;
143         int error;
144 
145         /*
146          * SEALING
147          * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
148          * but restrict access to a specific subset of file operations. Seals
149          * can only be added, but never removed. This way, mutually untrusted
150          * parties can share common memory regions with a well-defined policy.
151          * A malicious peer can thus never perform unwanted operations on a
152          * shared object.
153          *
154          * Seals are only supported on special tmpfs or hugetlbfs files and
155          * always affect the whole underlying inode. Once a seal is set, it
156          * may prevent some kinds of access to the file. Currently, the
157          * following seals are defined:
158          *   SEAL_SEAL: Prevent further seals from being set on this file
159          *   SEAL_SHRINK: Prevent the file from shrinking
160          *   SEAL_GROW: Prevent the file from growing
161          *   SEAL_WRITE: Prevent write access to the file
162          *
163          * As we don't require any trust relationship between two parties, we
164          * must prevent seals from being removed. Therefore, sealing a file
165          * only adds a given set of seals to the file, it never touches
166          * existing seals. Furthermore, the "setting seals"-operation can be
167          * sealed itself, which basically prevents any further seal from being
168          * added.
169          *
170          * Semantics of sealing are only defined on volatile files. Only
171          * anonymous tmpfs and hugetlbfs files support sealing. More
172          * importantly, seals are never written to disk. Therefore, there's
173          * no plan to support it on other file types.
174          */
175 
176         if (!(file->f_mode & FMODE_WRITE))
177                 return -EPERM;
178         if (seals & ~(unsigned int)F_ALL_SEALS)
179                 return -EINVAL;
180 
181         inode_lock(inode);
182 
183         file_seals = memfd_file_seals_ptr(file);
184         if (!file_seals) {
185                 error = -EINVAL;
186                 goto unlock;
187         }
188 
189         if (*file_seals & F_SEAL_SEAL) {
190                 error = -EPERM;
191                 goto unlock;
192         }
193 
194         if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
195                 error = mapping_deny_writable(file->f_mapping);
196                 if (error)
197                         goto unlock;
198 
199                 error = memfd_wait_for_pins(file->f_mapping);
200                 if (error) {
201                         mapping_allow_writable(file->f_mapping);
202                         goto unlock;
203                 }
204         }
205 
206         *file_seals |= seals;
207         error = 0;
208 
209 unlock:
210         inode_unlock(inode);
211         return error;
212 }
213 
214 static int memfd_get_seals(struct file *file)
215 {
216         unsigned int *seals = memfd_file_seals_ptr(file);
217 
218         return seals ? *seals : -EINVAL;
219 }
220 
221 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
222 {
223         long error;
224 
225         switch (cmd) {
226         case F_ADD_SEALS:
227                 /* disallow upper 32bit */
228                 if (arg > UINT_MAX)
229                         return -EINVAL;
230 
231                 error = memfd_add_seals(file, arg);
232                 break;
233         case F_GET_SEALS:
234                 error = memfd_get_seals(file);
235                 break;
236         default:
237                 error = -EINVAL;
238                 break;
239         }
240 
241         return error;
242 }
243 
244 #define MFD_NAME_PREFIX "memfd:"
245 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
246 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
247 
248 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
249 
250 SYSCALL_DEFINE2(memfd_create,
251                 const char __user *, uname,
252                 unsigned int, flags)
253 {
254         unsigned int *file_seals;
255         struct file *file;
256         int fd, error;
257         char *name;
258         long len;
259 
260         if (!(flags & MFD_HUGETLB)) {
261                 if (flags & ~(unsigned int)MFD_ALL_FLAGS)
262                         return -EINVAL;
263         } else {
264                 /* Allow huge page size encoding in flags. */
265                 if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
266                                 (MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
267                         return -EINVAL;
268         }
269 
270         /* length includes terminating zero */
271         len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
272         if (len <= 0)
273                 return -EFAULT;
274         if (len > MFD_NAME_MAX_LEN + 1)
275                 return -EINVAL;
276 
277         name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
278         if (!name)
279                 return -ENOMEM;
280 
281         strcpy(name, MFD_NAME_PREFIX);
282         if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
283                 error = -EFAULT;
284                 goto err_name;
285         }
286 
287         /* terminating-zero may have changed after strnlen_user() returned */
288         if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
289                 error = -EFAULT;
290                 goto err_name;
291         }
292 
293         fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
294         if (fd < 0) {
295                 error = fd;
296                 goto err_name;
297         }
298 
299         if (flags & MFD_HUGETLB) {
300                 file = hugetlb_file_setup(name, 0, VM_NORESERVE,
301                                         HUGETLB_ANONHUGE_INODE,
302                                         (flags >> MFD_HUGE_SHIFT) &
303                                         MFD_HUGE_MASK);
304         } else
305                 file = shmem_file_setup(name, 0, VM_NORESERVE);
306         if (IS_ERR(file)) {
307                 error = PTR_ERR(file);
308                 goto err_fd;
309         }
310         file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
311         file->f_flags |= O_LARGEFILE;
312 
313         if (flags & MFD_ALLOW_SEALING) {
314                 file_seals = memfd_file_seals_ptr(file);
315                 *file_seals &= ~F_SEAL_SEAL;
316         }
317 
318         fd_install(fd, file);
319         kfree(name);
320         return fd;
321 
322 err_fd:
323         put_unused_fd(fd);
324 err_name:
325         kfree(name);
326         return error;
327 }
328 

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