xref: /openbmc/linux/mm/memfd.c (revision 60630924bb5af8751adcecc896e7763c3783ca89)
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