xref: /openbmc/linux/mm/memfd.c (revision 2562d67b)
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 <linux/pid_namespace.h>
22 #include <uapi/linux/memfd.h>
23 
24 /*
25  * We need a tag: a new tag would expand every xa_node by 8 bytes,
26  * so reuse a tag which we firmly believe is never set or cleared on tmpfs
27  * or hugetlbfs because they are memory only filesystems.
28  */
29 #define MEMFD_TAG_PINNED        PAGECACHE_TAG_TOWRITE
30 #define LAST_SCAN               4       /* about 150ms max */
31 
memfd_tag_pins(struct xa_state * xas)32 static void memfd_tag_pins(struct xa_state *xas)
33 {
34 	struct page *page;
35 	int latency = 0;
36 	int cache_count;
37 
38 	lru_add_drain();
39 
40 	xas_lock_irq(xas);
41 	xas_for_each(xas, page, ULONG_MAX) {
42 		cache_count = 1;
43 		if (!xa_is_value(page) &&
44 		    PageTransHuge(page) && !PageHuge(page))
45 			cache_count = HPAGE_PMD_NR;
46 
47 		if (!xa_is_value(page) &&
48 		    page_count(page) - total_mapcount(page) != cache_count)
49 			xas_set_mark(xas, MEMFD_TAG_PINNED);
50 		if (cache_count != 1)
51 			xas_set(xas, page->index + cache_count);
52 
53 		latency += cache_count;
54 		if (latency < XA_CHECK_SCHED)
55 			continue;
56 		latency = 0;
57 
58 		xas_pause(xas);
59 		xas_unlock_irq(xas);
60 		cond_resched();
61 		xas_lock_irq(xas);
62 	}
63 	xas_unlock_irq(xas);
64 }
65 
66 /*
67  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
68  * via get_user_pages(), drivers might have some pending I/O without any active
69  * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
70  * and see whether it has an elevated ref-count. If so, we tag them and wait for
71  * them to be dropped.
72  * The caller must guarantee that no new user will acquire writable references
73  * to those pages to avoid races.
74  */
memfd_wait_for_pins(struct address_space * mapping)75 static int memfd_wait_for_pins(struct address_space *mapping)
76 {
77 	XA_STATE(xas, &mapping->i_pages, 0);
78 	struct page *page;
79 	int error, scan;
80 
81 	memfd_tag_pins(&xas);
82 
83 	error = 0;
84 	for (scan = 0; scan <= LAST_SCAN; scan++) {
85 		int latency = 0;
86 		int cache_count;
87 
88 		if (!xas_marked(&xas, MEMFD_TAG_PINNED))
89 			break;
90 
91 		if (!scan)
92 			lru_add_drain_all();
93 		else if (schedule_timeout_killable((HZ << scan) / 200))
94 			scan = LAST_SCAN;
95 
96 		xas_set(&xas, 0);
97 		xas_lock_irq(&xas);
98 		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
99 			bool clear = true;
100 
101 			cache_count = 1;
102 			if (!xa_is_value(page) &&
103 			    PageTransHuge(page) && !PageHuge(page))
104 				cache_count = HPAGE_PMD_NR;
105 
106 			if (!xa_is_value(page) && cache_count !=
107 			    page_count(page) - total_mapcount(page)) {
108 				/*
109 				 * On the last scan, we clean up all those tags
110 				 * we inserted; but make a note that we still
111 				 * found pages pinned.
112 				 */
113 				if (scan == LAST_SCAN)
114 					error = -EBUSY;
115 				else
116 					clear = false;
117 			}
118 			if (clear)
119 				xas_clear_mark(&xas, MEMFD_TAG_PINNED);
120 
121 			latency += cache_count;
122 			if (latency < XA_CHECK_SCHED)
123 				continue;
124 			latency = 0;
125 
126 			xas_pause(&xas);
127 			xas_unlock_irq(&xas);
128 			cond_resched();
129 			xas_lock_irq(&xas);
130 		}
131 		xas_unlock_irq(&xas);
132 	}
133 
134 	return error;
135 }
136 
memfd_file_seals_ptr(struct file * file)137 static unsigned int *memfd_file_seals_ptr(struct file *file)
138 {
139 	if (shmem_file(file))
140 		return &SHMEM_I(file_inode(file))->seals;
141 
142 #ifdef CONFIG_HUGETLBFS
143 	if (is_file_hugepages(file))
144 		return &HUGETLBFS_I(file_inode(file))->seals;
145 #endif
146 
147 	return NULL;
148 }
149 
150 #define F_ALL_SEALS (F_SEAL_SEAL | \
151 		     F_SEAL_EXEC | \
152 		     F_SEAL_SHRINK | \
153 		     F_SEAL_GROW | \
154 		     F_SEAL_WRITE | \
155 		     F_SEAL_FUTURE_WRITE)
156 
memfd_add_seals(struct file * file,unsigned int seals)157 static int memfd_add_seals(struct file *file, unsigned int seals)
158 {
159 	struct inode *inode = file_inode(file);
160 	unsigned int *file_seals;
161 	int error;
162 
163 	/*
164 	 * SEALING
165 	 * Sealing allows multiple parties to share a tmpfs or hugetlbfs file
166 	 * but restrict access to a specific subset of file operations. Seals
167 	 * can only be added, but never removed. This way, mutually untrusted
168 	 * parties can share common memory regions with a well-defined policy.
169 	 * A malicious peer can thus never perform unwanted operations on a
170 	 * shared object.
171 	 *
172 	 * Seals are only supported on special tmpfs or hugetlbfs files and
173 	 * always affect the whole underlying inode. Once a seal is set, it
174 	 * may prevent some kinds of access to the file. Currently, the
175 	 * following seals are defined:
176 	 *   SEAL_SEAL: Prevent further seals from being set on this file
177 	 *   SEAL_SHRINK: Prevent the file from shrinking
178 	 *   SEAL_GROW: Prevent the file from growing
179 	 *   SEAL_WRITE: Prevent write access to the file
180 	 *   SEAL_EXEC: Prevent modification of the exec bits in the file mode
181 	 *
182 	 * As we don't require any trust relationship between two parties, we
183 	 * must prevent seals from being removed. Therefore, sealing a file
184 	 * only adds a given set of seals to the file, it never touches
185 	 * existing seals. Furthermore, the "setting seals"-operation can be
186 	 * sealed itself, which basically prevents any further seal from being
187 	 * added.
188 	 *
189 	 * Semantics of sealing are only defined on volatile files. Only
190 	 * anonymous tmpfs and hugetlbfs files support sealing. More
191 	 * importantly, seals are never written to disk. Therefore, there's
192 	 * no plan to support it on other file types.
193 	 */
194 
195 	if (!(file->f_mode & FMODE_WRITE))
196 		return -EPERM;
197 	if (seals & ~(unsigned int)F_ALL_SEALS)
198 		return -EINVAL;
199 
200 	inode_lock(inode);
201 
202 	file_seals = memfd_file_seals_ptr(file);
203 	if (!file_seals) {
204 		error = -EINVAL;
205 		goto unlock;
206 	}
207 
208 	if (*file_seals & F_SEAL_SEAL) {
209 		error = -EPERM;
210 		goto unlock;
211 	}
212 
213 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
214 		error = mapping_deny_writable(file->f_mapping);
215 		if (error)
216 			goto unlock;
217 
218 		error = memfd_wait_for_pins(file->f_mapping);
219 		if (error) {
220 			mapping_allow_writable(file->f_mapping);
221 			goto unlock;
222 		}
223 	}
224 
225 	/*
226 	 * SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
227 	 */
228 	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
229 		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
230 
231 	*file_seals |= seals;
232 	error = 0;
233 
234 unlock:
235 	inode_unlock(inode);
236 	return error;
237 }
238 
memfd_get_seals(struct file * file)239 static int memfd_get_seals(struct file *file)
240 {
241 	unsigned int *seals = memfd_file_seals_ptr(file);
242 
243 	return seals ? *seals : -EINVAL;
244 }
245 
memfd_fcntl(struct file * file,unsigned int cmd,unsigned int arg)246 long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
247 {
248 	long error;
249 
250 	switch (cmd) {
251 	case F_ADD_SEALS:
252 		error = memfd_add_seals(file, arg);
253 		break;
254 	case F_GET_SEALS:
255 		error = memfd_get_seals(file);
256 		break;
257 	default:
258 		error = -EINVAL;
259 		break;
260 	}
261 
262 	return error;
263 }
264 
265 #define MFD_NAME_PREFIX "memfd:"
266 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
267 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
268 
269 #define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_NOEXEC_SEAL | MFD_EXEC)
270 
check_sysctl_memfd_noexec(unsigned int * flags)271 static int check_sysctl_memfd_noexec(unsigned int *flags)
272 {
273 #ifdef CONFIG_SYSCTL
274 	struct pid_namespace *ns = task_active_pid_ns(current);
275 	int sysctl = pidns_memfd_noexec_scope(ns);
276 
277 	if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
278 		if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL)
279 			*flags |= MFD_NOEXEC_SEAL;
280 		else
281 			*flags |= MFD_EXEC;
282 	}
283 
284 	if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) {
285 		pr_err_ratelimited(
286 			"%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n",
287 			current->comm, task_pid_nr(current), sysctl);
288 		return -EACCES;
289 	}
290 #endif
291 	return 0;
292 }
293 
SYSCALL_DEFINE2(memfd_create,const char __user *,uname,unsigned int,flags)294 SYSCALL_DEFINE2(memfd_create,
295 		const char __user *, uname,
296 		unsigned int, flags)
297 {
298 	unsigned int *file_seals;
299 	struct file *file;
300 	int fd, error;
301 	char *name;
302 	long len;
303 
304 	if (!(flags & MFD_HUGETLB)) {
305 		if (flags & ~(unsigned int)MFD_ALL_FLAGS)
306 			return -EINVAL;
307 	} else {
308 		/* Allow huge page size encoding in flags. */
309 		if (flags & ~(unsigned int)(MFD_ALL_FLAGS |
310 				(MFD_HUGE_MASK << MFD_HUGE_SHIFT)))
311 			return -EINVAL;
312 	}
313 
314 	/* Invalid if both EXEC and NOEXEC_SEAL are set.*/
315 	if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
316 		return -EINVAL;
317 
318 	if (!(flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
319 		pr_warn_once(
320 			"%s[%d]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set\n",
321 			current->comm, task_pid_nr(current));
322 	}
323 
324 	error = check_sysctl_memfd_noexec(&flags);
325 	if (error < 0)
326 		return error;
327 
328 	/* length includes terminating zero */
329 	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
330 	if (len <= 0)
331 		return -EFAULT;
332 	if (len > MFD_NAME_MAX_LEN + 1)
333 		return -EINVAL;
334 
335 	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);
336 	if (!name)
337 		return -ENOMEM;
338 
339 	strcpy(name, MFD_NAME_PREFIX);
340 	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {
341 		error = -EFAULT;
342 		goto err_name;
343 	}
344 
345 	/* terminating-zero may have changed after strnlen_user() returned */
346 	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {
347 		error = -EFAULT;
348 		goto err_name;
349 	}
350 
351 	fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0);
352 	if (fd < 0) {
353 		error = fd;
354 		goto err_name;
355 	}
356 
357 	if (flags & MFD_HUGETLB) {
358 		file = hugetlb_file_setup(name, 0, VM_NORESERVE,
359 					HUGETLB_ANONHUGE_INODE,
360 					(flags >> MFD_HUGE_SHIFT) &
361 					MFD_HUGE_MASK);
362 	} else
363 		file = shmem_file_setup(name, 0, VM_NORESERVE);
364 	if (IS_ERR(file)) {
365 		error = PTR_ERR(file);
366 		goto err_fd;
367 	}
368 	file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
369 	file->f_flags |= O_LARGEFILE;
370 
371 	if (flags & MFD_NOEXEC_SEAL) {
372 		struct inode *inode = file_inode(file);
373 
374 		inode->i_mode &= ~0111;
375 		file_seals = memfd_file_seals_ptr(file);
376 		if (file_seals) {
377 			*file_seals &= ~F_SEAL_SEAL;
378 			*file_seals |= F_SEAL_EXEC;
379 		}
380 	} else if (flags & MFD_ALLOW_SEALING) {
381 		/* MFD_EXEC and MFD_ALLOW_SEALING are set */
382 		file_seals = memfd_file_seals_ptr(file);
383 		if (file_seals)
384 			*file_seals &= ~F_SEAL_SEAL;
385 	}
386 
387 	fd_install(fd, file);
388 	kfree(name);
389 	return fd;
390 
391 err_fd:
392 	put_unused_fd(fd);
393 err_name:
394 	kfree(name);
395 	return error;
396 }
397