xref: /openbmc/linux/fs/devpts/inode.c (revision 4cff79e9)
1 /* -*- linux-c -*- --------------------------------------------------------- *
2  *
3  * linux/fs/devpts/inode.c
4  *
5  *  Copyright 1998-2004 H. Peter Anvin -- All Rights Reserved
6  *
7  * This file is part of the Linux kernel and is made available under
8  * the terms of the GNU General Public License, version 2, or at your
9  * option, any later version, incorporated herein by reference.
10  *
11  * ------------------------------------------------------------------------- */
12 
13 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
14 
15 #include <linux/module.h>
16 #include <linux/init.h>
17 #include <linux/fs.h>
18 #include <linux/sched.h>
19 #include <linux/namei.h>
20 #include <linux/slab.h>
21 #include <linux/mount.h>
22 #include <linux/tty.h>
23 #include <linux/mutex.h>
24 #include <linux/magic.h>
25 #include <linux/idr.h>
26 #include <linux/devpts_fs.h>
27 #include <linux/parser.h>
28 #include <linux/fsnotify.h>
29 #include <linux/seq_file.h>
30 
31 #define DEVPTS_DEFAULT_MODE 0600
32 /*
33  * ptmx is a new node in /dev/pts and will be unused in legacy (single-
34  * instance) mode. To prevent surprises in user space, set permissions of
35  * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
36  * permissions.
37  */
38 #define DEVPTS_DEFAULT_PTMX_MODE 0000
39 #define PTMX_MINOR	2
40 
41 /*
42  * sysctl support for setting limits on the number of Unix98 ptys allocated.
43  * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
44  */
45 static int pty_limit = NR_UNIX98_PTY_DEFAULT;
46 static int pty_reserve = NR_UNIX98_PTY_RESERVE;
47 static int pty_limit_min;
48 static int pty_limit_max = INT_MAX;
49 static int pty_count;
50 
51 static struct ctl_table pty_table[] = {
52 	{
53 		.procname	= "max",
54 		.maxlen		= sizeof(int),
55 		.mode		= 0644,
56 		.data		= &pty_limit,
57 		.proc_handler	= proc_dointvec_minmax,
58 		.extra1		= &pty_limit_min,
59 		.extra2		= &pty_limit_max,
60 	}, {
61 		.procname	= "reserve",
62 		.maxlen		= sizeof(int),
63 		.mode		= 0644,
64 		.data		= &pty_reserve,
65 		.proc_handler	= proc_dointvec_minmax,
66 		.extra1		= &pty_limit_min,
67 		.extra2		= &pty_limit_max,
68 	}, {
69 		.procname	= "nr",
70 		.maxlen		= sizeof(int),
71 		.mode		= 0444,
72 		.data		= &pty_count,
73 		.proc_handler	= proc_dointvec,
74 	},
75 	{}
76 };
77 
78 static struct ctl_table pty_kern_table[] = {
79 	{
80 		.procname	= "pty",
81 		.mode		= 0555,
82 		.child		= pty_table,
83 	},
84 	{}
85 };
86 
87 static struct ctl_table pty_root_table[] = {
88 	{
89 		.procname	= "kernel",
90 		.mode		= 0555,
91 		.child		= pty_kern_table,
92 	},
93 	{}
94 };
95 
96 static DEFINE_MUTEX(allocated_ptys_lock);
97 
98 struct pts_mount_opts {
99 	int setuid;
100 	int setgid;
101 	kuid_t   uid;
102 	kgid_t   gid;
103 	umode_t mode;
104 	umode_t ptmxmode;
105 	int reserve;
106 	int max;
107 };
108 
109 enum {
110 	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
111 	Opt_err
112 };
113 
114 static const match_table_t tokens = {
115 	{Opt_uid, "uid=%u"},
116 	{Opt_gid, "gid=%u"},
117 	{Opt_mode, "mode=%o"},
118 	{Opt_ptmxmode, "ptmxmode=%o"},
119 	{Opt_newinstance, "newinstance"},
120 	{Opt_max, "max=%d"},
121 	{Opt_err, NULL}
122 };
123 
124 struct pts_fs_info {
125 	struct ida allocated_ptys;
126 	struct pts_mount_opts mount_opts;
127 	struct super_block *sb;
128 	struct dentry *ptmx_dentry;
129 };
130 
131 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
132 {
133 	return sb->s_fs_info;
134 }
135 
136 static int devpts_ptmx_path(struct path *path)
137 {
138 	struct super_block *sb;
139 	int err;
140 
141 	/* Is a devpts filesystem at "pts" in the same directory? */
142 	err = path_pts(path);
143 	if (err)
144 		return err;
145 
146 	/* Is the path the root of a devpts filesystem? */
147 	sb = path->mnt->mnt_sb;
148 	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
149 	    (path->mnt->mnt_root != sb->s_root))
150 		return -ENODEV;
151 
152 	return 0;
153 }
154 
155 /*
156  * Try to find a suitable devpts filesystem. We support the following
157  * scenarios:
158  * - The ptmx device node is located in the same directory as the devpts
159  *   mount where the pts device nodes are located.
160  *   This is e.g. the case when calling open on the /dev/pts/ptmx device
161  *   node when the devpts filesystem is mounted at /dev/pts.
162  * - The ptmx device node is located outside the devpts filesystem mount
163  *   where the pts device nodes are located. For example, the ptmx device
164  *   is a symlink, separate device node, or bind-mount.
165  *   A supported scenario is bind-mounting /dev/pts/ptmx to /dev/ptmx and
166  *   then calling open on /dev/ptmx. In this case a suitable pts
167  *   subdirectory can be found in the common parent directory /dev of the
168  *   devpts mount and the ptmx bind-mount, after resolving the /dev/ptmx
169  *   bind-mount.
170  *   If no suitable pts subdirectory can be found this function will fail.
171  *   This is e.g. the case when bind-mounting /dev/pts/ptmx to /ptmx.
172  */
173 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
174 {
175 	struct path path;
176 	int err = 0;
177 
178 	path = filp->f_path;
179 	path_get(&path);
180 
181 	/* Walk upward while the start point is a bind mount of
182 	 * a single file.
183 	 */
184 	while (path.mnt->mnt_root == path.dentry)
185 		if (follow_up(&path) == 0)
186 			break;
187 
188 	/* devpts_ptmx_path() finds a devpts fs or returns an error. */
189 	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
190 	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
191 		err = devpts_ptmx_path(&path);
192 	dput(path.dentry);
193 	if (!err) {
194 		if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
195 			return path.mnt;
196 
197 		err = -ENODEV;
198 	}
199 
200 	mntput(path.mnt);
201 	return ERR_PTR(err);
202 }
203 
204 struct pts_fs_info *devpts_acquire(struct file *filp)
205 {
206 	struct pts_fs_info *result;
207 	struct path path;
208 	struct super_block *sb;
209 
210 	path = filp->f_path;
211 	path_get(&path);
212 
213 	/* Has the devpts filesystem already been found? */
214 	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
215 		int err;
216 
217 		err = devpts_ptmx_path(&path);
218 		if (err) {
219 			result = ERR_PTR(err);
220 			goto out;
221 		}
222 	}
223 
224 	/*
225 	 * pty code needs to hold extra references in case of last /dev/tty close
226 	 */
227 	sb = path.mnt->mnt_sb;
228 	atomic_inc(&sb->s_active);
229 	result = DEVPTS_SB(sb);
230 
231 out:
232 	path_put(&path);
233 	return result;
234 }
235 
236 void devpts_release(struct pts_fs_info *fsi)
237 {
238 	deactivate_super(fsi->sb);
239 }
240 
241 #define PARSE_MOUNT	0
242 #define PARSE_REMOUNT	1
243 
244 /*
245  * parse_mount_options():
246  *	Set @opts to mount options specified in @data. If an option is not
247  *	specified in @data, set it to its default value.
248  *
249  * Note: @data may be NULL (in which case all options are set to default).
250  */
251 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
252 {
253 	char *p;
254 	kuid_t uid;
255 	kgid_t gid;
256 
257 	opts->setuid  = 0;
258 	opts->setgid  = 0;
259 	opts->uid     = GLOBAL_ROOT_UID;
260 	opts->gid     = GLOBAL_ROOT_GID;
261 	opts->mode    = DEVPTS_DEFAULT_MODE;
262 	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
263 	opts->max     = NR_UNIX98_PTY_MAX;
264 
265 	/* Only allow instances mounted from the initial mount
266 	 * namespace to tap the reserve pool of ptys.
267 	 */
268 	if (op == PARSE_MOUNT)
269 		opts->reserve =
270 			(current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns);
271 
272 	while ((p = strsep(&data, ",")) != NULL) {
273 		substring_t args[MAX_OPT_ARGS];
274 		int token;
275 		int option;
276 
277 		if (!*p)
278 			continue;
279 
280 		token = match_token(p, tokens, args);
281 		switch (token) {
282 		case Opt_uid:
283 			if (match_int(&args[0], &option))
284 				return -EINVAL;
285 			uid = make_kuid(current_user_ns(), option);
286 			if (!uid_valid(uid))
287 				return -EINVAL;
288 			opts->uid = uid;
289 			opts->setuid = 1;
290 			break;
291 		case Opt_gid:
292 			if (match_int(&args[0], &option))
293 				return -EINVAL;
294 			gid = make_kgid(current_user_ns(), option);
295 			if (!gid_valid(gid))
296 				return -EINVAL;
297 			opts->gid = gid;
298 			opts->setgid = 1;
299 			break;
300 		case Opt_mode:
301 			if (match_octal(&args[0], &option))
302 				return -EINVAL;
303 			opts->mode = option & S_IALLUGO;
304 			break;
305 		case Opt_ptmxmode:
306 			if (match_octal(&args[0], &option))
307 				return -EINVAL;
308 			opts->ptmxmode = option & S_IALLUGO;
309 			break;
310 		case Opt_newinstance:
311 			break;
312 		case Opt_max:
313 			if (match_int(&args[0], &option) ||
314 			    option < 0 || option > NR_UNIX98_PTY_MAX)
315 				return -EINVAL;
316 			opts->max = option;
317 			break;
318 		default:
319 			pr_err("called with bogus options\n");
320 			return -EINVAL;
321 		}
322 	}
323 
324 	return 0;
325 }
326 
327 static int mknod_ptmx(struct super_block *sb)
328 {
329 	int mode;
330 	int rc = -ENOMEM;
331 	struct dentry *dentry;
332 	struct inode *inode;
333 	struct dentry *root = sb->s_root;
334 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
335 	struct pts_mount_opts *opts = &fsi->mount_opts;
336 	kuid_t ptmx_uid = current_fsuid();
337 	kgid_t ptmx_gid = current_fsgid();
338 
339 	inode_lock(d_inode(root));
340 
341 	/* If we have already created ptmx node, return */
342 	if (fsi->ptmx_dentry) {
343 		rc = 0;
344 		goto out;
345 	}
346 
347 	dentry = d_alloc_name(root, "ptmx");
348 	if (!dentry) {
349 		pr_err("Unable to alloc dentry for ptmx node\n");
350 		goto out;
351 	}
352 
353 	/*
354 	 * Create a new 'ptmx' node in this mount of devpts.
355 	 */
356 	inode = new_inode(sb);
357 	if (!inode) {
358 		pr_err("Unable to alloc inode for ptmx node\n");
359 		dput(dentry);
360 		goto out;
361 	}
362 
363 	inode->i_ino = 2;
364 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
365 
366 	mode = S_IFCHR|opts->ptmxmode;
367 	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
368 	inode->i_uid = ptmx_uid;
369 	inode->i_gid = ptmx_gid;
370 
371 	d_add(dentry, inode);
372 
373 	fsi->ptmx_dentry = dentry;
374 	rc = 0;
375 out:
376 	inode_unlock(d_inode(root));
377 	return rc;
378 }
379 
380 static void update_ptmx_mode(struct pts_fs_info *fsi)
381 {
382 	struct inode *inode;
383 	if (fsi->ptmx_dentry) {
384 		inode = d_inode(fsi->ptmx_dentry);
385 		inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
386 	}
387 }
388 
389 static int devpts_remount(struct super_block *sb, int *flags, char *data)
390 {
391 	int err;
392 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
393 	struct pts_mount_opts *opts = &fsi->mount_opts;
394 
395 	err = parse_mount_options(data, PARSE_REMOUNT, opts);
396 
397 	/*
398 	 * parse_mount_options() restores options to default values
399 	 * before parsing and may have changed ptmxmode. So, update the
400 	 * mode in the inode too. Bogus options don't fail the remount,
401 	 * so do this even on error return.
402 	 */
403 	update_ptmx_mode(fsi);
404 
405 	return err;
406 }
407 
408 static int devpts_show_options(struct seq_file *seq, struct dentry *root)
409 {
410 	struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
411 	struct pts_mount_opts *opts = &fsi->mount_opts;
412 
413 	if (opts->setuid)
414 		seq_printf(seq, ",uid=%u",
415 			   from_kuid_munged(&init_user_ns, opts->uid));
416 	if (opts->setgid)
417 		seq_printf(seq, ",gid=%u",
418 			   from_kgid_munged(&init_user_ns, opts->gid));
419 	seq_printf(seq, ",mode=%03o", opts->mode);
420 	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
421 	if (opts->max < NR_UNIX98_PTY_MAX)
422 		seq_printf(seq, ",max=%d", opts->max);
423 
424 	return 0;
425 }
426 
427 static const struct super_operations devpts_sops = {
428 	.statfs		= simple_statfs,
429 	.remount_fs	= devpts_remount,
430 	.show_options	= devpts_show_options,
431 };
432 
433 static void *new_pts_fs_info(struct super_block *sb)
434 {
435 	struct pts_fs_info *fsi;
436 
437 	fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
438 	if (!fsi)
439 		return NULL;
440 
441 	ida_init(&fsi->allocated_ptys);
442 	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
443 	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
444 	fsi->sb = sb;
445 
446 	return fsi;
447 }
448 
449 static int
450 devpts_fill_super(struct super_block *s, void *data, int silent)
451 {
452 	struct inode *inode;
453 	int error;
454 
455 	s->s_iflags &= ~SB_I_NODEV;
456 	s->s_blocksize = 1024;
457 	s->s_blocksize_bits = 10;
458 	s->s_magic = DEVPTS_SUPER_MAGIC;
459 	s->s_op = &devpts_sops;
460 	s->s_time_gran = 1;
461 
462 	error = -ENOMEM;
463 	s->s_fs_info = new_pts_fs_info(s);
464 	if (!s->s_fs_info)
465 		goto fail;
466 
467 	error = parse_mount_options(data, PARSE_MOUNT, &DEVPTS_SB(s)->mount_opts);
468 	if (error)
469 		goto fail;
470 
471 	error = -ENOMEM;
472 	inode = new_inode(s);
473 	if (!inode)
474 		goto fail;
475 	inode->i_ino = 1;
476 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
477 	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
478 	inode->i_op = &simple_dir_inode_operations;
479 	inode->i_fop = &simple_dir_operations;
480 	set_nlink(inode, 2);
481 
482 	s->s_root = d_make_root(inode);
483 	if (!s->s_root) {
484 		pr_err("get root dentry failed\n");
485 		goto fail;
486 	}
487 
488 	error = mknod_ptmx(s);
489 	if (error)
490 		goto fail_dput;
491 
492 	return 0;
493 fail_dput:
494 	dput(s->s_root);
495 	s->s_root = NULL;
496 fail:
497 	return error;
498 }
499 
500 /*
501  * devpts_mount()
502  *
503  *     Mount a new (private) instance of devpts.  PTYs created in this
504  *     instance are independent of the PTYs in other devpts instances.
505  */
506 static struct dentry *devpts_mount(struct file_system_type *fs_type,
507 	int flags, const char *dev_name, void *data)
508 {
509 	return mount_nodev(fs_type, flags, data, devpts_fill_super);
510 }
511 
512 static void devpts_kill_sb(struct super_block *sb)
513 {
514 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
515 
516 	if (fsi)
517 		ida_destroy(&fsi->allocated_ptys);
518 	kfree(fsi);
519 	kill_litter_super(sb);
520 }
521 
522 static struct file_system_type devpts_fs_type = {
523 	.name		= "devpts",
524 	.mount		= devpts_mount,
525 	.kill_sb	= devpts_kill_sb,
526 	.fs_flags	= FS_USERNS_MOUNT,
527 };
528 
529 /*
530  * The normal naming convention is simply /dev/pts/<number>; this conforms
531  * to the System V naming convention
532  */
533 
534 int devpts_new_index(struct pts_fs_info *fsi)
535 {
536 	int index;
537 	int ida_ret;
538 
539 retry:
540 	if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
541 		return -ENOMEM;
542 
543 	mutex_lock(&allocated_ptys_lock);
544 	if (pty_count >= (pty_limit -
545 			  (fsi->mount_opts.reserve ? 0 : pty_reserve))) {
546 		mutex_unlock(&allocated_ptys_lock);
547 		return -ENOSPC;
548 	}
549 
550 	ida_ret = ida_get_new(&fsi->allocated_ptys, &index);
551 	if (ida_ret < 0) {
552 		mutex_unlock(&allocated_ptys_lock);
553 		if (ida_ret == -EAGAIN)
554 			goto retry;
555 		return -EIO;
556 	}
557 
558 	if (index >= fsi->mount_opts.max) {
559 		ida_remove(&fsi->allocated_ptys, index);
560 		mutex_unlock(&allocated_ptys_lock);
561 		return -ENOSPC;
562 	}
563 	pty_count++;
564 	mutex_unlock(&allocated_ptys_lock);
565 	return index;
566 }
567 
568 void devpts_kill_index(struct pts_fs_info *fsi, int idx)
569 {
570 	mutex_lock(&allocated_ptys_lock);
571 	ida_remove(&fsi->allocated_ptys, idx);
572 	pty_count--;
573 	mutex_unlock(&allocated_ptys_lock);
574 }
575 
576 /**
577  * devpts_pty_new -- create a new inode in /dev/pts/
578  * @ptmx_inode: inode of the master
579  * @device: major+minor of the node to be created
580  * @index: used as a name of the node
581  * @priv: what's given back by devpts_get_priv
582  *
583  * The created inode is returned. Remove it from /dev/pts/ by devpts_pty_kill.
584  */
585 struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
586 {
587 	struct dentry *dentry;
588 	struct super_block *sb = fsi->sb;
589 	struct inode *inode;
590 	struct dentry *root;
591 	struct pts_mount_opts *opts;
592 	char s[12];
593 
594 	root = sb->s_root;
595 	opts = &fsi->mount_opts;
596 
597 	inode = new_inode(sb);
598 	if (!inode)
599 		return ERR_PTR(-ENOMEM);
600 
601 	inode->i_ino = index + 3;
602 	inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
603 	inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
604 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
605 	init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));
606 
607 	sprintf(s, "%d", index);
608 
609 	dentry = d_alloc_name(root, s);
610 	if (dentry) {
611 		dentry->d_fsdata = priv;
612 		d_add(dentry, inode);
613 		fsnotify_create(d_inode(root), dentry);
614 	} else {
615 		iput(inode);
616 		dentry = ERR_PTR(-ENOMEM);
617 	}
618 
619 	return dentry;
620 }
621 
622 /**
623  * devpts_get_priv -- get private data for a slave
624  * @pts_inode: inode of the slave
625  *
626  * Returns whatever was passed as priv in devpts_pty_new for a given inode.
627  */
628 void *devpts_get_priv(struct dentry *dentry)
629 {
630 	if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
631 		return NULL;
632 	return dentry->d_fsdata;
633 }
634 
635 /**
636  * devpts_pty_kill -- remove inode form /dev/pts/
637  * @inode: inode of the slave to be removed
638  *
639  * This is an inverse operation of devpts_pty_new.
640  */
641 void devpts_pty_kill(struct dentry *dentry)
642 {
643 	WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
644 
645 	dentry->d_fsdata = NULL;
646 	drop_nlink(dentry->d_inode);
647 	d_delete(dentry);
648 	dput(dentry);	/* d_alloc_name() in devpts_pty_new() */
649 }
650 
651 static int __init init_devpts_fs(void)
652 {
653 	int err = register_filesystem(&devpts_fs_type);
654 	if (!err) {
655 		register_sysctl_table(pty_root_table);
656 	}
657 	return err;
658 }
659 module_init(init_devpts_fs)
660