1// SPDX-License-Identifier: GPL-2.0-or-later
2/* -*- linux-c -*- --------------------------------------------------------- *
3 *
4 * linux/fs/devpts/inode.c
5 *
6 *  Copyright 1998-2004 H. Peter Anvin -- All Rights Reserved
7 *
8 * ------------------------------------------------------------------------- */
9
10#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
11
12#include <linux/module.h>
13#include <linux/init.h>
14#include <linux/fs.h>
15#include <linux/sched.h>
16#include <linux/namei.h>
17#include <linux/slab.h>
18#include <linux/mount.h>
19#include <linux/tty.h>
20#include <linux/mutex.h>
21#include <linux/magic.h>
22#include <linux/idr.h>
23#include <linux/devpts_fs.h>
24#include <linux/parser.h>
25#include <linux/fsnotify.h>
26#include <linux/seq_file.h>
27
28#define DEVPTS_DEFAULT_MODE 0600
29/*
30 * ptmx is a new node in /dev/pts and will be unused in legacy (single-
31 * instance) mode. To prevent surprises in user space, set permissions of
32 * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
33 * permissions.
34 */
35#define DEVPTS_DEFAULT_PTMX_MODE 0000
36#define PTMX_MINOR	2
37
38/*
39 * sysctl support for setting limits on the number of Unix98 ptys allocated.
40 * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
41 */
42static int pty_limit = NR_UNIX98_PTY_DEFAULT;
43static int pty_reserve = NR_UNIX98_PTY_RESERVE;
44static int pty_limit_min;
45static int pty_limit_max = INT_MAX;
46static atomic_t pty_count = ATOMIC_INIT(0);
47
48static struct ctl_table pty_table[] = {
49	{
50		.procname	= "max",
51		.maxlen		= sizeof(int),
52		.mode		= 0644,
53		.data		= &pty_limit,
54		.proc_handler	= proc_dointvec_minmax,
55		.extra1		= &pty_limit_min,
56		.extra2		= &pty_limit_max,
57	}, {
58		.procname	= "reserve",
59		.maxlen		= sizeof(int),
60		.mode		= 0644,
61		.data		= &pty_reserve,
62		.proc_handler	= proc_dointvec_minmax,
63		.extra1		= &pty_limit_min,
64		.extra2		= &pty_limit_max,
65	}, {
66		.procname	= "nr",
67		.maxlen		= sizeof(int),
68		.mode		= 0444,
69		.data		= &pty_count,
70		.proc_handler	= proc_dointvec,
71	},
72};
73
74struct pts_mount_opts {
75	int setuid;
76	int setgid;
77	kuid_t   uid;
78	kgid_t   gid;
79	umode_t mode;
80	umode_t ptmxmode;
81	int reserve;
82	int max;
83};
84
85enum {
86	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
87	Opt_err
88};
89
90static const match_table_t tokens = {
91	{Opt_uid, "uid=%u"},
92	{Opt_gid, "gid=%u"},
93	{Opt_mode, "mode=%o"},
94	{Opt_ptmxmode, "ptmxmode=%o"},
95	{Opt_newinstance, "newinstance"},
96	{Opt_max, "max=%d"},
97	{Opt_err, NULL}
98};
99
100struct pts_fs_info {
101	struct ida allocated_ptys;
102	struct pts_mount_opts mount_opts;
103	struct super_block *sb;
104	struct dentry *ptmx_dentry;
105};
106
107static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
108{
109	return sb->s_fs_info;
110}
111
112static int devpts_ptmx_path(struct path *path)
113{
114	struct super_block *sb;
115	int err;
116
117	/* Is a devpts filesystem at "pts" in the same directory? */
118	err = path_pts(path);
119	if (err)
120		return err;
121
122	/* Is the path the root of a devpts filesystem? */
123	sb = path->mnt->mnt_sb;
124	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
125	    (path->mnt->mnt_root != sb->s_root))
126		return -ENODEV;
127
128	return 0;
129}
130
131/*
132 * Try to find a suitable devpts filesystem. We support the following
133 * scenarios:
134 * - The ptmx device node is located in the same directory as the devpts
135 *   mount where the pts device nodes are located.
136 *   This is e.g. the case when calling open on the /dev/pts/ptmx device
137 *   node when the devpts filesystem is mounted at /dev/pts.
138 * - The ptmx device node is located outside the devpts filesystem mount
139 *   where the pts device nodes are located. For example, the ptmx device
140 *   is a symlink, separate device node, or bind-mount.
141 *   A supported scenario is bind-mounting /dev/pts/ptmx to /dev/ptmx and
142 *   then calling open on /dev/ptmx. In this case a suitable pts
143 *   subdirectory can be found in the common parent directory /dev of the
144 *   devpts mount and the ptmx bind-mount, after resolving the /dev/ptmx
145 *   bind-mount.
146 *   If no suitable pts subdirectory can be found this function will fail.
147 *   This is e.g. the case when bind-mounting /dev/pts/ptmx to /ptmx.
148 */
149struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
150{
151	struct path path;
152	int err = 0;
153
154	path = filp->f_path;
155	path_get(&path);
156
157	/* Walk upward while the start point is a bind mount of
158	 * a single file.
159	 */
160	while (path.mnt->mnt_root == path.dentry)
161		if (follow_up(&path) == 0)
162			break;
163
164	/* devpts_ptmx_path() finds a devpts fs or returns an error. */
165	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
166	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
167		err = devpts_ptmx_path(&path);
168	dput(path.dentry);
169	if (!err) {
170		if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
171			return path.mnt;
172
173		err = -ENODEV;
174	}
175
176	mntput(path.mnt);
177	return ERR_PTR(err);
178}
179
180struct pts_fs_info *devpts_acquire(struct file *filp)
181{
182	struct pts_fs_info *result;
183	struct path path;
184	struct super_block *sb;
185
186	path = filp->f_path;
187	path_get(&path);
188
189	/* Has the devpts filesystem already been found? */
190	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
191		int err;
192
193		err = devpts_ptmx_path(&path);
194		if (err) {
195			result = ERR_PTR(err);
196			goto out;
197		}
198	}
199
200	/*
201	 * pty code needs to hold extra references in case of last /dev/tty close
202	 */
203	sb = path.mnt->mnt_sb;
204	atomic_inc(&sb->s_active);
205	result = DEVPTS_SB(sb);
206
207out:
208	path_put(&path);
209	return result;
210}
211
212void devpts_release(struct pts_fs_info *fsi)
213{
214	deactivate_super(fsi->sb);
215}
216
217#define PARSE_MOUNT	0
218#define PARSE_REMOUNT	1
219
220/*
221 * parse_mount_options():
222 *	Set @opts to mount options specified in @data. If an option is not
223 *	specified in @data, set it to its default value.
224 *
225 * Note: @data may be NULL (in which case all options are set to default).
226 */
227static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
228{
229	char *p;
230	kuid_t uid;
231	kgid_t gid;
232
233	opts->setuid  = 0;
234	opts->setgid  = 0;
235	opts->uid     = GLOBAL_ROOT_UID;
236	opts->gid     = GLOBAL_ROOT_GID;
237	opts->mode    = DEVPTS_DEFAULT_MODE;
238	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
239	opts->max     = NR_UNIX98_PTY_MAX;
240
241	/* Only allow instances mounted from the initial mount
242	 * namespace to tap the reserve pool of ptys.
243	 */
244	if (op == PARSE_MOUNT)
245		opts->reserve =
246			(current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns);
247
248	while ((p = strsep(&data, ",")) != NULL) {
249		substring_t args[MAX_OPT_ARGS];
250		int token;
251		int option;
252
253		if (!*p)
254			continue;
255
256		token = match_token(p, tokens, args);
257		switch (token) {
258		case Opt_uid:
259			if (match_int(&args[0], &option))
260				return -EINVAL;
261			uid = make_kuid(current_user_ns(), option);
262			if (!uid_valid(uid))
263				return -EINVAL;
264			opts->uid = uid;
265			opts->setuid = 1;
266			break;
267		case Opt_gid:
268			if (match_int(&args[0], &option))
269				return -EINVAL;
270			gid = make_kgid(current_user_ns(), option);
271			if (!gid_valid(gid))
272				return -EINVAL;
273			opts->gid = gid;
274			opts->setgid = 1;
275			break;
276		case Opt_mode:
277			if (match_octal(&args[0], &option))
278				return -EINVAL;
279			opts->mode = option & S_IALLUGO;
280			break;
281		case Opt_ptmxmode:
282			if (match_octal(&args[0], &option))
283				return -EINVAL;
284			opts->ptmxmode = option & S_IALLUGO;
285			break;
286		case Opt_newinstance:
287			break;
288		case Opt_max:
289			if (match_int(&args[0], &option) ||
290			    option < 0 || option > NR_UNIX98_PTY_MAX)
291				return -EINVAL;
292			opts->max = option;
293			break;
294		default:
295			pr_err("called with bogus options\n");
296			return -EINVAL;
297		}
298	}
299
300	return 0;
301}
302
303static int mknod_ptmx(struct super_block *sb)
304{
305	int mode;
306	int rc = -ENOMEM;
307	struct dentry *dentry;
308	struct inode *inode;
309	struct dentry *root = sb->s_root;
310	struct pts_fs_info *fsi = DEVPTS_SB(sb);
311	struct pts_mount_opts *opts = &fsi->mount_opts;
312	kuid_t ptmx_uid = current_fsuid();
313	kgid_t ptmx_gid = current_fsgid();
314
315	inode_lock(d_inode(root));
316
317	/* If we have already created ptmx node, return */
318	if (fsi->ptmx_dentry) {
319		rc = 0;
320		goto out;
321	}
322
323	dentry = d_alloc_name(root, "ptmx");
324	if (!dentry) {
325		pr_err("Unable to alloc dentry for ptmx node\n");
326		goto out;
327	}
328
329	/*
330	 * Create a new 'ptmx' node in this mount of devpts.
331	 */
332	inode = new_inode(sb);
333	if (!inode) {
334		pr_err("Unable to alloc inode for ptmx node\n");
335		dput(dentry);
336		goto out;
337	}
338
339	inode->i_ino = 2;
340	simple_inode_init_ts(inode);
341
342	mode = S_IFCHR|opts->ptmxmode;
343	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
344	inode->i_uid = ptmx_uid;
345	inode->i_gid = ptmx_gid;
346
347	d_add(dentry, inode);
348
349	fsi->ptmx_dentry = dentry;
350	rc = 0;
351out:
352	inode_unlock(d_inode(root));
353	return rc;
354}
355
356static void update_ptmx_mode(struct pts_fs_info *fsi)
357{
358	struct inode *inode;
359	if (fsi->ptmx_dentry) {
360		inode = d_inode(fsi->ptmx_dentry);
361		inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
362	}
363}
364
365static int devpts_remount(struct super_block *sb, int *flags, char *data)
366{
367	int err;
368	struct pts_fs_info *fsi = DEVPTS_SB(sb);
369	struct pts_mount_opts *opts = &fsi->mount_opts;
370
371	err = parse_mount_options(data, PARSE_REMOUNT, opts);
372
373	/*
374	 * parse_mount_options() restores options to default values
375	 * before parsing and may have changed ptmxmode. So, update the
376	 * mode in the inode too. Bogus options don't fail the remount,
377	 * so do this even on error return.
378	 */
379	update_ptmx_mode(fsi);
380
381	return err;
382}
383
384static int devpts_show_options(struct seq_file *seq, struct dentry *root)
385{
386	struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
387	struct pts_mount_opts *opts = &fsi->mount_opts;
388
389	if (opts->setuid)
390		seq_printf(seq, ",uid=%u",
391			   from_kuid_munged(&init_user_ns, opts->uid));
392	if (opts->setgid)
393		seq_printf(seq, ",gid=%u",
394			   from_kgid_munged(&init_user_ns, opts->gid));
395	seq_printf(seq, ",mode=%03o", opts->mode);
396	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
397	if (opts->max < NR_UNIX98_PTY_MAX)
398		seq_printf(seq, ",max=%d", opts->max);
399
400	return 0;
401}
402
403static const struct super_operations devpts_sops = {
404	.statfs		= simple_statfs,
405	.remount_fs	= devpts_remount,
406	.show_options	= devpts_show_options,
407};
408
409static void *new_pts_fs_info(struct super_block *sb)
410{
411	struct pts_fs_info *fsi;
412
413	fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
414	if (!fsi)
415		return NULL;
416
417	ida_init(&fsi->allocated_ptys);
418	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
419	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
420	fsi->sb = sb;
421
422	return fsi;
423}
424
425static int
426devpts_fill_super(struct super_block *s, void *data, int silent)
427{
428	struct inode *inode;
429	int error;
430
431	s->s_iflags &= ~SB_I_NODEV;
432	s->s_blocksize = 1024;
433	s->s_blocksize_bits = 10;
434	s->s_magic = DEVPTS_SUPER_MAGIC;
435	s->s_op = &devpts_sops;
436	s->s_d_op = &simple_dentry_operations;
437	s->s_time_gran = 1;
438
439	error = -ENOMEM;
440	s->s_fs_info = new_pts_fs_info(s);
441	if (!s->s_fs_info)
442		goto fail;
443
444	error = parse_mount_options(data, PARSE_MOUNT, &DEVPTS_SB(s)->mount_opts);
445	if (error)
446		goto fail;
447
448	error = -ENOMEM;
449	inode = new_inode(s);
450	if (!inode)
451		goto fail;
452	inode->i_ino = 1;
453	simple_inode_init_ts(inode);
454	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
455	inode->i_op = &simple_dir_inode_operations;
456	inode->i_fop = &simple_dir_operations;
457	set_nlink(inode, 2);
458
459	s->s_root = d_make_root(inode);
460	if (!s->s_root) {
461		pr_err("get root dentry failed\n");
462		goto fail;
463	}
464
465	error = mknod_ptmx(s);
466	if (error)
467		goto fail_dput;
468
469	return 0;
470fail_dput:
471	dput(s->s_root);
472	s->s_root = NULL;
473fail:
474	return error;
475}
476
477/*
478 * devpts_mount()
479 *
480 *     Mount a new (private) instance of devpts.  PTYs created in this
481 *     instance are independent of the PTYs in other devpts instances.
482 */
483static struct dentry *devpts_mount(struct file_system_type *fs_type,
484	int flags, const char *dev_name, void *data)
485{
486	return mount_nodev(fs_type, flags, data, devpts_fill_super);
487}
488
489static void devpts_kill_sb(struct super_block *sb)
490{
491	struct pts_fs_info *fsi = DEVPTS_SB(sb);
492
493	if (fsi)
494		ida_destroy(&fsi->allocated_ptys);
495	kfree(fsi);
496	kill_litter_super(sb);
497}
498
499static struct file_system_type devpts_fs_type = {
500	.name		= "devpts",
501	.mount		= devpts_mount,
502	.kill_sb	= devpts_kill_sb,
503	.fs_flags	= FS_USERNS_MOUNT,
504};
505
506/*
507 * The normal naming convention is simply /dev/pts/<number>; this conforms
508 * to the System V naming convention
509 */
510
511int devpts_new_index(struct pts_fs_info *fsi)
512{
513	int index = -ENOSPC;
514
515	if (atomic_inc_return(&pty_count) >= (pty_limit -
516			  (fsi->mount_opts.reserve ? 0 : pty_reserve)))
517		goto out;
518
519	index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
520			GFP_KERNEL);
521
522out:
523	if (index < 0)
524		atomic_dec(&pty_count);
525	return index;
526}
527
528void devpts_kill_index(struct pts_fs_info *fsi, int idx)
529{
530	ida_free(&fsi->allocated_ptys, idx);
531	atomic_dec(&pty_count);
532}
533
534/**
535 * devpts_pty_new -- create a new inode in /dev/pts/
536 * @fsi: Filesystem info for this instance.
537 * @index: used as a name of the node
538 * @priv: what's given back by devpts_get_priv
539 *
540 * The dentry for the created inode is returned.
541 * Remove it from /dev/pts/ with devpts_pty_kill().
542 */
543struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
544{
545	struct dentry *dentry;
546	struct super_block *sb = fsi->sb;
547	struct inode *inode;
548	struct dentry *root;
549	struct pts_mount_opts *opts;
550	char s[12];
551
552	root = sb->s_root;
553	opts = &fsi->mount_opts;
554
555	inode = new_inode(sb);
556	if (!inode)
557		return ERR_PTR(-ENOMEM);
558
559	inode->i_ino = index + 3;
560	inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
561	inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
562	simple_inode_init_ts(inode);
563	init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));
564
565	sprintf(s, "%d", index);
566
567	dentry = d_alloc_name(root, s);
568	if (dentry) {
569		dentry->d_fsdata = priv;
570		d_add(dentry, inode);
571		fsnotify_create(d_inode(root), dentry);
572	} else {
573		iput(inode);
574		dentry = ERR_PTR(-ENOMEM);
575	}
576
577	return dentry;
578}
579
580/**
581 * devpts_get_priv -- get private data for a slave
582 * @dentry: dentry of the slave
583 *
584 * Returns whatever was passed as priv in devpts_pty_new for a given inode.
585 */
586void *devpts_get_priv(struct dentry *dentry)
587{
588	if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
589		return NULL;
590	return dentry->d_fsdata;
591}
592
593/**
594 * devpts_pty_kill -- remove inode form /dev/pts/
595 * @dentry: dentry of the slave to be removed
596 *
597 * This is an inverse operation of devpts_pty_new.
598 */
599void devpts_pty_kill(struct dentry *dentry)
600{
601	WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
602
603	dentry->d_fsdata = NULL;
604	drop_nlink(dentry->d_inode);
605	d_drop(dentry);
606	fsnotify_unlink(d_inode(dentry->d_parent), dentry);
607	dput(dentry);	/* d_alloc_name() in devpts_pty_new() */
608}
609
610static int __init init_devpts_fs(void)
611{
612	int err = register_filesystem(&devpts_fs_type);
613	if (!err) {
614		register_sysctl("kernel/pty", pty_table);
615	}
616	return err;
617}
618module_init(init_devpts_fs)
619