xref: /openbmc/linux/fs/devpts/inode.c (revision ec05b126)
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  */
42 static int pty_limit = NR_UNIX98_PTY_DEFAULT;
43 static int pty_reserve = NR_UNIX98_PTY_RESERVE;
44 static int pty_limit_min;
45 static int pty_limit_max = INT_MAX;
46 static atomic_t pty_count = ATOMIC_INIT(0);
47 
48 static 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 };
74 
75 struct pts_mount_opts {
76 	int setuid;
77 	int setgid;
78 	kuid_t   uid;
79 	kgid_t   gid;
80 	umode_t mode;
81 	umode_t ptmxmode;
82 	int reserve;
83 	int max;
84 };
85 
86 enum {
87 	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
88 	Opt_err
89 };
90 
91 static const match_table_t tokens = {
92 	{Opt_uid, "uid=%u"},
93 	{Opt_gid, "gid=%u"},
94 	{Opt_mode, "mode=%o"},
95 	{Opt_ptmxmode, "ptmxmode=%o"},
96 	{Opt_newinstance, "newinstance"},
97 	{Opt_max, "max=%d"},
98 	{Opt_err, NULL}
99 };
100 
101 struct pts_fs_info {
102 	struct ida allocated_ptys;
103 	struct pts_mount_opts mount_opts;
104 	struct super_block *sb;
105 	struct dentry *ptmx_dentry;
106 };
107 
DEVPTS_SB(struct super_block * sb)108 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
109 {
110 	return sb->s_fs_info;
111 }
112 
devpts_ptmx_path(struct path * path)113 static int devpts_ptmx_path(struct path *path)
114 {
115 	struct super_block *sb;
116 	int err;
117 
118 	/* Is a devpts filesystem at "pts" in the same directory? */
119 	err = path_pts(path);
120 	if (err)
121 		return err;
122 
123 	/* Is the path the root of a devpts filesystem? */
124 	sb = path->mnt->mnt_sb;
125 	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
126 	    (path->mnt->mnt_root != sb->s_root))
127 		return -ENODEV;
128 
129 	return 0;
130 }
131 
132 /*
133  * Try to find a suitable devpts filesystem. We support the following
134  * scenarios:
135  * - The ptmx device node is located in the same directory as the devpts
136  *   mount where the pts device nodes are located.
137  *   This is e.g. the case when calling open on the /dev/pts/ptmx device
138  *   node when the devpts filesystem is mounted at /dev/pts.
139  * - The ptmx device node is located outside the devpts filesystem mount
140  *   where the pts device nodes are located. For example, the ptmx device
141  *   is a symlink, separate device node, or bind-mount.
142  *   A supported scenario is bind-mounting /dev/pts/ptmx to /dev/ptmx and
143  *   then calling open on /dev/ptmx. In this case a suitable pts
144  *   subdirectory can be found in the common parent directory /dev of the
145  *   devpts mount and the ptmx bind-mount, after resolving the /dev/ptmx
146  *   bind-mount.
147  *   If no suitable pts subdirectory can be found this function will fail.
148  *   This is e.g. the case when bind-mounting /dev/pts/ptmx to /ptmx.
149  */
devpts_mntget(struct file * filp,struct pts_fs_info * fsi)150 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
151 {
152 	struct path path;
153 	int err = 0;
154 
155 	path = filp->f_path;
156 	path_get(&path);
157 
158 	/* Walk upward while the start point is a bind mount of
159 	 * a single file.
160 	 */
161 	while (path.mnt->mnt_root == path.dentry)
162 		if (follow_up(&path) == 0)
163 			break;
164 
165 	/* devpts_ptmx_path() finds a devpts fs or returns an error. */
166 	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
167 	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
168 		err = devpts_ptmx_path(&path);
169 	dput(path.dentry);
170 	if (!err) {
171 		if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
172 			return path.mnt;
173 
174 		err = -ENODEV;
175 	}
176 
177 	mntput(path.mnt);
178 	return ERR_PTR(err);
179 }
180 
devpts_acquire(struct file * filp)181 struct pts_fs_info *devpts_acquire(struct file *filp)
182 {
183 	struct pts_fs_info *result;
184 	struct path path;
185 	struct super_block *sb;
186 
187 	path = filp->f_path;
188 	path_get(&path);
189 
190 	/* Has the devpts filesystem already been found? */
191 	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
192 		int err;
193 
194 		err = devpts_ptmx_path(&path);
195 		if (err) {
196 			result = ERR_PTR(err);
197 			goto out;
198 		}
199 	}
200 
201 	/*
202 	 * pty code needs to hold extra references in case of last /dev/tty close
203 	 */
204 	sb = path.mnt->mnt_sb;
205 	atomic_inc(&sb->s_active);
206 	result = DEVPTS_SB(sb);
207 
208 out:
209 	path_put(&path);
210 	return result;
211 }
212 
devpts_release(struct pts_fs_info * fsi)213 void devpts_release(struct pts_fs_info *fsi)
214 {
215 	deactivate_super(fsi->sb);
216 }
217 
218 #define PARSE_MOUNT	0
219 #define PARSE_REMOUNT	1
220 
221 /*
222  * parse_mount_options():
223  *	Set @opts to mount options specified in @data. If an option is not
224  *	specified in @data, set it to its default value.
225  *
226  * Note: @data may be NULL (in which case all options are set to default).
227  */
parse_mount_options(char * data,int op,struct pts_mount_opts * opts)228 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
229 {
230 	char *p;
231 	kuid_t uid;
232 	kgid_t gid;
233 
234 	opts->setuid  = 0;
235 	opts->setgid  = 0;
236 	opts->uid     = GLOBAL_ROOT_UID;
237 	opts->gid     = GLOBAL_ROOT_GID;
238 	opts->mode    = DEVPTS_DEFAULT_MODE;
239 	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
240 	opts->max     = NR_UNIX98_PTY_MAX;
241 
242 	/* Only allow instances mounted from the initial mount
243 	 * namespace to tap the reserve pool of ptys.
244 	 */
245 	if (op == PARSE_MOUNT)
246 		opts->reserve =
247 			(current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns);
248 
249 	while ((p = strsep(&data, ",")) != NULL) {
250 		substring_t args[MAX_OPT_ARGS];
251 		int token;
252 		int option;
253 
254 		if (!*p)
255 			continue;
256 
257 		token = match_token(p, tokens, args);
258 		switch (token) {
259 		case Opt_uid:
260 			if (match_int(&args[0], &option))
261 				return -EINVAL;
262 			uid = make_kuid(current_user_ns(), option);
263 			if (!uid_valid(uid))
264 				return -EINVAL;
265 			opts->uid = uid;
266 			opts->setuid = 1;
267 			break;
268 		case Opt_gid:
269 			if (match_int(&args[0], &option))
270 				return -EINVAL;
271 			gid = make_kgid(current_user_ns(), option);
272 			if (!gid_valid(gid))
273 				return -EINVAL;
274 			opts->gid = gid;
275 			opts->setgid = 1;
276 			break;
277 		case Opt_mode:
278 			if (match_octal(&args[0], &option))
279 				return -EINVAL;
280 			opts->mode = option & S_IALLUGO;
281 			break;
282 		case Opt_ptmxmode:
283 			if (match_octal(&args[0], &option))
284 				return -EINVAL;
285 			opts->ptmxmode = option & S_IALLUGO;
286 			break;
287 		case Opt_newinstance:
288 			break;
289 		case Opt_max:
290 			if (match_int(&args[0], &option) ||
291 			    option < 0 || option > NR_UNIX98_PTY_MAX)
292 				return -EINVAL;
293 			opts->max = option;
294 			break;
295 		default:
296 			pr_err("called with bogus options\n");
297 			return -EINVAL;
298 		}
299 	}
300 
301 	return 0;
302 }
303 
mknod_ptmx(struct super_block * sb)304 static int mknod_ptmx(struct super_block *sb)
305 {
306 	int mode;
307 	int rc = -ENOMEM;
308 	struct dentry *dentry;
309 	struct inode *inode;
310 	struct dentry *root = sb->s_root;
311 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
312 	struct pts_mount_opts *opts = &fsi->mount_opts;
313 	kuid_t ptmx_uid = current_fsuid();
314 	kgid_t ptmx_gid = current_fsgid();
315 
316 	inode_lock(d_inode(root));
317 
318 	/* If we have already created ptmx node, return */
319 	if (fsi->ptmx_dentry) {
320 		rc = 0;
321 		goto out;
322 	}
323 
324 	dentry = d_alloc_name(root, "ptmx");
325 	if (!dentry) {
326 		pr_err("Unable to alloc dentry for ptmx node\n");
327 		goto out;
328 	}
329 
330 	/*
331 	 * Create a new 'ptmx' node in this mount of devpts.
332 	 */
333 	inode = new_inode(sb);
334 	if (!inode) {
335 		pr_err("Unable to alloc inode for ptmx node\n");
336 		dput(dentry);
337 		goto out;
338 	}
339 
340 	inode->i_ino = 2;
341 	inode->i_mtime = inode->i_atime = inode_set_ctime_current(inode);
342 
343 	mode = S_IFCHR|opts->ptmxmode;
344 	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
345 	inode->i_uid = ptmx_uid;
346 	inode->i_gid = ptmx_gid;
347 
348 	d_add(dentry, inode);
349 
350 	fsi->ptmx_dentry = dentry;
351 	rc = 0;
352 out:
353 	inode_unlock(d_inode(root));
354 	return rc;
355 }
356 
update_ptmx_mode(struct pts_fs_info * fsi)357 static void update_ptmx_mode(struct pts_fs_info *fsi)
358 {
359 	struct inode *inode;
360 	if (fsi->ptmx_dentry) {
361 		inode = d_inode(fsi->ptmx_dentry);
362 		inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
363 	}
364 }
365 
devpts_remount(struct super_block * sb,int * flags,char * data)366 static int devpts_remount(struct super_block *sb, int *flags, char *data)
367 {
368 	int err;
369 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
370 	struct pts_mount_opts *opts = &fsi->mount_opts;
371 
372 	err = parse_mount_options(data, PARSE_REMOUNT, opts);
373 
374 	/*
375 	 * parse_mount_options() restores options to default values
376 	 * before parsing and may have changed ptmxmode. So, update the
377 	 * mode in the inode too. Bogus options don't fail the remount,
378 	 * so do this even on error return.
379 	 */
380 	update_ptmx_mode(fsi);
381 
382 	return err;
383 }
384 
devpts_show_options(struct seq_file * seq,struct dentry * root)385 static int devpts_show_options(struct seq_file *seq, struct dentry *root)
386 {
387 	struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
388 	struct pts_mount_opts *opts = &fsi->mount_opts;
389 
390 	if (opts->setuid)
391 		seq_printf(seq, ",uid=%u",
392 			   from_kuid_munged(&init_user_ns, opts->uid));
393 	if (opts->setgid)
394 		seq_printf(seq, ",gid=%u",
395 			   from_kgid_munged(&init_user_ns, opts->gid));
396 	seq_printf(seq, ",mode=%03o", opts->mode);
397 	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
398 	if (opts->max < NR_UNIX98_PTY_MAX)
399 		seq_printf(seq, ",max=%d", opts->max);
400 
401 	return 0;
402 }
403 
404 static const struct super_operations devpts_sops = {
405 	.statfs		= simple_statfs,
406 	.remount_fs	= devpts_remount,
407 	.show_options	= devpts_show_options,
408 };
409 
new_pts_fs_info(struct super_block * sb)410 static void *new_pts_fs_info(struct super_block *sb)
411 {
412 	struct pts_fs_info *fsi;
413 
414 	fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
415 	if (!fsi)
416 		return NULL;
417 
418 	ida_init(&fsi->allocated_ptys);
419 	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
420 	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
421 	fsi->sb = sb;
422 
423 	return fsi;
424 }
425 
426 static int
devpts_fill_super(struct super_block * s,void * data,int silent)427 devpts_fill_super(struct super_block *s, void *data, int silent)
428 {
429 	struct inode *inode;
430 	int error;
431 
432 	s->s_iflags &= ~SB_I_NODEV;
433 	s->s_blocksize = 1024;
434 	s->s_blocksize_bits = 10;
435 	s->s_magic = DEVPTS_SUPER_MAGIC;
436 	s->s_op = &devpts_sops;
437 	s->s_d_op = &simple_dentry_operations;
438 	s->s_time_gran = 1;
439 
440 	error = -ENOMEM;
441 	s->s_fs_info = new_pts_fs_info(s);
442 	if (!s->s_fs_info)
443 		goto fail;
444 
445 	error = parse_mount_options(data, PARSE_MOUNT, &DEVPTS_SB(s)->mount_opts);
446 	if (error)
447 		goto fail;
448 
449 	error = -ENOMEM;
450 	inode = new_inode(s);
451 	if (!inode)
452 		goto fail;
453 	inode->i_ino = 1;
454 	inode->i_mtime = inode->i_atime = inode_set_ctime_current(inode);
455 	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
456 	inode->i_op = &simple_dir_inode_operations;
457 	inode->i_fop = &simple_dir_operations;
458 	set_nlink(inode, 2);
459 
460 	s->s_root = d_make_root(inode);
461 	if (!s->s_root) {
462 		pr_err("get root dentry failed\n");
463 		goto fail;
464 	}
465 
466 	error = mknod_ptmx(s);
467 	if (error)
468 		goto fail_dput;
469 
470 	return 0;
471 fail_dput:
472 	dput(s->s_root);
473 	s->s_root = NULL;
474 fail:
475 	return error;
476 }
477 
478 /*
479  * devpts_mount()
480  *
481  *     Mount a new (private) instance of devpts.  PTYs created in this
482  *     instance are independent of the PTYs in other devpts instances.
483  */
devpts_mount(struct file_system_type * fs_type,int flags,const char * dev_name,void * data)484 static struct dentry *devpts_mount(struct file_system_type *fs_type,
485 	int flags, const char *dev_name, void *data)
486 {
487 	return mount_nodev(fs_type, flags, data, devpts_fill_super);
488 }
489 
devpts_kill_sb(struct super_block * sb)490 static void devpts_kill_sb(struct super_block *sb)
491 {
492 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
493 
494 	if (fsi)
495 		ida_destroy(&fsi->allocated_ptys);
496 	kfree(fsi);
497 	kill_litter_super(sb);
498 }
499 
500 static struct file_system_type devpts_fs_type = {
501 	.name		= "devpts",
502 	.mount		= devpts_mount,
503 	.kill_sb	= devpts_kill_sb,
504 	.fs_flags	= FS_USERNS_MOUNT,
505 };
506 
507 /*
508  * The normal naming convention is simply /dev/pts/<number>; this conforms
509  * to the System V naming convention
510  */
511 
devpts_new_index(struct pts_fs_info * fsi)512 int devpts_new_index(struct pts_fs_info *fsi)
513 {
514 	int index = -ENOSPC;
515 
516 	if (atomic_inc_return(&pty_count) >= (pty_limit -
517 			  (fsi->mount_opts.reserve ? 0 : pty_reserve)))
518 		goto out;
519 
520 	index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
521 			GFP_KERNEL);
522 
523 out:
524 	if (index < 0)
525 		atomic_dec(&pty_count);
526 	return index;
527 }
528 
devpts_kill_index(struct pts_fs_info * fsi,int idx)529 void devpts_kill_index(struct pts_fs_info *fsi, int idx)
530 {
531 	ida_free(&fsi->allocated_ptys, idx);
532 	atomic_dec(&pty_count);
533 }
534 
535 /**
536  * devpts_pty_new -- create a new inode in /dev/pts/
537  * @fsi: Filesystem info for this instance.
538  * @index: used as a name of the node
539  * @priv: what's given back by devpts_get_priv
540  *
541  * The dentry for the created inode is returned.
542  * Remove it from /dev/pts/ with devpts_pty_kill().
543  */
devpts_pty_new(struct pts_fs_info * fsi,int index,void * priv)544 struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
545 {
546 	struct dentry *dentry;
547 	struct super_block *sb = fsi->sb;
548 	struct inode *inode;
549 	struct dentry *root;
550 	struct pts_mount_opts *opts;
551 	char s[12];
552 
553 	root = sb->s_root;
554 	opts = &fsi->mount_opts;
555 
556 	inode = new_inode(sb);
557 	if (!inode)
558 		return ERR_PTR(-ENOMEM);
559 
560 	inode->i_ino = index + 3;
561 	inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
562 	inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
563 	inode->i_mtime = inode->i_atime = inode_set_ctime_current(inode);
564 	init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));
565 
566 	sprintf(s, "%d", index);
567 
568 	dentry = d_alloc_name(root, s);
569 	if (dentry) {
570 		dentry->d_fsdata = priv;
571 		d_add(dentry, inode);
572 		fsnotify_create(d_inode(root), dentry);
573 	} else {
574 		iput(inode);
575 		dentry = ERR_PTR(-ENOMEM);
576 	}
577 
578 	return dentry;
579 }
580 
581 /**
582  * devpts_get_priv -- get private data for a slave
583  * @dentry: dentry of the slave
584  *
585  * Returns whatever was passed as priv in devpts_pty_new for a given inode.
586  */
devpts_get_priv(struct dentry * dentry)587 void *devpts_get_priv(struct dentry *dentry)
588 {
589 	if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
590 		return NULL;
591 	return dentry->d_fsdata;
592 }
593 
594 /**
595  * devpts_pty_kill -- remove inode form /dev/pts/
596  * @dentry: dentry of the slave to be removed
597  *
598  * This is an inverse operation of devpts_pty_new.
599  */
devpts_pty_kill(struct dentry * dentry)600 void devpts_pty_kill(struct dentry *dentry)
601 {
602 	WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
603 
604 	dentry->d_fsdata = NULL;
605 	drop_nlink(dentry->d_inode);
606 	d_drop(dentry);
607 	fsnotify_unlink(d_inode(dentry->d_parent), dentry);
608 	dput(dentry);	/* d_alloc_name() in devpts_pty_new() */
609 }
610 
init_devpts_fs(void)611 static int __init init_devpts_fs(void)
612 {
613 	int err = register_filesystem(&devpts_fs_type);
614 	if (!err) {
615 		register_sysctl("kernel/pty", pty_table);
616 	}
617 	return err;
618 }
619 module_init(init_devpts_fs)
620