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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/fs_context.h>
16#include <linux/fs_parser.h>
17#include <linux/sched.h>
18#include <linux/namei.h>
19#include <linux/slab.h>
20#include <linux/mount.h>
21#include <linux/tty.h>
22#include <linux/mutex.h>
23#include <linux/magic.h>
24#include <linux/idr.h>
25#include <linux/devpts_fs.h>
26#include <linux/fsnotify.h>
27#include <linux/seq_file.h>
28
29#define DEVPTS_DEFAULT_MODE 0600
30/*
31 * ptmx is a new node in /dev/pts and will be unused in legacy (single-
32 * instance) mode. To prevent surprises in user space, set permissions of
33 * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
34 * permissions.
35 */
36#define DEVPTS_DEFAULT_PTMX_MODE 0000
37#define PTMX_MINOR 2
38
39/*
40 * sysctl support for setting limits on the number of Unix98 ptys allocated.
41 * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
42 */
43static int pty_limit = NR_UNIX98_PTY_DEFAULT;
44static int pty_reserve = NR_UNIX98_PTY_RESERVE;
45static int pty_limit_min;
46static int pty_limit_max = INT_MAX;
47static atomic_t pty_count = ATOMIC_INIT(0);
48
49static const struct ctl_table pty_table[] = {
50 {
51 .procname = "max",
52 .maxlen = sizeof(int),
53 .mode = 0644,
54 .data = &pty_limit,
55 .proc_handler = proc_dointvec_minmax,
56 .extra1 = &pty_limit_min,
57 .extra2 = &pty_limit_max,
58 }, {
59 .procname = "reserve",
60 .maxlen = sizeof(int),
61 .mode = 0644,
62 .data = &pty_reserve,
63 .proc_handler = proc_dointvec_minmax,
64 .extra1 = &pty_limit_min,
65 .extra2 = &pty_limit_max,
66 }, {
67 .procname = "nr",
68 .maxlen = sizeof(int),
69 .mode = 0444,
70 .data = &pty_count,
71 .proc_handler = proc_dointvec,
72 },
73};
74
75struct 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
86enum {
87 Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance, Opt_max,
88 Opt_err
89};
90
91static const struct fs_parameter_spec devpts_param_specs[] = {
92 fsparam_gid ("gid", Opt_gid),
93 fsparam_s32 ("max", Opt_max),
94 fsparam_u32oct ("mode", Opt_mode),
95 fsparam_flag ("newinstance", Opt_newinstance),
96 fsparam_u32oct ("ptmxmode", Opt_ptmxmode),
97 fsparam_uid ("uid", Opt_uid),
98 {}
99};
100
101struct pts_fs_info {
102 struct ida allocated_ptys;
103 struct pts_mount_opts mount_opts;
104 struct super_block *sb;
105 struct inode *ptmx_inode; // borrowed
106};
107
108static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
109{
110 return sb->s_fs_info;
111}
112
113static 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 */
150struct 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
181struct 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
208out:
209 path_put(&path);
210 return result;
211}
212
213void devpts_release(struct pts_fs_info *fsi)
214{
215 deactivate_super(fsi->sb);
216}
217
218/*
219 * devpts_parse_param - Parse mount parameters
220 */
221static int devpts_parse_param(struct fs_context *fc, struct fs_parameter *param)
222{
223 struct pts_fs_info *fsi = fc->s_fs_info;
224 struct pts_mount_opts *opts = &fsi->mount_opts;
225 struct fs_parse_result result;
226 int opt;
227
228 opt = fs_parse(fc, devpts_param_specs, param, &result);
229 if (opt < 0)
230 return opt;
231
232 switch (opt) {
233 case Opt_uid:
234 opts->uid = result.uid;
235 opts->setuid = 1;
236 break;
237 case Opt_gid:
238 opts->gid = result.gid;
239 opts->setgid = 1;
240 break;
241 case Opt_mode:
242 opts->mode = result.uint_32 & S_IALLUGO;
243 break;
244 case Opt_ptmxmode:
245 opts->ptmxmode = result.uint_32 & S_IALLUGO;
246 break;
247 case Opt_newinstance:
248 break;
249 case Opt_max:
250 if (result.uint_32 > NR_UNIX98_PTY_MAX)
251 return invalf(fc, "max out of range");
252 if (result.uint_32 == 0)
253 return invalf(fc, "max must be greater than 0");
254 opts->max = result.uint_32;
255 break;
256 }
257
258 return 0;
259}
260
261static int mknod_ptmx(struct super_block *sb, struct fs_context *fc)
262{
263 int mode;
264 struct dentry *dentry;
265 struct inode *inode;
266 struct dentry *root = sb->s_root;
267 struct pts_fs_info *fsi = DEVPTS_SB(sb);
268 struct pts_mount_opts *opts = &fsi->mount_opts;
269 kuid_t ptmx_uid = current_fsuid();
270 kgid_t ptmx_gid = current_fsgid();
271
272 dentry = simple_start_creating(root, "ptmx");
273 if (IS_ERR(dentry)) {
274 pr_err("Unable to alloc dentry for ptmx node\n");
275 return PTR_ERR(dentry);
276 }
277
278 /*
279 * Create a new 'ptmx' node in this mount of devpts.
280 */
281 inode = new_inode(sb);
282 if (!inode) {
283 simple_done_creating(dentry);
284 pr_err("Unable to alloc inode for ptmx node\n");
285 return -ENOMEM;
286 }
287
288 inode->i_ino = 2;
289 simple_inode_init_ts(inode);
290
291 mode = S_IFCHR|opts->ptmxmode;
292 init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
293 inode->i_uid = ptmx_uid;
294 inode->i_gid = ptmx_gid;
295 fsi->ptmx_inode = inode;
296
297 d_make_persistent(dentry, inode);
298
299 simple_done_creating(dentry);
300
301 return 0;
302}
303
304static void update_ptmx_mode(struct pts_fs_info *fsi)
305{
306 fsi->ptmx_inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
307}
308
309static int devpts_reconfigure(struct fs_context *fc)
310{
311 struct pts_fs_info *fsi = DEVPTS_SB(fc->root->d_sb);
312 struct pts_fs_info *new = fc->s_fs_info;
313
314 /* Apply the revised options. We don't want to change ->reserve.
315 * Ideally, we'd update each option conditionally on it having been
316 * explicitly changed, but the default is to reset everything so that
317 * would break UAPI...
318 */
319 fsi->mount_opts.setuid = new->mount_opts.setuid;
320 fsi->mount_opts.setgid = new->mount_opts.setgid;
321 fsi->mount_opts.uid = new->mount_opts.uid;
322 fsi->mount_opts.gid = new->mount_opts.gid;
323 fsi->mount_opts.mode = new->mount_opts.mode;
324 fsi->mount_opts.ptmxmode = new->mount_opts.ptmxmode;
325 fsi->mount_opts.max = new->mount_opts.max;
326
327 /*
328 * parse_mount_options() restores options to default values
329 * before parsing and may have changed ptmxmode. So, update the
330 * mode in the inode too. Bogus options don't fail the remount,
331 * so do this even on error return.
332 */
333 update_ptmx_mode(fsi);
334
335 return 0;
336}
337
338static int devpts_show_options(struct seq_file *seq, struct dentry *root)
339{
340 struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
341 struct pts_mount_opts *opts = &fsi->mount_opts;
342
343 if (opts->setuid)
344 seq_printf(seq, ",uid=%u",
345 from_kuid_munged(&init_user_ns, opts->uid));
346 if (opts->setgid)
347 seq_printf(seq, ",gid=%u",
348 from_kgid_munged(&init_user_ns, opts->gid));
349 seq_printf(seq, ",mode=%03o", opts->mode);
350 seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
351 if (opts->max < NR_UNIX98_PTY_MAX)
352 seq_printf(seq, ",max=%d", opts->max);
353
354 return 0;
355}
356
357static const struct super_operations devpts_sops = {
358 .statfs = simple_statfs,
359 .show_options = devpts_show_options,
360};
361
362static int devpts_fill_super(struct super_block *s, struct fs_context *fc)
363{
364 struct pts_fs_info *fsi = DEVPTS_SB(s);
365 struct inode *inode;
366
367 s->s_iflags &= ~SB_I_NODEV;
368 s->s_blocksize = 1024;
369 s->s_blocksize_bits = 10;
370 s->s_magic = DEVPTS_SUPER_MAGIC;
371 s->s_op = &devpts_sops;
372 s->s_d_flags = DCACHE_DONTCACHE;
373 s->s_time_gran = 1;
374 fsi->sb = s;
375
376 inode = new_inode(s);
377 if (!inode)
378 return -ENOMEM;
379 inode->i_ino = 1;
380 simple_inode_init_ts(inode);
381 inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
382 inode->i_op = &simple_dir_inode_operations;
383 inode->i_fop = &simple_dir_operations;
384 set_nlink(inode, 2);
385
386 s->s_root = d_make_root(inode);
387 if (!s->s_root) {
388 pr_err("get root dentry failed\n");
389 return -ENOMEM;
390 }
391
392 return mknod_ptmx(s, fc);
393}
394
395/*
396 * devpts_get_tree()
397 *
398 * Mount a new (private) instance of devpts. PTYs created in this
399 * instance are independent of the PTYs in other devpts instances.
400 */
401static int devpts_get_tree(struct fs_context *fc)
402{
403 return get_tree_nodev(fc, devpts_fill_super);
404}
405
406static void devpts_free_fc(struct fs_context *fc)
407{
408 kfree(fc->s_fs_info);
409}
410
411static const struct fs_context_operations devpts_context_ops = {
412 .free = devpts_free_fc,
413 .parse_param = devpts_parse_param,
414 .get_tree = devpts_get_tree,
415 .reconfigure = devpts_reconfigure,
416};
417
418/*
419 * Set up the filesystem mount context.
420 */
421static int devpts_init_fs_context(struct fs_context *fc)
422{
423 struct pts_fs_info *fsi;
424
425 fsi = kzalloc_obj(struct pts_fs_info);
426 if (!fsi)
427 return -ENOMEM;
428
429 ida_init(&fsi->allocated_ptys);
430 fsi->mount_opts.uid = GLOBAL_ROOT_UID;
431 fsi->mount_opts.gid = GLOBAL_ROOT_GID;
432 fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
433 fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
434 fsi->mount_opts.max = NR_UNIX98_PTY_MAX;
435
436 if (fc->purpose == FS_CONTEXT_FOR_MOUNT &&
437 current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns)
438 fsi->mount_opts.reserve = true;
439
440 fc->s_fs_info = fsi;
441 fc->ops = &devpts_context_ops;
442 return 0;
443}
444
445static void devpts_kill_sb(struct super_block *sb)
446{
447 struct pts_fs_info *fsi = DEVPTS_SB(sb);
448
449 if (fsi)
450 ida_destroy(&fsi->allocated_ptys);
451 kfree(fsi);
452 kill_anon_super(sb);
453}
454
455static struct file_system_type devpts_fs_type = {
456 .name = "devpts",
457 .init_fs_context = devpts_init_fs_context,
458 .parameters = devpts_param_specs,
459 .kill_sb = devpts_kill_sb,
460 .fs_flags = FS_USERNS_MOUNT,
461};
462
463/*
464 * The normal naming convention is simply /dev/pts/<number>; this conforms
465 * to the System V naming convention
466 */
467
468int devpts_new_index(struct pts_fs_info *fsi)
469{
470 int index = -ENOSPC;
471
472 if (atomic_inc_return(&pty_count) >= (pty_limit -
473 (fsi->mount_opts.reserve ? 0 : pty_reserve)))
474 goto out;
475
476 index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
477 GFP_KERNEL);
478
479out:
480 if (index < 0)
481 atomic_dec(&pty_count);
482 return index;
483}
484
485void devpts_kill_index(struct pts_fs_info *fsi, int idx)
486{
487 ida_free(&fsi->allocated_ptys, idx);
488 atomic_dec(&pty_count);
489}
490
491/**
492 * devpts_pty_new -- create a new inode in /dev/pts/
493 * @fsi: Filesystem info for this instance.
494 * @index: used as a name of the node
495 * @priv: what's given back by devpts_get_priv
496 *
497 * The dentry for the created inode is returned.
498 * Remove it from /dev/pts/ with devpts_pty_kill().
499 */
500struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
501{
502 struct dentry *dentry;
503 struct super_block *sb = fsi->sb;
504 struct inode *inode;
505 struct dentry *root;
506 struct pts_mount_opts *opts;
507 char s[12];
508
509 root = sb->s_root;
510 opts = &fsi->mount_opts;
511
512 inode = new_inode(sb);
513 if (!inode)
514 return ERR_PTR(-ENOMEM);
515
516 inode->i_ino = index + 3;
517 inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
518 inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
519 simple_inode_init_ts(inode);
520 init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));
521
522 sprintf(s, "%d", index);
523
524 dentry = d_alloc_name(root, s);
525 if (!dentry) {
526 iput(inode);
527 return ERR_PTR(-ENOMEM);
528 }
529 dentry->d_fsdata = priv;
530 d_make_persistent(dentry, inode);
531 fsnotify_create(d_inode(root), dentry);
532 dput(dentry);
533 return dentry; // borrowed
534}
535
536/**
537 * devpts_get_priv -- get private data for a slave
538 * @dentry: dentry of the slave
539 *
540 * Returns whatever was passed as priv in devpts_pty_new for a given inode.
541 */
542void *devpts_get_priv(struct dentry *dentry)
543{
544 if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
545 return NULL;
546 return dentry->d_fsdata;
547}
548
549/**
550 * devpts_pty_kill -- remove inode form /dev/pts/
551 * @dentry: dentry of the slave to be removed
552 *
553 * This is an inverse operation of devpts_pty_new.
554 */
555void devpts_pty_kill(struct dentry *dentry)
556{
557 WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
558
559 dentry->d_fsdata = NULL;
560 drop_nlink(dentry->d_inode);
561 d_drop(dentry);
562 fsnotify_unlink(d_inode(dentry->d_parent), dentry);
563 d_make_discardable(dentry);
564}
565
566static int __init init_devpts_fs(void)
567{
568 int err = register_filesystem(&devpts_fs_type);
569 if (!err) {
570 register_sysctl("kernel/pty", pty_table);
571 }
572 return err;
573}
574module_init(init_devpts_fs)