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Linux/fs/devpts/inode.c

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

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