security/landlock/syscalls.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Landlock LSM - System call implementations and user space interfaces
   4  *
   5  * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
   6  * Copyright © 2018-2020 ANSSI
   7  */
   8
   9 #include <asm/current.h>
  10 #include <linux/anon_inodes.h>
  11 #include <linux/build_bug.h>
  12 #include <linux/capability.h>
  13 #include <linux/compiler_types.h>
  14 #include <linux/dcache.h>
  15 #include <linux/err.h>
  16 #include <linux/errno.h>
  17 #include <linux/fs.h>
  18 #include <linux/limits.h>
  19 #include <linux/mount.h>
  20 #include <linux/path.h>
  21 #include <linux/sched.h>
  22 #include <linux/security.h>
  23 #include <linux/stddef.h>
  24 #include <linux/syscalls.h>
  25 #include <linux/types.h>
  26 #include <linux/uaccess.h>
  27 #include <uapi/linux/landlock.h>
  28
  29 #include "cred.h"
  30 #include "fs.h"
  31 #include "limits.h"
  32 #include "ruleset.h"
  33 #include "setup.h"
  34
  35 /**
  36  * copy_min_struct_from_user - Safe future-proof argument copying
  37  *
  38  * Extend copy_struct_from_user() to check for consistent user buffer.
  39  *
  40  * @dst: Kernel space pointer or NULL.
  41  * @ksize: Actual size of the data pointed to by @dst.
  42  * @ksize_min: Minimal required size to be copied.
  43  * @src: User space pointer or NULL.
  44  * @usize: (Alleged) size of the data pointed to by @src.
  45  */
  46 static __always_inline int copy_min_struct_from_user(void *const dst,
  47                 const size_t ksize, const size_t ksize_min,
  48                 const void __user *const src, const size_t usize)
  49 {
  50         /* Checks buffer inconsistencies. */
  51         BUILD_BUG_ON(!dst);
  52         if (!src)
  53                 return -EFAULT;
  54
  55         /* Checks size ranges. */
  56         BUILD_BUG_ON(ksize <= 0);
  57         BUILD_BUG_ON(ksize < ksize_min);
  58         if (usize < ksize_min)
  59                 return -EINVAL;
  60         if (usize > PAGE_SIZE)
  61                 return -E2BIG;
  62
  63         /* Copies user buffer and fills with zeros. */
  64         return copy_struct_from_user(dst, ksize, src, usize);
  65 }
  66
  67 /*
  68  * This function only contains arithmetic operations with constants, leading to
  69  * BUILD_BUG_ON().  The related code is evaluated and checked at build time,
  70  * but it is then ignored thanks to compiler optimizations.
  71  */
  72 static void build_check_abi(void)
  73 {
  74         struct landlock_ruleset_attr ruleset_attr;
  75         struct landlock_path_beneath_attr path_beneath_attr;
  76         size_t ruleset_size, path_beneath_size;
  77
  78         /*
  79          * For each user space ABI structures, first checks that there is no
  80          * hole in them, then checks that all architectures have the same
  81          * struct size.
  82          */
  83         ruleset_size = sizeof(ruleset_attr.handled_access_fs);
  84         BUILD_BUG_ON(sizeof(ruleset_attr) != ruleset_size);
  85         BUILD_BUG_ON(sizeof(ruleset_attr) != 8);
  86
  87         path_beneath_size = sizeof(path_beneath_attr.allowed_access);
  88         path_beneath_size += sizeof(path_beneath_attr.parent_fd);
  89         BUILD_BUG_ON(sizeof(path_beneath_attr) != path_beneath_size);
  90         BUILD_BUG_ON(sizeof(path_beneath_attr) != 12);
  91 }
  92
  93 /* Ruleset handling */
  94
  95 static int fop_ruleset_release(struct inode *const inode,
  96                 struct file *const filp)
  97 {
  98         struct landlock_ruleset *ruleset = filp->private_data;
  99
 100         landlock_put_ruleset(ruleset);
 101         return 0;
 102 }
 103
 104 static ssize_t fop_dummy_read(struct file *const filp, char __user *const buf,
 105                 const size_t size, loff_t *const ppos)
 106 {
 107         /* Dummy handler to enable FMODE_CAN_READ. */
 108         return -EINVAL;
 109 }
 110
 111 static ssize_t fop_dummy_write(struct file *const filp,
 112                 const char __user *const buf, const size_t size,
 113                 loff_t *const ppos)
 114 {
 115         /* Dummy handler to enable FMODE_CAN_WRITE. */
 116         return -EINVAL;
 117 }
 118
 119 /*
 120  * A ruleset file descriptor enables to build a ruleset by adding (i.e.
 121  * writing) rule after rule, without relying on the task's context.  This
 122  * reentrant design is also used in a read way to enforce the ruleset on the
 123  * current task.
 124  */
 125 static const struct file_operations ruleset_fops = {
 126         .release = fop_ruleset_release,
 127         .read = fop_dummy_read,
 128         .write = fop_dummy_write,
 129 };
 130
 131 #define LANDLOCK_ABI_VERSION    1
 132
 133 /**
 134  * sys_landlock_create_ruleset - Create a new ruleset
 135  *
 136  * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of
 137  *        the new ruleset.
 138  * @size: Size of the pointed &struct landlock_ruleset_attr (needed for
 139  *        backward and forward compatibility).
 140  * @flags: Supported value: %LANDLOCK_CREATE_RULESET_VERSION.
 141  *
 142  * This system call enables to create a new Landlock ruleset, and returns the
 143  * related file descriptor on success.
 144  *
 145  * If @flags is %LANDLOCK_CREATE_RULESET_VERSION and @attr is NULL and @size is
 146  * 0, then the returned value is the highest supported Landlock ABI version
 147  * (starting at 1).
 148  *
 149  * Possible returned errors are:
 150  *
 151  * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
 152  * - EINVAL: unknown @flags, or unknown access, or too small @size;
 153  * - E2BIG or EFAULT: @attr or @size inconsistencies;
 154  * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs.
 155  */
 156 SYSCALL_DEFINE3(landlock_create_ruleset,
 157                 const struct landlock_ruleset_attr __user *const, attr,
 158                 const size_t, size, const __u32, flags)
 159 {
 160         struct landlock_ruleset_attr ruleset_attr;
 161         struct landlock_ruleset *ruleset;
 162         int err, ruleset_fd;
 163
 164         /* Build-time checks. */
 165         build_check_abi();
 166
 167         if (!landlock_initialized)
 168                 return -EOPNOTSUPP;
 169
 170         if (flags) {
 171                 if ((flags == LANDLOCK_CREATE_RULESET_VERSION)
 172                                 && !attr && !size)
 173                         return LANDLOCK_ABI_VERSION;
 174                 return -EINVAL;
 175         }
 176
 177         /* Copies raw user space buffer. */
 178         err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr),
 179                         offsetofend(typeof(ruleset_attr), handled_access_fs),
 180                         attr, size);
 181         if (err)
 182                 return err;
 183
 184         /* Checks content (and 32-bits cast). */
 185         if ((ruleset_attr.handled_access_fs | LANDLOCK_MASK_ACCESS_FS) !=
 186                         LANDLOCK_MASK_ACCESS_FS)
 187                 return -EINVAL;
 188
 189         /* Checks arguments and transforms to kernel struct. */
 190         ruleset = landlock_create_ruleset(ruleset_attr.handled_access_fs);
 191         if (IS_ERR(ruleset))
 192                 return PTR_ERR(ruleset);
 193
 194         /* Creates anonymous FD referring to the ruleset. */
 195         ruleset_fd = anon_inode_getfd("[landlock-ruleset]", &ruleset_fops,
 196                         ruleset, O_RDWR | O_CLOEXEC);
 197         if (ruleset_fd < 0)
 198                 landlock_put_ruleset(ruleset);
 199         return ruleset_fd;
 200 }
 201
 202 /*
 203  * Returns an owned ruleset from a FD. It is thus needed to call
 204  * landlock_put_ruleset() on the return value.
 205  */
 206 static struct landlock_ruleset *get_ruleset_from_fd(const int fd,
 207                 const fmode_t mode)
 208 {
 209         struct fd ruleset_f;
 210         struct landlock_ruleset *ruleset;
 211
 212         ruleset_f = fdget(fd);
 213         if (!ruleset_f.file)
 214                 return ERR_PTR(-EBADF);
 215
 216         /* Checks FD type and access right. */
 217         if (ruleset_f.file->f_op != &ruleset_fops) {
 218                 ruleset = ERR_PTR(-EBADFD);
 219                 goto out_fdput;
 220         }
 221         if (!(ruleset_f.file->f_mode & mode)) {
 222                 ruleset = ERR_PTR(-EPERM);
 223                 goto out_fdput;
 224         }
 225         ruleset = ruleset_f.file->private_data;
 226         if (WARN_ON_ONCE(ruleset->num_layers != 1)) {
 227                 ruleset = ERR_PTR(-EINVAL);
 228                 goto out_fdput;
 229         }
 230         landlock_get_ruleset(ruleset);
 231
 232 out_fdput:
 233         fdput(ruleset_f);
 234         return ruleset;
 235 }
 236
 237 /* Path handling */
 238
 239 /*
 240  * @path: Must call put_path(@path) after the call if it succeeded.
 241  */
 242 static int get_path_from_fd(const s32 fd, struct path *const path)
 243 {
 244         struct fd f;
 245         int err = 0;
 246
 247         BUILD_BUG_ON(!__same_type(fd,
 248                 ((struct landlock_path_beneath_attr *)NULL)->parent_fd));
 249
 250         /* Handles O_PATH. */
 251         f = fdget_raw(fd);
 252         if (!f.file)
 253                 return -EBADF;
 254         /*
 255          * Forbids ruleset FDs, internal filesystems (e.g. nsfs), including
 256          * pseudo filesystems that will never be mountable (e.g. sockfs,
 257          * pipefs).
 258          */
 259         if ((f.file->f_op == &ruleset_fops) ||
 260                         (f.file->f_path.mnt->mnt_flags & MNT_INTERNAL) ||
 261                         (f.file->f_path.dentry->d_sb->s_flags & SB_NOUSER) ||
 262                         d_is_negative(f.file->f_path.dentry) ||
 263                         IS_PRIVATE(d_backing_inode(f.file->f_path.dentry))) {
 264                 err = -EBADFD;
 265                 goto out_fdput;
 266         }
 267         *path = f.file->f_path;
 268         path_get(path);
 269
 270 out_fdput:
 271         fdput(f);
 272         return err;
 273 }
 274
 275 /**
 276  * sys_landlock_add_rule - Add a new rule to a ruleset
 277  *
 278  * @ruleset_fd: File descriptor tied to the ruleset that should be extended
 279  *              with the new rule.
 280  * @rule_type: Identify the structure type pointed to by @rule_attr (only
 281  *             LANDLOCK_RULE_PATH_BENEATH for now).
 282  * @rule_attr: Pointer to a rule (only of type &struct
 283  *             landlock_path_beneath_attr for now).
 284  * @flags: Must be 0.
 285  *
 286  * This system call enables to define a new rule and add it to an existing
 287  * ruleset.
 288  *
 289  * Possible returned errors are:
 290  *
 291  * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
 292  * - EINVAL: @flags is not 0, or inconsistent access in the rule (i.e.
 293  *   &landlock_path_beneath_attr.allowed_access is not a subset of the rule's
 294  *   accesses);
 295  * - ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access);
 296  * - EBADF: @ruleset_fd is not a file descriptor for the current thread, or a
 297  *   member of @rule_attr is not a file descriptor as expected;
 298  * - EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of
 299  *   @rule_attr is not the expected file descriptor type (e.g. file open
 300  *   without O_PATH);
 301  * - EPERM: @ruleset_fd has no write access to the underlying ruleset;
 302  * - EFAULT: @rule_attr inconsistency.
 303  */
 304 SYSCALL_DEFINE4(landlock_add_rule,
 305                 const int, ruleset_fd, const enum landlock_rule_type, rule_type,
 306                 const void __user *const, rule_attr, const __u32, flags)
 307 {
 308         struct landlock_path_beneath_attr path_beneath_attr;
 309         struct path path;
 310         struct landlock_ruleset *ruleset;
 311         int res, err;
 312
 313         if (!landlock_initialized)
 314                 return -EOPNOTSUPP;
 315
 316         /* No flag for now. */
 317         if (flags)
 318                 return -EINVAL;
 319
 320         if (rule_type != LANDLOCK_RULE_PATH_BENEATH)
 321                 return -EINVAL;
 322
 323         /* Copies raw user space buffer, only one type for now. */
 324         res = copy_from_user(&path_beneath_attr, rule_attr,
 325                         sizeof(path_beneath_attr));
 326         if (res)
 327                 return -EFAULT;
 328
 329         /* Gets and checks the ruleset. */
 330         ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_WRITE);
 331         if (IS_ERR(ruleset))
 332                 return PTR_ERR(ruleset);
 333
 334         /*
 335          * Informs about useless rule: empty allowed_access (i.e. deny rules)
 336          * are ignored in path walks.
 337          */
 338         if (!path_beneath_attr.allowed_access) {
 339                 err = -ENOMSG;
 340                 goto out_put_ruleset;
 341         }
 342         /*
 343          * Checks that allowed_access matches the @ruleset constraints
 344          * (ruleset->fs_access_masks[0] is automatically upgraded to 64-bits).
 345          */
 346         if ((path_beneath_attr.allowed_access | ruleset->fs_access_masks[0]) !=
 347                         ruleset->fs_access_masks[0]) {
 348                 err = -EINVAL;
 349                 goto out_put_ruleset;
 350         }
 351
 352         /* Gets and checks the new rule. */
 353         err = get_path_from_fd(path_beneath_attr.parent_fd, &path);
 354         if (err)
 355                 goto out_put_ruleset;
 356
 357         /* Imports the new rule. */
 358         err = landlock_append_fs_rule(ruleset, &path,
 359                         path_beneath_attr.allowed_access);
 360         path_put(&path);
 361
 362 out_put_ruleset:
 363         landlock_put_ruleset(ruleset);
 364         return err;
 365 }
 366
 367 /* Enforcement */
 368
 369 /**
 370  * sys_landlock_restrict_self - Enforce a ruleset on the calling thread
 371  *
 372  * @ruleset_fd: File descriptor tied to the ruleset to merge with the target.
 373  * @flags: Must be 0.
 374  *
 375  * This system call enables to enforce a Landlock ruleset on the current
 376  * thread.  Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its
 377  * namespace or is running with no_new_privs.  This avoids scenarios where
 378  * unprivileged tasks can affect the behavior of privileged children.
 379  *
 380  * Possible returned errors are:
 381  *
 382  * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
 383  * - EINVAL: @flags is not 0.
 384  * - EBADF: @ruleset_fd is not a file descriptor for the current thread;
 385  * - EBADFD: @ruleset_fd is not a ruleset file descriptor;
 386  * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the
 387  *   current thread is not running with no_new_privs, or it doesn't have
 388  *   CAP_SYS_ADMIN in its namespace.
 389  * - E2BIG: The maximum number of stacked rulesets is reached for the current
 390  *   thread.
 391  */
 392 SYSCALL_DEFINE2(landlock_restrict_self,
 393                 const int, ruleset_fd, const __u32, flags)
 394 {
 395         struct landlock_ruleset *new_dom, *ruleset;
 396         struct cred *new_cred;
 397         struct landlock_cred_security *new_llcred;
 398         int err;
 399
 400         if (!landlock_initialized)
 401                 return -EOPNOTSUPP;
 402
 403         /* No flag for now. */
 404         if (flags)
 405                 return -EINVAL;
 406
 407         /*
 408          * Similar checks as for seccomp(2), except that an -EPERM may be
 409          * returned.
 410          */
 411         if (!task_no_new_privs(current) &&
 412                         !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN))
 413                 return -EPERM;
 414
 415         /* Gets and checks the ruleset. */
 416         ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ);
 417         if (IS_ERR(ruleset))
 418                 return PTR_ERR(ruleset);
 419
 420         /* Prepares new credentials. */
 421         new_cred = prepare_creds();
 422         if (!new_cred) {
 423                 err = -ENOMEM;
 424                 goto out_put_ruleset;
 425         }
 426         new_llcred = landlock_cred(new_cred);
 427
 428         /*
 429          * There is no possible race condition while copying and manipulating
 430          * the current credentials because they are dedicated per thread.
 431          */
 432         new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset);
 433         if (IS_ERR(new_dom)) {
 434                 err = PTR_ERR(new_dom);
 435                 goto out_put_creds;
 436         }
 437
 438         /* Replaces the old (prepared) domain. */
 439         landlock_put_ruleset(new_llcred->domain);
 440         new_llcred->domain = new_dom;
 441
 442         landlock_put_ruleset(ruleset);
 443         return commit_creds(new_cred);
 444
 445 out_put_creds:
 446         abort_creds(new_cred);
 447
 448 out_put_ruleset:
 449         landlock_put_ruleset(ruleset);
 450         return err;
 451 }