--- /dev/null
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - Filesystem management and hooks
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#include <linux/atomic.h>
+#include <linux/bitops.h>
+#include <linux/bits.h>
+#include <linux/compiler_types.h>
+#include <linux/dcache.h>
+#include <linux/err.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/limits.h>
+#include <linux/list.h>
+#include <linux/lsm_hooks.h>
+#include <linux/mount.h>
+#include <linux/namei.h>
+#include <linux/path.h>
+#include <linux/rcupdate.h>
+#include <linux/spinlock.h>
+#include <linux/stat.h>
+#include <linux/types.h>
+#include <linux/wait_bit.h>
+#include <linux/workqueue.h>
+#include <uapi/linux/landlock.h>
+
+#include "common.h"
+#include "cred.h"
+#include "fs.h"
+#include "limits.h"
+#include "object.h"
+#include "ruleset.h"
+#include "setup.h"
+
+/* Underlying object management */
+
+static void release_inode(struct landlock_object *const object)
+ __releases(object->lock)
+{
+ struct inode *const inode = object->underobj;
+ struct super_block *sb;
+
+ if (!inode) {
+ spin_unlock(&object->lock);
+ return;
+ }
+
+ /*
+ * Protects against concurrent use by hook_sb_delete() of the reference
+ * to the underlying inode.
+ */
+ object->underobj = NULL;
+ /*
+ * Makes sure that if the filesystem is concurrently unmounted,
+ * hook_sb_delete() will wait for us to finish iput().
+ */
+ sb = inode->i_sb;
+ atomic_long_inc(&landlock_superblock(sb)->inode_refs);
+ spin_unlock(&object->lock);
+ /*
+ * Because object->underobj was not NULL, hook_sb_delete() and
+ * get_inode_object() guarantee that it is safe to reset
+ * landlock_inode(inode)->object while it is not NULL. It is therefore
+ * not necessary to lock inode->i_lock.
+ */
+ rcu_assign_pointer(landlock_inode(inode)->object, NULL);
+ /*
+ * Now, new rules can safely be tied to @inode with get_inode_object().
+ */
+
+ iput(inode);
+ if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs))
+ wake_up_var(&landlock_superblock(sb)->inode_refs);
+}
+
+static const struct landlock_object_underops landlock_fs_underops = {
+ .release = release_inode
+};
+
+/* Ruleset management */
+
+static struct landlock_object *get_inode_object(struct inode *const inode)
+{
+ struct landlock_object *object, *new_object;
+ struct landlock_inode_security *inode_sec = landlock_inode(inode);
+
+ rcu_read_lock();
+retry:
+ object = rcu_dereference(inode_sec->object);
+ if (object) {
+ if (likely(refcount_inc_not_zero(&object->usage))) {
+ rcu_read_unlock();
+ return object;
+ }
+ /*
+ * We are racing with release_inode(), the object is going
+ * away. Wait for release_inode(), then retry.
+ */
+ spin_lock(&object->lock);
+ spin_unlock(&object->lock);
+ goto retry;
+ }
+ rcu_read_unlock();
+
+ /*
+ * If there is no object tied to @inode, then create a new one (without
+ * holding any locks).
+ */
+ new_object = landlock_create_object(&landlock_fs_underops, inode);
+ if (IS_ERR(new_object))
+ return new_object;
+
+ /*
+ * Protects against concurrent calls to get_inode_object() or
+ * hook_sb_delete().
+ */
+ spin_lock(&inode->i_lock);
+ if (unlikely(rcu_access_pointer(inode_sec->object))) {
+ /* Someone else just created the object, bail out and retry. */
+ spin_unlock(&inode->i_lock);
+ kfree(new_object);
+
+ rcu_read_lock();
+ goto retry;
+ }
+
+ /*
+ * @inode will be released by hook_sb_delete() on its superblock
+ * shutdown, or by release_inode() when no more ruleset references the
+ * related object.
+ */
+ ihold(inode);
+ rcu_assign_pointer(inode_sec->object, new_object);
+ spin_unlock(&inode->i_lock);
+ return new_object;
+}
+
+/* All access rights that can be tied to files. */
+#define ACCESS_FILE ( \
+ LANDLOCK_ACCESS_FS_EXECUTE | \
+ LANDLOCK_ACCESS_FS_WRITE_FILE | \
+ LANDLOCK_ACCESS_FS_READ_FILE)
+
+/*
+ * @path: Should have been checked by get_path_from_fd().
+ */
+int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
+ const struct path *const path, u32 access_rights)
+{
+ int err;
+ struct landlock_object *object;
+
+ /* Files only get access rights that make sense. */
+ if (!d_is_dir(path->dentry) && (access_rights | ACCESS_FILE) !=
+ ACCESS_FILE)
+ return -EINVAL;
+ if (WARN_ON_ONCE(ruleset->num_layers != 1))
+ return -EINVAL;
+
+ /* Transforms relative access rights to absolute ones. */
+ access_rights |= LANDLOCK_MASK_ACCESS_FS & ~ruleset->fs_access_masks[0];
+ object = get_inode_object(d_backing_inode(path->dentry));
+ if (IS_ERR(object))
+ return PTR_ERR(object);
+ mutex_lock(&ruleset->lock);
+ err = landlock_insert_rule(ruleset, object, access_rights);
+ mutex_unlock(&ruleset->lock);
+ /*
+ * No need to check for an error because landlock_insert_rule()
+ * increments the refcount for the new object if needed.
+ */
+ landlock_put_object(object);
+ return err;
+}
+
+/* Access-control management */
+
+static inline u64 unmask_layers(
+ const struct landlock_ruleset *const domain,
+ const struct path *const path, const u32 access_request,
+ u64 layer_mask)
+{
+ const struct landlock_rule *rule;
+ const struct inode *inode;
+ size_t i;
+
+ if (d_is_negative(path->dentry))
+ /* Ignore nonexistent leafs. */
+ return layer_mask;
+ inode = d_backing_inode(path->dentry);
+ rcu_read_lock();
+ rule = landlock_find_rule(domain,
+ rcu_dereference(landlock_inode(inode)->object));
+ rcu_read_unlock();
+ if (!rule)
+ return layer_mask;
+
+ /*
+ * An access is granted if, for each policy layer, at least one rule
+ * encountered on the pathwalk grants the requested accesses,
+ * regardless of their position in the layer stack. We must then check
+ * the remaining layers for each inode, from the first added layer to
+ * the last one.
+ */
+ for (i = 0; i < rule->num_layers; i++) {
+ const struct landlock_layer *const layer = &rule->layers[i];
+ const u64 layer_level = BIT_ULL(layer->level - 1);
+
+ /* Checks that the layer grants access to the full request. */
+ if ((layer->access & access_request) == access_request) {
+ layer_mask &= ~layer_level;
+
+ if (layer_mask == 0)
+ return layer_mask;
+ }
+ }
+ return layer_mask;
+}
+
+static int check_access_path(const struct landlock_ruleset *const domain,
+ const struct path *const path, u32 access_request)
+{
+ bool allowed = false;
+ struct path walker_path;
+ u64 layer_mask;
+ size_t i;
+
+ /* Make sure all layers can be checked. */
+ BUILD_BUG_ON(BITS_PER_TYPE(layer_mask) < LANDLOCK_MAX_NUM_LAYERS);
+
+ if (!access_request)
+ return 0;
+ if (WARN_ON_ONCE(!domain || !path))
+ return 0;
+ /*
+ * Allows access to pseudo filesystems that will never be mountable
+ * (e.g. sockfs, pipefs), but can still be reachable through
+ * /proc/<pid>/fd/<file-descriptor> .
+ */
+ if ((path->dentry->d_sb->s_flags & SB_NOUSER) ||
+ (d_is_positive(path->dentry) &&
+ unlikely(IS_PRIVATE(d_backing_inode(path->dentry)))))
+ return 0;
+ if (WARN_ON_ONCE(domain->num_layers < 1))
+ return -EACCES;
+
+ /* Saves all layers handling a subset of requested accesses. */
+ layer_mask = 0;
+ for (i = 0; i < domain->num_layers; i++) {
+ if (domain->fs_access_masks[i] & access_request)
+ layer_mask |= BIT_ULL(i);
+ }
+ /* An access request not handled by the domain is allowed. */
+ if (layer_mask == 0)
+ return 0;
+
+ walker_path = *path;
+ path_get(&walker_path);
+ /*
+ * We need to walk through all the hierarchy to not miss any relevant
+ * restriction.
+ */
+ while (true) {
+ struct dentry *parent_dentry;
+
+ layer_mask = unmask_layers(domain, &walker_path,
+ access_request, layer_mask);
+ if (layer_mask == 0) {
+ /* Stops when a rule from each layer grants access. */
+ allowed = true;
+ break;
+ }
+
+jump_up:
+ if (walker_path.dentry == walker_path.mnt->mnt_root) {
+ if (follow_up(&walker_path)) {
+ /* Ignores hidden mount points. */
+ goto jump_up;
+ } else {
+ /*
+ * Stops at the real root. Denies access
+ * because not all layers have granted access.
+ */
+ allowed = false;
+ break;
+ }
+ }
+ if (unlikely(IS_ROOT(walker_path.dentry))) {
+ /*
+ * Stops at disconnected root directories. Only allows
+ * access to internal filesystems (e.g. nsfs, which is
+ * reachable through /proc/<pid>/ns/<namespace>).
+ */
+ allowed = !!(walker_path.mnt->mnt_flags & MNT_INTERNAL);
+ break;
+ }
+ parent_dentry = dget_parent(walker_path.dentry);
+ dput(walker_path.dentry);
+ walker_path.dentry = parent_dentry;
+ }
+ path_put(&walker_path);
+ return allowed ? 0 : -EACCES;
+}
+
+static inline int current_check_access_path(const struct path *const path,
+ const u32 access_request)
+{
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+ return check_access_path(dom, path, access_request);
+}
+
+/* Inode hooks */
+
+static void hook_inode_free_security(struct inode *const inode)
+{
+ /*
+ * All inodes must already have been untied from their object by
+ * release_inode() or hook_sb_delete().
+ */
+ WARN_ON_ONCE(landlock_inode(inode)->object);
+}
+
+/* Super-block hooks */
+
+/*
+ * Release the inodes used in a security policy.
+ *
+ * Cf. fsnotify_unmount_inodes() and invalidate_inodes()
+ */
+static void hook_sb_delete(struct super_block *const sb)
+{
+ struct inode *inode, *prev_inode = NULL;
+
+ if (!landlock_initialized)
+ return;
+
+ spin_lock(&sb->s_inode_list_lock);
+ list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+ struct landlock_object *object;
+
+ /* Only handles referenced inodes. */
+ if (!atomic_read(&inode->i_count))
+ continue;
+
+ /*
+ * Protects against concurrent modification of inode (e.g.
+ * from get_inode_object()).
+ */
+ spin_lock(&inode->i_lock);
+ /*
+ * Checks I_FREEING and I_WILL_FREE to protect against a race
+ * condition when release_inode() just called iput(), which
+ * could lead to a NULL dereference of inode->security or a
+ * second call to iput() for the same Landlock object. Also
+ * checks I_NEW because such inode cannot be tied to an object.
+ */
+ if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
+ spin_unlock(&inode->i_lock);
+ continue;
+ }
+
+ rcu_read_lock();
+ object = rcu_dereference(landlock_inode(inode)->object);
+ if (!object) {
+ rcu_read_unlock();
+ spin_unlock(&inode->i_lock);
+ continue;
+ }
+ /* Keeps a reference to this inode until the next loop walk. */
+ __iget(inode);
+ spin_unlock(&inode->i_lock);
+
+ /*
+ * If there is no concurrent release_inode() ongoing, then we
+ * are in charge of calling iput() on this inode, otherwise we
+ * will just wait for it to finish.
+ */
+ spin_lock(&object->lock);
+ if (object->underobj == inode) {
+ object->underobj = NULL;
+ spin_unlock(&object->lock);
+ rcu_read_unlock();
+
+ /*
+ * Because object->underobj was not NULL,
+ * release_inode() and get_inode_object() guarantee
+ * that it is safe to reset
+ * landlock_inode(inode)->object while it is not NULL.
+ * It is therefore not necessary to lock inode->i_lock.
+ */
+ rcu_assign_pointer(landlock_inode(inode)->object, NULL);
+ /*
+ * At this point, we own the ihold() reference that was
+ * originally set up by get_inode_object() and the
+ * __iget() reference that we just set in this loop
+ * walk. Therefore the following call to iput() will
+ * not sleep nor drop the inode because there is now at
+ * least two references to it.
+ */
+ iput(inode);
+ } else {
+ spin_unlock(&object->lock);
+ rcu_read_unlock();
+ }
+
+ if (prev_inode) {
+ /*
+ * At this point, we still own the __iget() reference
+ * that we just set in this loop walk. Therefore we
+ * can drop the list lock and know that the inode won't
+ * disappear from under us until the next loop walk.
+ */
+ spin_unlock(&sb->s_inode_list_lock);
+ /*
+ * We can now actually put the inode reference from the
+ * previous loop walk, which is not needed anymore.
+ */
+ iput(prev_inode);
+ cond_resched();
+ spin_lock(&sb->s_inode_list_lock);
+ }
+ prev_inode = inode;
+ }
+ spin_unlock(&sb->s_inode_list_lock);
+
+ /* Puts the inode reference from the last loop walk, if any. */
+ if (prev_inode)
+ iput(prev_inode);
+ /* Waits for pending iput() in release_inode(). */
+ wait_var_event(&landlock_superblock(sb)->inode_refs, !atomic_long_read(
+ &landlock_superblock(sb)->inode_refs));
+}
+
+/*
+ * Because a Landlock security policy is defined according to the filesystem
+ * topology (i.e. the mount namespace), changing it may grant access to files
+ * not previously allowed.
+ *
+ * To make it simple, deny any filesystem topology modification by landlocked
+ * processes. Non-landlocked processes may still change the namespace of a
+ * landlocked process, but this kind of threat must be handled by a system-wide
+ * access-control security policy.
+ *
+ * This could be lifted in the future if Landlock can safely handle mount
+ * namespace updates requested by a landlocked process. Indeed, we could
+ * update the current domain (which is currently read-only) by taking into
+ * account the accesses of the source and the destination of a new mount point.
+ * However, it would also require to make all the child domains dynamically
+ * inherit these new constraints. Anyway, for backward compatibility reasons,
+ * a dedicated user space option would be required (e.g. as a ruleset flag).
+ */
+static int hook_sb_mount(const char *const dev_name,
+ const struct path *const path, const char *const type,
+ const unsigned long flags, void *const data)
+{
+ if (!landlock_get_current_domain())
+ return 0;
+ return -EPERM;
+}
+
+static int hook_move_mount(const struct path *const from_path,
+ const struct path *const to_path)
+{
+ if (!landlock_get_current_domain())
+ return 0;
+ return -EPERM;
+}
+
+/*
+ * Removing a mount point may reveal a previously hidden file hierarchy, which
+ * may then grant access to files, which may have previously been forbidden.
+ */
+static int hook_sb_umount(struct vfsmount *const mnt, const int flags)
+{
+ if (!landlock_get_current_domain())
+ return 0;
+ return -EPERM;
+}
+
+static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
+{
+ if (!landlock_get_current_domain())
+ return 0;
+ return -EPERM;
+}
+
+/*
+ * pivot_root(2), like mount(2), changes the current mount namespace. It must
+ * then be forbidden for a landlocked process.
+ *
+ * However, chroot(2) may be allowed because it only changes the relative root
+ * directory of the current process. Moreover, it can be used to restrict the
+ * view of the filesystem.
+ */
+static int hook_sb_pivotroot(const struct path *const old_path,
+ const struct path *const new_path)
+{
+ if (!landlock_get_current_domain())
+ return 0;
+ return -EPERM;
+}
+
+/* Path hooks */
+
+static inline u32 get_mode_access(const umode_t mode)
+{
+ switch (mode & S_IFMT) {
+ case S_IFLNK:
+ return LANDLOCK_ACCESS_FS_MAKE_SYM;
+ case 0:
+ /* A zero mode translates to S_IFREG. */
+ case S_IFREG:
+ return LANDLOCK_ACCESS_FS_MAKE_REG;
+ case S_IFDIR:
+ return LANDLOCK_ACCESS_FS_MAKE_DIR;
+ case S_IFCHR:
+ return LANDLOCK_ACCESS_FS_MAKE_CHAR;
+ case S_IFBLK:
+ return LANDLOCK_ACCESS_FS_MAKE_BLOCK;
+ case S_IFIFO:
+ return LANDLOCK_ACCESS_FS_MAKE_FIFO;
+ case S_IFSOCK:
+ return LANDLOCK_ACCESS_FS_MAKE_SOCK;
+ default:
+ WARN_ON_ONCE(1);
+ return 0;
+ }
+}
+
+/*
+ * Creating multiple links or renaming may lead to privilege escalations if not
+ * handled properly. Indeed, we must be sure that the source doesn't gain more
+ * privileges by being accessible from the destination. This is getting more
+ * complex when dealing with multiple layers. The whole picture can be seen as
+ * a multilayer partial ordering problem. A future version of Landlock will
+ * deal with that.
+ */
+static int hook_path_link(struct dentry *const old_dentry,
+ const struct path *const new_dir,
+ struct dentry *const new_dentry)
+{
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+ /* The mount points are the same for old and new paths, cf. EXDEV. */
+ if (old_dentry->d_parent != new_dir->dentry)
+ /* Gracefully forbids reparenting. */
+ return -EXDEV;
+ if (unlikely(d_is_negative(old_dentry)))
+ return -ENOENT;
+ return check_access_path(dom, new_dir,
+ get_mode_access(d_backing_inode(old_dentry)->i_mode));
+}
+
+static inline u32 maybe_remove(const struct dentry *const dentry)
+{
+ if (d_is_negative(dentry))
+ return 0;
+ return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR :
+ LANDLOCK_ACCESS_FS_REMOVE_FILE;
+}
+
+static int hook_path_rename(const struct path *const old_dir,
+ struct dentry *const old_dentry,
+ const struct path *const new_dir,
+ struct dentry *const new_dentry)
+{
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+ /* The mount points are the same for old and new paths, cf. EXDEV. */
+ if (old_dir->dentry != new_dir->dentry)
+ /* Gracefully forbids reparenting. */
+ return -EXDEV;
+ if (unlikely(d_is_negative(old_dentry)))
+ return -ENOENT;
+ /* RENAME_EXCHANGE is handled because directories are the same. */
+ return check_access_path(dom, old_dir, maybe_remove(old_dentry) |
+ maybe_remove(new_dentry) |
+ get_mode_access(d_backing_inode(old_dentry)->i_mode));
+}
+
+static int hook_path_mkdir(const struct path *const dir,
+ struct dentry *const dentry, const umode_t mode)
+{
+ return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR);
+}
+
+static int hook_path_mknod(const struct path *const dir,
+ struct dentry *const dentry, const umode_t mode,
+ const unsigned int dev)
+{
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+ return check_access_path(dom, dir, get_mode_access(mode));
+}
+
+static int hook_path_symlink(const struct path *const dir,
+ struct dentry *const dentry, const char *const old_name)
+{
+ return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM);
+}
+
+static int hook_path_unlink(const struct path *const dir,
+ struct dentry *const dentry)
+{
+ return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE);
+}
+
+static int hook_path_rmdir(const struct path *const dir,
+ struct dentry *const dentry)
+{
+ return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
+}
+
+/* File hooks */
+
+static inline u32 get_file_access(const struct file *const file)
+{
+ u32 access = 0;
+
+ if (file->f_mode & FMODE_READ) {
+ /* A directory can only be opened in read mode. */
+ if (S_ISDIR(file_inode(file)->i_mode))
+ return LANDLOCK_ACCESS_FS_READ_DIR;
+ access = LANDLOCK_ACCESS_FS_READ_FILE;
+ }
+ if (file->f_mode & FMODE_WRITE)
+ access |= LANDLOCK_ACCESS_FS_WRITE_FILE;
+ /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */
+ if (file->f_flags & __FMODE_EXEC)
+ access |= LANDLOCK_ACCESS_FS_EXECUTE;
+ return access;
+}
+
+static int hook_file_open(struct file *const file)
+{
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+ /*
+ * Because a file may be opened with O_PATH, get_file_access() may
+ * return 0. This case will be handled with a future Landlock
+ * evolution.
+ */
+ return check_access_path(dom, &file->f_path, get_file_access(file));
+}
+
+static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
+ LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
+
+ LSM_HOOK_INIT(sb_delete, hook_sb_delete),
+ LSM_HOOK_INIT(sb_mount, hook_sb_mount),
+ LSM_HOOK_INIT(move_mount, hook_move_mount),
+ LSM_HOOK_INIT(sb_umount, hook_sb_umount),
+ LSM_HOOK_INIT(sb_remount, hook_sb_remount),
+ LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot),
+
+ LSM_HOOK_INIT(path_link, hook_path_link),
+ LSM_HOOK_INIT(path_rename, hook_path_rename),
+ LSM_HOOK_INIT(path_mkdir, hook_path_mkdir),
+ LSM_HOOK_INIT(path_mknod, hook_path_mknod),
+ LSM_HOOK_INIT(path_symlink, hook_path_symlink),
+ LSM_HOOK_INIT(path_unlink, hook_path_unlink),
+ LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
+
+ LSM_HOOK_INIT(file_open, hook_file_open),
+};
+
+__init void landlock_add_fs_hooks(void)
+{
+ security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),
+ LANDLOCK_NAME);
+}
projects / linux.git / commitdiff