// 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);
}