[PATCH v30 07/12] landlock: Support filesystem access-control

Mickaël Salaün
2021-03-16T20:44:00Z

From: Mickaël Salaün <mic@linux.microsoft.com>

Using Landlock objects and ruleset, it is possible to tag inodes
according to a process's domain. To enable an unprivileged process to
express a file hierarchy, it first needs to open a directory (or a file)
and pass this file descriptor to the kernel through
landlock_add_rule(2). When checking if a file access request is
allowed, we walk from the requested dentry to the real root, following
the different mount layers. The access to each "tagged" inodes are
collected according to their rule layer level, and ANDed to create
access to the requested file hierarchy. This makes possible to identify
a lot of files without tagging every inodes nor modifying the
filesystem, while still following the view and understanding the user
has from the filesystem.

Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not
keep the same struct inodes for the same inodes whereas these inodes are
in use.

This commit adds a minimal set of supported filesystem access-control
which doesn't enable to restrict all file-related actions. This is the
result of multiple discussions to minimize the code of Landlock to ease
review. Thanks to the Landlock design, extending this access-control
without breaking user space will not be a problem. Moreover, seccomp
filters can be used to restrict the use of syscall families which may
not be currently handled by Landlock.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210316204252.427806-8-mic@digikod.net
---

Changes since v29:
* Remove a useless unlock/lock for the first loop walk in
hook_sb_delete(). This also makes the code clearer but doesn't change
the garantees for iput().
* Rename iput_inode to prev_inode, which shows its origin.

Changes since v28:
* Fix race conditions that could be caused by concurrent calls to
release_inode() and hook_sb_delete().
* Avoid livelock when a lot of inodes are tagged.
* Improve concurrency locking and add comments to explain the specific
lock rules.
* Add an inode_free_security hook to check that release_inode() and
hook_sb_delete() do their job.
* Add early return to check_access_path() to check if the access request
is empty. This doesn't change the semantic.
* Reword the first description sentence (suggested by Serge Hallyn).

Changes since v27:
* Fix domains with layers of non-overlapping access rights (cf.
layout1.non_overlapping_accesses test) thanks to a stack of access
rights per layer (replacing ORed access rights). This avoids
too-restrictive domains.
* Cosmetic fixes and updates in comments and Kconfig.

Changes since v26:
* Check each rule of a path to enable a more permissive and pragmatic
access control per layer. Suggested by Jann Horn:
https://lore.kernel.org/lkml/CAG48ez1O0VTwEiRd3KqexoF78WR+cmP5bGk5Kh5Cs7aPepiDVg@mail.gmail.com/
* Rename check_access_path_continue() to unmask_layers() and make it
return the new layer mask.
* Avoid double domain check in hook_file_open().
* In the documentation, add utime(2) as another example of unhandled
syscalls. Indeed, using `touch` to test write access may be tempting.
* Remove outdated comment about OverlayFS.
* Rename the landlock.h ifdef to align with most similar files.
* Fix spelling.

Changes since v25:
* Move build_check_layer() to ruleset.c, and add built-time checks for
the fs_access_mask and access variables according to
_LANDLOCK_ACCESS_FS_MASK.
* Move limits to a dedicated file and rename them:
_LANDLOCK_ACCESS_FS_LAST and _LANDLOCK_ACCESS_FS_MASK.
* Set build_check_layer() as non-inline to trigger a warning if it is
not called.
* Use BITS_PER_TYPE() macro.
* Rename function to landlock_add_fs_hooks().
* Cosmetic variable renames.

Changes since v24:
* Use the new struct landlock_rule and landlock_layer to not mix
accesses from different layers. Revert "Enforce deterministic
interleaved path rules" from v24, and fix the layer check. This
enables to follow a sane semantic: an access is granted if, for each
policy layer, at least one rule encountered on the pathwalk grants the
access, regardless of their position in the layer stack (suggested by
Jann Horn). See layout1.interleaved_masked_accesses tests from
tools/testing/selftests/landlock/fs_test.c for corner cases.
* Add build-time checks for layers.
* Use the new landlock_insert_rule() API.

Changes since v23:
* Enforce deterministic interleaved path rules. To have consistent
layered rules, granting access to a path implies that all accesses
tied to inodes, from the requested file to the real root, must be
checked. Otherwise, stacked rules may result to overzealous
restrictions. By excluding the ability to add exceptions in the same
layer (e.g. /a allowed, /a/b denied, and /a/b/c allowed), we get
deterministic interleaved path rules. This removes an optimization
which could be replaced by a proper cache mechanism. This also
further simplifies and explain check_access_path_continue().
* Fix memory allocation error handling in landlock_create_object()
calls. This prevent to inadvertently hold an inode.
* In get_inode_object(), improve comments, make code more readable and
move kfree() call out of the lock window.
* Use the simplified landlock_insert_rule() API.

Changes since v22:
* Simplify check_access_path_continue() (suggested by Jann Horn).
* Remove prefetch() call for now (suggested by Jann Horn).
* Fix spelling and remove superfluous comment (spotted by Jann Horn).
* Cosmetic variable renaming.

Changes since v21:
* Rename ARCH_EPHEMERAL_STATES to ARCH_EPHEMERAL_INODES (suggested by
James Morris).
* Remove the LANDLOCK_ACCESS_FS_CHROOT right because chroot(2) (which
requires CAP_SYS_CHROOT) doesn't enable to bypass Landlock (as tests
demonstrate it), and because it is often used by sandboxes, it would
be counterproductive to forbid it. This also reduces the code size.
* Clean up documentation.

Changes since v19:
* Fix spelling (spotted by Randy Dunlap).

Changes since v18:
* Remove useless include.
* Fix spelling.

Changes since v17:
* Replace landlock_release_inodes() with security_sb_delete() (requested
by James Morris).
* Replace struct super_block->s_landlock_inode_refs with the LSM
infrastructure management of the superblock (requested by James
Morris).
* Fix mknod restriction with a zero mode (spotted by Vincent Dagonneau).
* Minimize executed code in path_mknod and file_open hooks when the
current tasks is not sandboxed.
* Remove useless checks on the file pointer and inode in
hook_file_open() .
* Constify domain pointers.
* Rename inode_landlock() to landlock_inode().
* Import include/uapi/linux/landlock.h and _LANDLOCK_ACCESS_FS_* from
the ruleset and domain management patch.
* Explain the rational of this minimal set of access-control.
https://lore.kernel.org/lkml/f646e1c7-33cf-333f-070c-0a40ad0468cd@digikod.net/

Changes since v16:
* Add ARCH_EPHEMERAL_STATES and enable it for UML.

Changes since v15:
* Replace layer_levels and layer_depth with a bitfield of layers: this
enables to properly manage superset and subset of access rights,
whatever their order in the stack of layers.
Cf. https://lore.kernel.org/lkml/e07fe473-1801-01cc-12ae-b3167f95250e@digikod.net/
* Allow to open pipes and similar special files through /proc/self/fd/.
* Properly handle internal filesystems such as nsfs: always allow these
kind of roots because disconnected path cannot be evaluated.
* Remove the LANDLOCK_ACCESS_FS_LINK_TO and
LANDLOCK_ACCESS_FS_RENAME_{TO,FROM}, but use the
LANDLOCK_ACCESS_FS_REMOVE_{FILE,DIR} and LANDLOCK_ACCESS_FS_MAKE_*
instead. Indeed, it is not possible for now (and not really useful)
to express the semantic of a source and a destination.
* Check access rights to remove a directory or a file with rename(2).
* Forbid reparenting when linking or renaming. This is needed to easily
protect against possible privilege escalation by changing the place of
a file or directory in relation to an enforced access policy (from the
set of layers). This will be relaxed in the future.
* Update hooks to take into account replacement of the object's self and
beneath access bitfields with one. Simplify the code.
* Check file related access rights.
* Check d_is_negative() instead of !d_backing_inode() in
check_access_path_continue(), and continue the path walk while there
is no mapped inode e.g., with rename(2).
* Check private inode in check_access_path().
* Optimize get_file_access() when dealing with a directory.
* Add missing atomic.h .

Changes since v14:
* Simplify the object, rule and ruleset management at the expense of a
less aggressive memory freeing (contributed by Jann Horn, with
additional modifications):
- Rewrite release_inode() to use inode->sb->s_landlock_inode_refs.
- Remove useless checks in landlock_release_inodes(), clean object
pointer according to the new struct landlock_object and wait for all
iput() to complete.
- Rewrite get_inode_object() according to the new struct
landlock_object. If there is a race-condition when cleaning up an
object, we retry until the concurrent thread finished the object
cleaning.
Cf. https://lore.kernel.org/lkml/CAG48ez21bEn0wL1bbmTiiu8j9jP5iEWtHOwz4tURUJ+ki0ydYw@mail.gmail.com/
* Fix nested domains by implementing a notion of layer level and depth:
- Check for matching level ranges when walking through a file path.
- Only allow access if every layer granted the access request.
* Handles files without mount points (e.g. pipes).
* Hardens path walk by checking inode pointer values.
* Prefetches d_parent when walking to the root directory.
* Remove useless inode_alloc_security hook() (suggested by Jann Horn):
already initialized by lsm_inode_alloc().
* Remove the inode_free_security hook.
* Remove access checks that may be required for FD-only requests:
truncate, getattr, lock, chmod, chown, chgrp, ioctl. This will be
handle in a future evolution of Landlock, but right now the goal is to
lighten the code to ease review.
* Constify variables.
* Move ABI checks into syscall.c .
* Cosmetic variable renames.

Changes since v11:
* Add back, revamp and make a fully working filesystem access-control
based on paths and inodes.
* Remove the eBPF dependency.

Previous changes:
https://lore.kernel.org/lkml/20190721213116.23476-6-mic@digikod.net/
---
MAINTAINERS | 1 +
arch/Kconfig | 7 +
arch/um/Kconfig | 1 +
include/uapi/linux/landlock.h | 75 ++++
security/landlock/Kconfig | 2 +-
security/landlock/Makefile | 2 +-
security/landlock/fs.c | 687 ++++++++++++++++++++++++++++++++++
security/landlock/fs.h | 56 +++
security/landlock/limits.h | 4 +
security/landlock/ruleset.c | 4 +
security/landlock/setup.c | 7 +
security/landlock/setup.h | 2 +
12 files changed, 846 insertions(+), 2 deletions(-)
create mode 100644 include/uapi/linux/landlock.h
create mode 100644 security/landlock/fs.c
create mode 100644 security/landlock/fs.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 87a2738dfdec..70ec117efa8a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10003,6 +10003,7 @@ L: linux-security-module@vger.kernel.org
S: Supported
W: https://landlock.io
T: git https://github.com/landlock-lsm/linux.git
+F: include/uapi/linux/landlock.h
F: security/landlock/
K: landlock
K: LANDLOCK
diff --git a/arch/Kconfig b/arch/Kconfig
index ecfd3520b676..8160ab7e3e03 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1013,6 +1013,13 @@ config COMPAT_32BIT_TIME
config ARCH_NO_PREEMPT
bool

+config ARCH_EPHEMERAL_INODES
+ def_bool n
+ help
+ An arch should select this symbol if it doesn't keep track of inode
+ instances on its own, but instead relies on something else (e.g. the
+ host kernel for an UML kernel).
+
config ARCH_SUPPORTS_RT
bool

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index c3030db3325f..57cfd9a1c082 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -5,6 +5,7 @@ menu "UML-specific options"
config UML
bool
default y
+ select ARCH_EPHEMERAL_INODES
select ARCH_HAS_KCOV
select ARCH_NO_PREEMPT
select HAVE_ARCH_AUDITSYSCALL
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
new file mode 100644
index 000000000000..f69877099c8e
--- /dev/null
+++ b/include/uapi/linux/landlock.h
@@ -0,0 +1,75 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Landlock - User space API
+ *
+ * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#ifndef _UAPI_LINUX_LANDLOCK_H
+#define _UAPI_LINUX_LANDLOCK_H
+
+/**
+ * DOC: fs_access
+ *
+ * A set of actions on kernel objects may be defined by an attribute (e.g.
+ * &struct landlock_path_beneath_attr) including a bitmask of access.
+ *
+ * Filesystem flags
+ * ~~~~~~~~~~~~~~~~
+ *
+ * These flags enable to restrict a sandboxed process to a set of actions on
+ * files and directories. Files or directories opened before the sandboxing
+ * are not subject to these restrictions.
+ *
+ * A file can only receive these access rights:
+ *
+ * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
+ * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.
+ * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
+ *
+ * A directory can receive access rights related to files or directories. The
+ * following access right is applied to the directory itself, and the
+ * directories beneath it:
+ *
+ * - %LANDLOCK_ACCESS_FS_READ_DIR: Open a directory or list its content.
+ *
+ * However, the following access rights only apply to the content of a
+ * directory, not the directory itself:
+ *
+ * - %LANDLOCK_ACCESS_FS_REMOVE_DIR: Remove an empty directory or rename one.
+ * - %LANDLOCK_ACCESS_FS_REMOVE_FILE: Unlink (or rename) a file.
+ * - %LANDLOCK_ACCESS_FS_MAKE_CHAR: Create (or rename or link) a character
+ * device.
+ * - %LANDLOCK_ACCESS_FS_MAKE_DIR: Create (or rename) a directory.
+ * - %LANDLOCK_ACCESS_FS_MAKE_REG: Create (or rename or link) a regular file.
+ * - %LANDLOCK_ACCESS_FS_MAKE_SOCK: Create (or rename or link) a UNIX domain
+ * socket.
+ * - %LANDLOCK_ACCESS_FS_MAKE_FIFO: Create (or rename or link) a named pipe.
+ * - %LANDLOCK_ACCESS_FS_MAKE_BLOCK: Create (or rename or link) a block device.
+ * - %LANDLOCK_ACCESS_FS_MAKE_SYM: Create (or rename or link) a symbolic link.
+ *
+ * .. warning::
+ *
+ * It is currently not possible to restrict some file-related actions
+ * accessible through these syscall families: :manpage:`chdir(2)`,
+ * :manpage:`truncate(2)`, :manpage:`stat(2)`, :manpage:`flock(2)`,
+ * :manpage:`chmod(2)`, :manpage:`chown(2)`, :manpage:`setxattr(2)`,
+ * :manpage:`utime(2)`, :manpage:`ioctl(2)`, :manpage:`fcntl(2)`.
+ * Future Landlock evolutions will enable to restrict them.
+ */
+#define LANDLOCK_ACCESS_FS_EXECUTE (1ULL << 0)
+#define LANDLOCK_ACCESS_FS_WRITE_FILE (1ULL << 1)
+#define LANDLOCK_ACCESS_FS_READ_FILE (1ULL << 2)
+#define LANDLOCK_ACCESS_FS_READ_DIR (1ULL << 3)
+#define LANDLOCK_ACCESS_FS_REMOVE_DIR (1ULL << 4)
+#define LANDLOCK_ACCESS_FS_REMOVE_FILE (1ULL << 5)
+#define LANDLOCK_ACCESS_FS_MAKE_CHAR (1ULL << 6)
+#define LANDLOCK_ACCESS_FS_MAKE_DIR (1ULL << 7)
+#define LANDLOCK_ACCESS_FS_MAKE_REG (1ULL << 8)
+#define LANDLOCK_ACCESS_FS_MAKE_SOCK (1ULL << 9)
+#define LANDLOCK_ACCESS_FS_MAKE_FIFO (1ULL << 10)
+#define LANDLOCK_ACCESS_FS_MAKE_BLOCK (1ULL << 11)
+#define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12)
+
+#endif /* _UAPI_LINUX_LANDLOCK_H */
diff --git a/security/landlock/Kconfig b/security/landlock/Kconfig
index c1e862a38410..8e33c4e8ffb8 100644
--- a/security/landlock/Kconfig
+++ b/security/landlock/Kconfig
@@ -2,7 +2,7 @@

config SECURITY_LANDLOCK
bool "Landlock support"
- depends on SECURITY
+ depends on SECURITY && !ARCH_EPHEMERAL_INODES
select SECURITY_PATH
help
Landlock is a sandboxing mechanism that enables processes to restrict
diff --git a/security/landlock/Makefile b/security/landlock/Makefile
index f1d1eb72fa76..92e3d80ab8ed 100644
--- a/security/landlock/Makefile
+++ b/security/landlock/Makefile
@@ -1,4 +1,4 @@
obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o

landlock-y := setup.o object.o ruleset.o \
- cred.o ptrace.o
+ cred.o ptrace.o fs.o
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
new file mode 100644
index 000000000000..e3b710d6f56d
--- /dev/null
+++ b/security/landlock/fs.c
@@ -0,0 +1,687 @@
+// 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 get_inode_object() calls. */
+ spin_lock(&inode->i_lock);
+ object = rcu_dereference_protected(inode_sec->object,
+ lockdep_is_held(&inode->i_lock));
+ if (unlikely(object)) {
+ /* Someone else just created the object, bail out and retry. */
+ spin_unlock(&inode->i_lock);
+ kfree(new_object);
+
+ rcu_read_lock();
+ goto retry;
+ }
+
+ rcu_assign_pointer(inode_sec->object, new_object);
+ /*
+ * @inode will be released by hook_sb_delete() on its superblock
+ * shutdown.
+ */
+ ihold(inode);
+ 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))
+ /* Continues to walk while there is no mapped inode. */
+ 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/self/fd .
+ */
+ 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/self/ns).
+ */
+ 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;
+
+ /*
+ * 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.
+ */
+ spin_lock(&inode->i_lock);
+ 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
+ * layout (i.e. the mount namespace), changing it may grant access to files not
+ * previously allowed.
+ *
+ * To make it simple, deny any filesystem layout 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 command
+ * option).
+ */
+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)
+ /* For now, forbids reparenting. */
+ return -EACCES;
+ if (unlikely(d_is_negative(old_dentry)))
+ return -EACCES;
+ 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)
+ /* For now, forbids reparenting. */
+ return -EACCES;
+ if (WARN_ON_ONCE(d_is_negative(old_dentry)))
+ return -EACCES;
+ /* 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);
+}
diff --git a/security/landlock/fs.h b/security/landlock/fs.h
new file mode 100644
index 000000000000..9f14ec4d8d48
--- /dev/null
+++ b/security/landlock/fs.h
@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Landlock LSM - Filesystem management and hooks
+ *
+ * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#ifndef _SECURITY_LANDLOCK_FS_H
+#define _SECURITY_LANDLOCK_FS_H
+
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/rcupdate.h>
+
+#include "ruleset.h"
+#include "setup.h"
+
+struct landlock_inode_security {
+ /*
+ * @object: Weak pointer to an allocated object. All writes (i.e.
+ * creating a new object or removing one) are protected by the
+ * underlying inode->i_lock. Disassociating @object from the inode is
+ * additionally protected by @object->lock, from the time @object's
+ * usage refcount drops to zero to the time this pointer is nulled out.
+ * Cf. release_inode().
+ */
+ struct landlock_object __rcu *object;
+};
+
+struct landlock_superblock_security {
+ /*
+ * @inode_refs: References to Landlock underlying objects.
+ * Cf. struct super_block->s_fsnotify_inode_refs .
+ */
+ atomic_long_t inode_refs;
+};
+
+static inline struct landlock_inode_security *landlock_inode(
+ const struct inode *const inode)
+{
+ return inode->i_security + landlock_blob_sizes.lbs_inode;
+}
+
+static inline struct landlock_superblock_security *landlock_superblock(
+ const struct super_block *const superblock)
+{
+ return superblock->s_security + landlock_blob_sizes.lbs_superblock;
+}
+
+__init void landlock_add_fs_hooks(void);
+
+int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
+ const struct path *const path, u32 access_hierarchy);
+
+#endif /* _SECURITY_LANDLOCK_FS_H */
diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index b734f597bb0e..2a0a1095ee27 100644
--- a/security/landlock/limits.h
+++ b/security/landlock/limits.h
@@ -10,8 +10,12 @@
#define _SECURITY_LANDLOCK_LIMITS_H

#include <linux/limits.h>
+#include <uapi/linux/landlock.h>

#define LANDLOCK_MAX_NUM_LAYERS 64
#define LANDLOCK_MAX_NUM_RULES U32_MAX

+#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_MAKE_SYM
+#define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
+
#endif /* _SECURITY_LANDLOCK_LIMITS_H */
diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
index 59c86126ea1c..9c15aea180a1 100644
--- a/security/landlock/ruleset.c
+++ b/security/landlock/ruleset.c
@@ -116,9 +116,11 @@ static void build_check_ruleset(void)
.num_rules = ~0,
.num_layers = ~0,
};
+ typeof(ruleset.fs_access_masks[0]) fs_access_mask = ~0;

BUILD_BUG_ON(ruleset.num_rules < LANDLOCK_MAX_NUM_RULES);
BUILD_BUG_ON(ruleset.num_layers < LANDLOCK_MAX_NUM_LAYERS);
+ BUILD_BUG_ON(fs_access_mask < LANDLOCK_MASK_ACCESS_FS);
}

/**
@@ -217,9 +219,11 @@ static void build_check_layer(void)
{
const struct landlock_layer layer = {
.level = ~0,
+ .access = ~0,
};

BUILD_BUG_ON(layer.level < LANDLOCK_MAX_NUM_LAYERS);
+ BUILD_BUG_ON(layer.access < LANDLOCK_MASK_ACCESS_FS);
}

/* @ruleset must be locked by the caller. */
diff --git a/security/landlock/setup.c b/security/landlock/setup.c
index a5d6ef334991..f8e8e980454c 100644
--- a/security/landlock/setup.c
+++ b/security/landlock/setup.c
@@ -11,17 +11,24 @@

#include "common.h"
#include "cred.h"
+#include "fs.h"
#include "ptrace.h"
#include "setup.h"

+bool landlock_initialized __lsm_ro_after_init = false;
+
struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
.lbs_cred = sizeof(struct landlock_cred_security),
+ .lbs_inode = sizeof(struct landlock_inode_security),
+ .lbs_superblock = sizeof(struct landlock_superblock_security),
};

static int __init landlock_init(void)
{
landlock_add_cred_hooks();
landlock_add_ptrace_hooks();
+ landlock_add_fs_hooks();
+ landlock_initialized = true;
pr_info("Up and running.\n");
return 0;
}
diff --git a/security/landlock/setup.h b/security/landlock/setup.h
index 9fdbf33fcc33..1daffab1ab4b 100644
--- a/security/landlock/setup.h
+++ b/security/landlock/setup.h
@@ -11,6 +11,8 @@

#include <linux/lsm_hooks.h>

+extern bool landlock_initialized;
+
extern struct lsm_blob_sizes landlock_blob_sizes;

#endif /* _SECURITY_LANDLOCK_SETUP_H */
--
2.30.2