Jann, Kees, are you OK with this patch and the next one?
On 24/03/2021 20:15, Mickaël Salaün wrote: > From: Mickaël Salaün <m...@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 <v...@zeniv.linux.org.uk> > Cc: Anton Ivanov <anton.iva...@cambridgegreys.com> > Cc: James Morris <jmor...@namei.org> > Cc: Jann Horn <ja...@google.com> > Cc: Jeff Dike <jd...@addtoit.com> > Cc: Kees Cook <keesc...@chromium.org> > Cc: Richard Weinberger <rich...@nod.at> > Cc: Serge E. Hallyn <se...@hallyn.com> > Signed-off-by: Mickaël Salaün <m...@linux.microsoft.com> > Link: https://lore.kernel.org/r/20210324191520.125779-8-...@digikod.net > --- > > Changes since v30: > * Fix a (theoretical) data race in get_inode_object() (spotted by Jann > Horn) and update the related comments. Also move the ihold() call to > better reflect the logical order (opposite to release_inode() and > hook_sb_delete()), even if it is still protected by inode->i_lock. > * Improve comments related to inode and superblock management. > > 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+cmp5bgk5kh5cs7apepi...@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-0a40ad046...@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-b3167f952...@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+ki0y...@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-...@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 | 693 ++++++++++++++++++++++++++++++++++ > security/landlock/fs.h | 70 ++++ > security/landlock/limits.h | 4 + > security/landlock/ruleset.c | 4 + > security/landlock/setup.c | 7 + > security/landlock/setup.h | 2 + > 12 files changed, 866 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-mod...@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 <m...@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..3f221d0e5c90 > --- /dev/null > +++ b/security/landlock/fs.c > @@ -0,0 +1,693 @@ > +// SPDX-License-Identifier: GPL-2.0-only > +/* > + * Landlock LSM - Filesystem management and hooks > + * > + * Copyright © 2016-2020 Mickaël Salaün <m...@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)) > + /* 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/<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 > + * 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..187284b421c9 > --- /dev/null > +++ b/security/landlock/fs.h > @@ -0,0 +1,70 @@ > +/* SPDX-License-Identifier: GPL-2.0-only */ > +/* > + * Landlock LSM - Filesystem management and hooks > + * > + * Copyright © 2017-2020 Mickaël Salaün <m...@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 - Inode security blob > + * > + * Enable to reference a &struct landlock_object tied to an inode (i.e. > + * underlying object). > + */ > +struct landlock_inode_security { > + /** > + * @object: Weak pointer to an allocated object. All assignments of a > + * new object are protected by the underlying inode->i_lock. However, > + * atomically disassociating @object from the inode is only 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() and > + * hook_sb_delete()). Indeed, such disassociation doesn't require > + * inode->i_lock thanks to the careful rcu_access_pointer() check > + * performed by get_inode_object(). > + */ > + struct landlock_object __rcu *object; > +}; > + > +/** > + * struct landlock_superblock_security - Superblock security blob > + * > + * Enable hook_sb_delete() to wait for concurrent calls to release_inode(). > + */ > +struct landlock_superblock_security { > + /** > + * @inode_refs: Number of pending inodes (from this superblock) that > + * are being released by release_inode(). > + * 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 2e616f6d5274..ec72b9262bf3 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 */ >
