[PATCH 2/2] landlock: add support for private bind mount

Shervin Oloumi
2024-12-31T01:47:07Z

This patch introduces a new LandLock rule, LANDLOCK_ACCESS_FS_MOUNT for
controlling bind mount operations. While this patch focuses on bind
mounts only, the long-term goal is to utilize this rule for controlling
regular device mounts as well. To perform a successful bind mount, both
the source parent and the destination need to have the REFER rule and
the destination needs to carry the MOUNT rule. Note that this does imply
that the MOUNT rule needs to also exist in the source hierarchy, to
avoid rejection due to privilege escalation. The same path access logic
as REFER (used for rename/move and link) is used to check for any
possible privilege escalations before allowing the bind mount.

Additionally, only private bind mounts are allowed. This is to avoid
filesystem hierarchy changes that happen through mount propagation
between peer mount groups. For instance, bind mounting a directory in a
shared mount namespace, can result in hierarchy changes propagating
across the peer groups in that namespace. Such changes cannot be checked
for privilege escalation as the REFER check only covers the source and
the destination surfaced in the bind mount hook function.

If a bind mount request carries a --make-shared flag, or any flag other
than --make-private, the bind mount succeeds while the subsequent
propagation type change fails. This is because, the kernel calls the
mount hook once with the --bind mount flag, and then once more with the
flag for the propagation type change. If the bind mount request carries
the --make-private flag, both the mount operation and the subsequent
propagation type change succeed. Any mount request with the
--make-private flag only is also allowed. Such requests operate on
existing mount points, changing the propagation type to private. In this
case any previously propagated mounts would continue to exist, but
additional bind mounts under the newly privatized mount point, would not
propagate anymore.

Finally, any existing mounts or bind mounts before the process enters a
LandLock domain remain as they are. Such mounts can be of the shared
propagation type, and they would continue to share updates with the rest
of their peer group. While this is an existing behavior, after this
patch such mounts can also be remounted as private, or be unmounted
after the process enters the sandbox. Existing mounts are outside the
scope of LandLock and should be considered before entering the sandbox.

Tests: Regular mount is rejected. Bind mount is rejected if either the
source parent or the mount point do not have the REFER rule, or if the
destination does not have the MOUNT rule. Bind mount is allowed only if
the REFER check passes, i.e.: no restrictions are dropped. Bind mounting
a directory onto itself is always allowed. Mount calls with the flag
--make-private are always allowed. Unmounting is always allowed. Link
and rename functionality is unaffected. All LandLock self-tests pass.

Link: https://github.com/landlock-lsm/linux/issues/14
Signed-off-by: Shervin Oloumi <enlightened@chromium.org>
---
include/uapi/linux/landlock.h | 1 +
security/landlock/fs.c | 105 +++++++++++-------
security/landlock/limits.h | 2 +-
security/landlock/ruleset.h | 2 +-
tools/testing/selftests/landlock/fs_test.c | 119 +++++++++++++++++++--
5 files changed, 181 insertions(+), 48 deletions(-)

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 33745642f787..10541a001f2f 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -259,6 +259,7 @@ struct landlock_net_port_attr {
#define LANDLOCK_ACCESS_FS_REFER (1ULL << 13)
#define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14)
#define LANDLOCK_ACCESS_FS_IOCTL_DEV (1ULL << 15)
+#define LANDLOCK_ACCESS_FS_MOUNT (1ULL << 16)
/* clang-format on */

/**
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index e31b97a9f175..1ae18324bd02 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -35,6 +35,7 @@
#include <linux/workqueue.h>
#include <uapi/linux/fiemap.h>
#include <uapi/linux/landlock.h>
+#include <uapi/linux/mount.h>

#include "common.h"
#include "cred.h"
@@ -1033,34 +1034,36 @@ static bool collect_domain_accesses(
}

/**
- * current_check_refer_path - Check if a rename or link action is allowed
+ * current_check_reparent_path - Check if a reparent action is allowed
*
- * @old_dentry: File or directory requested to be moved or linked.
- * @new_dir: Destination parent directory.
+ * @old_dentry: Source file or directory for move, link or bind mount.
+ * @new_dir: Destination parent directory for move and link, or destination
+ * directory itself for bind mount (mount point).
* @new_dentry: Destination file or directory.
* @removable: Sets to true if it is a rename operation.
* @exchange: Sets to true if it is a rename operation with RENAME_EXCHANGE.
+ * @bind_mount: Sets to true if it is a bind mount operation.
*
* Because of its unprivileged constraints, Landlock relies on file hierarchies
- * (and not only inodes) to tie access rights to files. Being able to link or
- * rename a file hierarchy brings some challenges. Indeed, moving or linking a
- * file (i.e. creating a new reference to an inode) can have an impact on the
- * actions allowed for a set of files if it would change its parent directory
- * (i.e. reparenting).
+ * (and not only inodes) to tie access rights to files. Being able to link,
+ * rename or bind mount a file hierarchy brings some challenges. Indeed, these
+ * operations create a new reference to an inode, which can have an impact on
+ * the actions allowed for a set of files if it would change its parent
+ * directory (i.e. reparenting).
*
* To avoid trivial access right bypasses, Landlock first checks if the file or
* directory requested to be moved would gain new access rights inherited from
* its new hierarchy. Before returning any error, Landlock then checks that
* the parent source hierarchy and the destination hierarchy would allow the
- * link or rename action. If it is not the case, an error with EACCES is
- * returned to inform user space that there is no way to remove or create the
- * requested source file type. If it should be allowed but the new inherited
- * access rights would be greater than the source access rights, then the
- * kernel returns an error with EXDEV. Prioritizing EACCES over EXDEV enables
+ * link, rename or bind mount action. If it is not the case, an error with
+ * EACCES is returned to inform user space that there is no way to remove or
+ * create the requested source file type. If it should be allowed but the new
+ * inherited access rights would be greater than the source access rights, then
+ * the kernel returns an error with EXDEV or EPERM. Prioritizing EACCES enables
* user space to abort the whole operation if there is no way to do it, or to
* manually copy the source to the destination if this remains allowed, e.g.
* because file creation is allowed on the destination directory but not direct
- * linking.
+ * linking, or bind mounting.
*
* To achieve this goal, the kernel needs to compare two file hierarchies: the
* one identifying the source file or directory (including itself), and the
@@ -1082,13 +1085,18 @@ static bool collect_domain_accesses(
*
* Returns:
* - 0 if access is allowed;
- * - -EXDEV if @old_dentry would inherit new access rights from @new_dir;
+ * - -EXDEV if @old_dentry would inherit new access rights from @new_dir through
+ * link or rename.
+ * - -EPERM if @old_dentry would inherit new access rights from @new_dir through
+ * bind mount.
* - -EACCES if file removal or creation is denied.
*/
-static int current_check_refer_path(struct dentry *const old_dentry,
- const struct path *const new_dir,
- struct dentry *const new_dentry,
- const bool removable, const bool exchange)
+static int current_check_reparent_path(struct dentry *const old_dentry,
+ const struct path *const new_dir,
+ struct dentry *const new_dentry,
+ const bool removable,
+ const bool exchange,
+ const bool bind_mount)
{
const struct landlock_ruleset *const dom = get_current_fs_domain();
bool allow_parent1, allow_parent2;
@@ -1120,7 +1128,8 @@ static int current_check_refer_path(struct dentry *const old_dentry,
}

/* The mount points are the same for old and new paths, cf. EXDEV. */
- if (old_dentry->d_parent == new_dir->dentry) {
+ if ((bind_mount && old_dentry == new_dentry) ||
+ (!bind_mount && old_dentry->d_parent == new_dentry->d_parent)) {
/*
* The LANDLOCK_ACCESS_FS_REFER access right is not required
* for same-directory referer (i.e. no reparenting).
@@ -1160,6 +1169,14 @@ static int current_check_refer_path(struct dentry *const old_dentry,
if (allow_parent1 && allow_parent2)
return 0;

+ if (bind_mount) {
+ if (!is_access_to_paths_allowed(
+ dom, new_dir, LANDLOCK_ACCESS_FS_MOUNT,
+ &layer_masks_parent2, NULL, 0, NULL, NULL)) {
+ return -EPERM;
+ }
+ }
+
/*
* To be able to compare source and destination domain access rights,
* take into account the @old_dentry access rights aggregated with its
@@ -1173,7 +1190,7 @@ static int current_check_refer_path(struct dentry *const old_dentry,
return 0;

/*
- * This prioritizes EACCES over EXDEV for all actions, including
+ * This prioritizes EACCES over EXDEV/EPERM for all actions, including
* renames with RENAME_EXCHANGE.
*/
if (likely(is_eacces(&layer_masks_parent1, access_request_parent1) ||
@@ -1186,6 +1203,8 @@ static int current_check_refer_path(struct dentry *const old_dentry,
* hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the
* source or the destination.
*/
+ if (bind_mount)
+ return -EPERM;
return -EXDEV;
}

@@ -1319,9 +1338,11 @@ static void hook_sb_delete(struct super_block *const sb)
* 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
+ * Currently, we can safely handle private bind mounts, as the source and
+ * destination restrictions can be compared, however all other types of mount
+ * system calls are blocked, including shared bind mounts and device mounts.
+ * 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
@@ -1338,22 +1359,27 @@ static int hook_sb_mount(const char *const dev_name,
{
if (!get_current_fs_domain())
return 0;
+
+ /*
+ * Only allow mount system calls with the --bind flag or --make-private
+ * flag. Further security checks are performed in the bind mount hook.
+ */
+ if (flags && (!(~MS_BIND & flags) || !(~MS_PRIVATE & flags)))
+ return 0;
return -EPERM;
}

-static int hook_move_mount(const struct path *const from_path,
- const struct path *const to_path)
+static int hook_sb_bindmount(const struct path *const old_path,
+ const struct path *const path)
{
- if (!get_current_fs_domain())
- return 0;
- return -EPERM;
+ return current_check_reparent_path(old_path->dentry,
+ path,
+ path->dentry,
+ false, false, true);
}

-/*
- * 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)
+static int hook_move_mount(const struct path *const from_path,
+ const struct path *const to_path)
{
if (!get_current_fs_domain())
return 0;
@@ -1389,8 +1415,8 @@ static int hook_path_link(struct dentry *const old_dentry,
const struct path *const new_dir,
struct dentry *const new_dentry)
{
- return current_check_refer_path(old_dentry, new_dir, new_dentry, false,
- false);
+ return current_check_reparent_path(old_dentry, new_dir, new_dentry,
+ false, false, false);
}

static int hook_path_rename(const struct path *const old_dir,
@@ -1400,8 +1426,9 @@ static int hook_path_rename(const struct path *const old_dir,
const unsigned int flags)
{
/* old_dir refers to old_dentry->d_parent and new_dir->mnt */
- return current_check_refer_path(old_dentry, new_dir, new_dentry, true,
- !!(flags & RENAME_EXCHANGE));
+ return current_check_reparent_path(old_dentry, new_dir, new_dentry,
+ true, !!(flags & RENAME_EXCHANGE),
+ false);
}

static int hook_path_mkdir(const struct path *const dir,
@@ -1652,8 +1679,8 @@ static struct security_hook_list landlock_hooks[] __ro_after_init = {

LSM_HOOK_INIT(sb_delete, hook_sb_delete),
LSM_HOOK_INIT(sb_mount, hook_sb_mount),
+ LSM_HOOK_INIT(sb_bindmount, hook_sb_bindmount),
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),

diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index 15f7606066c8..b495b15df25d 100644
--- a/security/landlock/limits.h
+++ b/security/landlock/limits.h
@@ -18,7 +18,7 @@
#define LANDLOCK_MAX_NUM_LAYERS 16
#define LANDLOCK_MAX_NUM_RULES U32_MAX

-#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_IOCTL_DEV
+#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_MOUNT
#define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
#define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS)

diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index 631e24d4ffe9..eeff66a207de 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h
@@ -31,7 +31,7 @@
LANDLOCK_ACCESS_FS_REFER)
/* clang-format on */

-typedef u16 access_mask_t;
+typedef u32 access_mask_t;
/* Makes sure all filesystem access rights can be stored. */
static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
/* Makes sure all network access rights can be stored. */
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 6788762188fe..2553b473a02c 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -274,6 +274,11 @@ static int mount_opt(const struct mnt_opt *const mnt, const char *const target)
mnt->data);
}

+static int bindmount(const char *const source, const char *const target)
+{
+ return mount(source, target, NULL, MS_BIND, NULL);
+}
+
static void prepare_layout_opt(struct __test_metadata *const _metadata,
const struct mnt_opt *const mnt)
{
@@ -558,7 +563,7 @@ TEST_F_FORK(layout1, inval)
LANDLOCK_ACCESS_FS_TRUNCATE | \
LANDLOCK_ACCESS_FS_IOCTL_DEV)

-#define ACCESS_LAST LANDLOCK_ACCESS_FS_IOCTL_DEV
+#define ACCESS_LAST LANDLOCK_ACCESS_FS_MOUNT

#define ACCESS_ALL ( \
ACCESS_FILE | \
@@ -572,7 +577,8 @@ TEST_F_FORK(layout1, inval)
LANDLOCK_ACCESS_FS_MAKE_FIFO | \
LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
LANDLOCK_ACCESS_FS_MAKE_SYM | \
- LANDLOCK_ACCESS_FS_REFER)
+ LANDLOCK_ACCESS_FS_REFER | \
+ LANDLOCK_ACCESS_FS_MOUNT)

/* clang-format on */

@@ -1735,14 +1741,14 @@ TEST_F_FORK(layout1, topology_changes_with_net_only)
enforce_ruleset(_metadata, ruleset_fd);
ASSERT_EQ(0, close(ruleset_fd));

- /* Mount, remount, move_mount, umount, and pivot_root checks. */
+ /* Mount, remount, move_mount, pivot_root and umount checks. */
set_cap(_metadata, CAP_SYS_ADMIN);
ASSERT_EQ(0, mount_opt(&mnt_tmp, dir_s1d2));
ASSERT_EQ(0, mount(NULL, dir_s1d2, NULL, MS_PRIVATE | MS_REC, NULL));
ASSERT_EQ(0, syscall(__NR_move_mount, AT_FDCWD, dir_s1d2, AT_FDCWD,
dir_s2d2, 0));
- ASSERT_EQ(0, umount(dir_s2d2));
ASSERT_EQ(0, syscall(__NR_pivot_root, dir_s3d2, dir_s3d3));
+ ASSERT_EQ(0, umount(dir_s2d2));
ASSERT_EQ(0, chdir("/"));
clear_cap(_metadata, CAP_SYS_ADMIN);
}
@@ -1763,7 +1769,7 @@ TEST_F_FORK(layout1, topology_changes_with_net_and_fs)
enforce_ruleset(_metadata, ruleset_fd);
ASSERT_EQ(0, close(ruleset_fd));

- /* Mount, remount, move_mount, umount, and pivot_root checks. */
+ /* Mount, remount, move_mount, pivot_root and umount checks. */
set_cap(_metadata, CAP_SYS_ADMIN);
ASSERT_EQ(-1, mount_opt(&mnt_tmp, dir_s1d2));
ASSERT_EQ(EPERM, errno);
@@ -1772,10 +1778,9 @@ TEST_F_FORK(layout1, topology_changes_with_net_and_fs)
ASSERT_EQ(-1, syscall(__NR_move_mount, AT_FDCWD, dir_s3d2, AT_FDCWD,
dir_s2d2, 0));
ASSERT_EQ(EPERM, errno);
- ASSERT_EQ(-1, umount(dir_s3d2));
- ASSERT_EQ(EPERM, errno);
ASSERT_EQ(-1, syscall(__NR_pivot_root, dir_s3d2, dir_s3d3));
ASSERT_EQ(EPERM, errno);
+ ASSERT_EQ(0, umount(dir_s3d2));
clear_cap(_metadata, CAP_SYS_ADMIN);
}

@@ -3029,6 +3034,106 @@ TEST_F_FORK(layout1, reparent_remove)
ASSERT_EQ(EACCES, errno);
}

+TEST_F_FORK(layout1, reparent_bindmount_deny)
+{
+ const struct rule layer1[] = {
+ {
+ .path = dir_s1d1,
+ .access = LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_MOUNT |
+ LANDLOCK_ACCESS_FS_READ_DIR,
+ },
+ {
+ .path = dir_s2d1,
+ .access = LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_MOUNT |
+ LANDLOCK_ACCESS_FS_READ_DIR |
+ LANDLOCK_ACCESS_FS_MAKE_DIR,
+ },
+ {
+ .path = dir_s3d1,
+ .access = LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_READ_DIR |
+ LANDLOCK_ACCESS_FS_MAKE_DIR,
+ },
+ {},
+ };
+ const int ruleset_fd = create_ruleset(
+ _metadata,
+ LANDLOCK_ACCESS_FS_REFER | LANDLOCK_ACCESS_FS_MOUNT |
+ LANDLOCK_ACCESS_FS_READ_DIR | LANDLOCK_ACCESS_FS_MAKE_DIR,
+ layer1);
+
+ ASSERT_LE(0, ruleset_fd);
+ enforce_ruleset(_metadata, ruleset_fd);
+ ASSERT_EQ(0, close(ruleset_fd));
+
+ set_cap(_metadata, CAP_SYS_ADMIN);
+
+ /* Privilege escalation (FS_MAKE_DIR). */
+ ASSERT_EQ(-1, bindmount(dir_s1d2, dir_s2d2));
+ ASSERT_EQ(EPERM, errno);
+
+ /* Mount point missing FS_MOUNT. */
+ ASSERT_EQ(-1, bindmount(dir_s2d2, dir_s3d2));
+ ASSERT_EQ(EPERM, errno);
+
+ /* Mount point missing permissions other than FS_MOUNT. */
+ ASSERT_EQ(-1, bindmount(dir_s2d2, dir_s1d2));
+ ASSERT_EQ(EACCES, errno);
+
+ /* Missing permissions other than FS_MOUNT, for a self bind mount. */
+ ASSERT_EQ(-1, bindmount(dir_s1d2, dir_s1d2));
+ ASSERT_EQ(EACCES, errno);
+
+
+ clear_cap(_metadata, CAP_SYS_ADMIN);
+}
+
+TEST_F_FORK(layout1, reparent_bindmount_allow)
+{
+ const struct rule layer1[] = {
+ {
+ .path = dir_s1d1,
+ .access = LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_MOUNT,
+ },
+ {
+ .path = dir_s2d1,
+ .access = LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_MOUNT |
+ LANDLOCK_ACCESS_FS_EXECUTE,
+ },
+ {
+ .path = dir_s3d1,
+ .access = LANDLOCK_ACCESS_FS_EXECUTE,
+ },
+ {},
+ };
+ const int ruleset_fd = create_ruleset(
+ _metadata,
+ LANDLOCK_ACCESS_FS_REFER | LANDLOCK_ACCESS_FS_MOUNT |
+ LANDLOCK_ACCESS_FS_EXECUTE,
+ layer1);
+
+ ASSERT_LE(0, ruleset_fd);
+ enforce_ruleset(_metadata, ruleset_fd);
+ ASSERT_EQ(0, close(ruleset_fd));
+
+ set_cap(_metadata, CAP_SYS_ADMIN);
+
+ /* Mount point has more restrictions than the source. */
+ ASSERT_EQ(0, bindmount(dir_s2d2, dir_s1d2));
+ ASSERT_EQ(0, umount(dir_s1d2));
+
+ /* Bind mounting a directory on itself with no FS_MOUNT. */
+ ASSERT_EQ(0, bindmount(dir_s3d2, dir_s3d2));
+ ASSERT_EQ(0, umount(dir_s3d2));
+
+ clear_cap(_metadata, CAP_SYS_ADMIN);
+}
+
+
TEST_F_FORK(layout1, reparent_dom_superset)
{
const struct rule layer1[] = {
--
2.47.1.613.gc27f4b7a9f-goog