[PATCH v4 1/4] landlock: Support file truncation
Günther Noack
gnoack3000 at gmail.com
Sun Aug 14 19:26:00 UTC 2022
Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation.
This flag hooks into the path_truncate LSM hook and covers file
truncation using truncate(2), ftruncate(2), open(2) with O_TRUNC, as
well as creat().
This change also increments the Landlock ABI version, updates
corresponding selftests, and includes minor documentation changes to
document the flag.
Signed-off-by: Günther Noack <gnoack3000 at gmail.com>
---
Documentation/userspace-api/landlock.rst | 10 ++++++++++
include/uapi/linux/landlock.h | 17 ++++++++++++-----
security/landlock/fs.c | 9 ++++++++-
security/landlock/limits.h | 2 +-
security/landlock/syscalls.c | 2 +-
tools/testing/selftests/landlock/base_test.c | 2 +-
tools/testing/selftests/landlock/fs_test.c | 7 ++++---
7 files changed, 37 insertions(+), 12 deletions(-)
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index b8ea59493964..6648e59fabe7 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -380,6 +380,16 @@ by the Documentation/admin-guide/cgroup-v1/memory.rst.
Previous limitations
====================
+File truncation (ABI < 3)
+-------------------------
+
+File truncation could not be denied before the third Landlock ABI, so it is
+always allowed when using a kernel that only supports the first or second ABI.
+
+Starting with the Landlock ABI version 3, it is now possible to securely
+control truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access
+right.
+
File renaming and linking (ABI 1)
---------------------------------
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 23df4e0e8ace..a2fef267bf34 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -95,8 +95,15 @@ struct landlock_path_beneath_attr {
* 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_WRITE_FILE: Open a file with write access. Note that
+ * you might additionally need the `LANDLOCK_ACCESS_FS_TRUNCATE` right in
+ * order to overwrite files with :manpage:`open(2)` using `O_TRUNC` or
+ * :manpage:`creat(2)`.
* - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
+ * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file through file truncation APIs
+ * like :manpage:`truncate(2)`, :manpage:`ftruncate(2)`, or
+ * :manpage:`open(2)` with `O_TRUNC` or :manpage:`creat(2)`. This access right
+ * is available since the third version of the Landlock ABI.
*
* A directory can receive access rights related to files or directories. The
* following access right is applied to the directory itself, and the
@@ -139,10 +146,9 @@ struct landlock_path_beneath_attr {
*
* 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)`,
- * :manpage:`access(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)`, :manpage:`access(2)`.
* Future Landlock evolutions will enable to restrict them.
*/
/* clang-format off */
@@ -160,6 +166,7 @@ struct landlock_path_beneath_attr {
#define LANDLOCK_ACCESS_FS_MAKE_BLOCK (1ULL << 11)
#define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12)
#define LANDLOCK_ACCESS_FS_REFER (1ULL << 13)
+#define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14)
/* clang-format on */
#endif /* _UAPI_LINUX_LANDLOCK_H */
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index ec5a6247cd3e..c57f581a9cd5 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -146,7 +146,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
#define ACCESS_FILE ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
- LANDLOCK_ACCESS_FS_READ_FILE)
+ LANDLOCK_ACCESS_FS_READ_FILE | \
+ LANDLOCK_ACCESS_FS_TRUNCATE)
/* clang-format on */
/*
@@ -1140,6 +1141,11 @@ static int hook_path_rmdir(const struct path *const dir,
return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
}
+static int hook_path_truncate(const struct path *const path)
+{
+ return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE);
+}
+
/* File hooks */
static inline access_mask_t get_file_access(const struct file *const file)
@@ -1192,6 +1198,7 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
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(path_truncate, hook_path_truncate),
LSM_HOOK_INIT(file_open, hook_file_open),
};
diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index b54184ab9439..82288f0e9e5e 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_REFER
+#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_TRUNCATE
#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/syscalls.c b/security/landlock/syscalls.c
index 735a0865ea11..f4d6fc7ed17f 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -129,7 +129,7 @@ static const struct file_operations ruleset_fops = {
.write = fop_dummy_write,
};
-#define LANDLOCK_ABI_VERSION 2
+#define LANDLOCK_ABI_VERSION 3
/**
* sys_landlock_create_ruleset - Create a new ruleset
diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
index da9290817866..72cdae277b02 100644
--- a/tools/testing/selftests/landlock/base_test.c
+++ b/tools/testing/selftests/landlock/base_test.c
@@ -75,7 +75,7 @@ TEST(abi_version)
const struct landlock_ruleset_attr ruleset_attr = {
.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
};
- ASSERT_EQ(2, landlock_create_ruleset(NULL, 0,
+ ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
LANDLOCK_CREATE_RULESET_VERSION));
ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 21a2ce8fa739..cb77eaa01c91 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -399,9 +399,10 @@ TEST_F_FORK(layout1, inval)
#define ACCESS_FILE ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
- LANDLOCK_ACCESS_FS_READ_FILE)
+ LANDLOCK_ACCESS_FS_READ_FILE | \
+ LANDLOCK_ACCESS_FS_TRUNCATE)
-#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER
+#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
#define ACCESS_ALL ( \
ACCESS_FILE | \
@@ -415,7 +416,7 @@ TEST_F_FORK(layout1, inval)
LANDLOCK_ACCESS_FS_MAKE_FIFO | \
LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
LANDLOCK_ACCESS_FS_MAKE_SYM | \
- ACCESS_LAST)
+ LANDLOCK_ACCESS_FS_REFER)
/* clang-format on */
--
2.37.2
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