[PATCH v4 1/4] landlock: Support file truncation
Mickaël Salaün
mic at digikod.net
Tue Aug 16 19:20:06 UTC 2022
On 14/08/2022 21:26, Günther Noack wrote:
> 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.
> +
This should be in the forth patch, below the file renaming and linking
section.
> 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
s/through file truncation APIs like/with/
> + * :manpage:`open(2)` with `O_TRUNC` or :manpage:`creat(2)`. This access right
:manpage:`creat(2)`, or :manpage:`open(2)` using `O_TRUNC`.
With only one "or".
> + * 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 */
>
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